In this Issue
our regular monthly meetings following a 2-month hiatus because of the
Auburn home football games, on Friday November 1, at 7:45PM, in
room 215 of Davis Hall,
the Aerospace Engineering Building, on the main
campus of Auburn University. The program this month will be a
presentation by Maggie Murphy, a senior in mechanical engineering
at Auburn, on her telescope project. In response to our invitation
to speak, Maggie wrote:
would love to present the telescope project at the November meeting! I
can bring in the stand, mirror blanks, grit, etc and hopefully I will have
finished the Foucault testing device and made a pitch lap, in case anyone
wants to try polishing the mirror! Maybe we could look at shadowgrams and
everyone would be more than welcome to have a go at re-figuring the paraboloid?
I have some pictures I'd love to share [from] the Stellafane convention
over my break at home...it was such a riotous blast!! The weekend
convention was such a wonderful time and a great learning experience! I
brought the mirrors and got a chance to talk to some of Springfield's Amateur
Telescope Makers' members after a mirror making presentation they did.
They were so kind and helpful, and reassured me that the mirrors are in
a fine condition to continue forth with the grinding process!
a very good crowd because we have had to cancel the past two meetings and
especially due to the excellent subject matter of the presentation.
star party will be on the Saturday following our meeting, Nov 02, either
at the Conecuh
National Forest or at Cliff
Hill’s farm — clouds permitting, of course. We’ll discuss these
options at Friday’s meeting.
Troop Stargaze Report
5, Frank Ward’s son's scout troop had their stargaze in Lowndes
County. Frank had arrived earlier so the scouts could view the
sun with the AAS PST solar scope. I arrived at sunset to find Frank’s
12-inch Lightbridge Dobsonian already set up. I was greeted by some
curious cattle checking out their strange visitors. The cows had
made us some pies to make us feel welcome. We almost didn’t step
in them. The scout’s prepared a steak supper for us. By the
time the boys arrived at the observing site from their camp site, we had
50% - 75% cloud cover, but we managed to have the boys view an assortment
of deep-sky objects and binary stars that were required for their astronomy
badges. Frank wrapped up the viewing suggesting to the boys that
they concentrate their studies in the fields of math and science.
Clouds not withstanding, the sky is quite dark at this site with the exception
of one cell tower strobe about a mile away. Frank has received permission
to use this site from the property owner.
Airmen National Historic Site Stargaze
now holds the record for most cancellations beginning in May of 2012.
This time, the government shut-down was to blame. Ranger Christine
so sorry that we could not host the stargaze planned for October 12th.
This time it was Congress' fault for shutting us down. I hope nobody
came out. I had no way of contacting you because we had to turn in
our keys, laptops, etc. We will have to regroup and see what
management wants to do. I'll be in touch.
It's good to have
you back. I know you had no choice in this matter and no need for
the apology. We hated it too. We had let all of our folks know
why the event had to be cancelled. Let us know when you want to try
Lolley has asked that we postpone her group’s stargaze until January
or February. Jennifer writes:
would love to host one with the club to try it out on December 7th (without
crowds). I do have permission to use Kiesel-. I went out and it is so nice
and dark. We have bathrooms, parking, the gazebo to show the movies
and have snacks. Would like to do this in January or February if possible
with crowds- have student help then for set-up. I also am trying to bring
in a Starlab planetarium to try out- best for in January. If this won’t
work for January I will try to recruit help for December.
our December dark-sky star party for this dry run before Jennifer invites
her Foresr Preserve members.
Combs: Jupiter - October 16, 2013
Owsley: Take a look at the website below & check out the
great coursework that is available (for free). https://www.coursera.org/
Morrison: Thought you might like to see what someone did with
just a camera on a tripod. Wow! http://www.flickr.com/photos/latent0image/6866355961/
. And, I think you’ll like this: http://www.xkcd.com/1276/
Zachry, will be collecting for club subscriptions to Astronomy
Sky & Telescope magazines at the November meeting.
The cost is the same as last year: Astronomy (Regular $42.95),
Club rate $34.00. No minimum number of subscriptions required.
& Telescope (Regular $42.95), Club rate $32.95.
We must have 5 subscriptions/renewals to qualify.
made out to Auburn Astronomical Society, to:
John B. Zachry, Secretary/Treasurer
you are a 2013 Auburn Astronomical Society member and want a club subscription
to either magazine but can't attend the November meeting, please
send me an e-mail telling me what you want. You can pay me later. Don't
make me wait for your check and then have to write a separate letter in
your behalf, make a separate trip to the bank to make a deposit, and do
an additional update to my club subscription files on my computer.
Halloween astro-imaging: http://www.astrobin.com/full/59859/
your cooperation. - John
Phil also posted
on the AAS facebook
been getting much sleep with all the clear nights we've been having this
past week. Here's a little something from last night. IC1805 / Melotte
15. It's a combination of RGB and a luminance layer created with a blend
of H-alpha data and the red channel, about 4 hours integration. Be sure
to look at the full resolution version. I have plans to shoot this again
in November with the 8" RC so I can get in closer to the structure in the
center of the Melotte 15 cluster.
would really like to get everything set up to enable to club to meet at
Trinity in the future so we can avoid the football parking problems for
everyone. In the future, I believe meetings would run smoothly at
Trinity Lutheran Church on Gay Street and I talked with Pastor Hudspith
and the church would be happy for us to meet there. There is always
parking on Friday nights and towing does not happen Friday nights.
I have a key as well. We would have large meeting room and access
to a video player and even a large flatscreen TV and/or projector, whichever
I think we're good for the rest of this year, but it's a really good chance
we'll be taking you up on this offer next year. Thanks for your help.
I'll be graduating
after this year, but if the same problem comes up next year, I am sure
the church would still be happy to host AAS.
On a related
topic, Rodger Morrison writes:
propose we change our club meetings to be always two Fridays after our
new moon stargaze.
It seems we tried
this early in the club’s history. We can discuss this at the meeting.
am trying to help Camryn find resources for her science fair project. Camryn
has hypothesized that the differences between light pollution in cities
compared to that of rural areas is not just the result of a higher concentration
of artificial light. She wants to focus on other things in the atmosphere
that effect the concentration of light and give special focus to how those
things effect the color appearance of the light pollution.
it all Together:
schools last week and was placed in an advanced science class that requires
her to complete this project as a large part of her test grade. She
told the teacher that she would have no problem meeting the deadlines already
given to her class and immediately gave the teacher the hypothesis. You
have to love her ambition. She must turn in a research plan by October
I have sent
a message to Rodger Morrison, and I was hoping you may know others
that might be able to help guide her in the right direction. Thanks
to the Orion 10” f/4.7 OTA and related equipment.
Saturday October 12th was a productive evening at Cliff Hill's farm.
I have been laid up from knee surgery and spent a couple of weeks making
modifications to the 10” Orion f/4.7 that I purchased last Christmas.
The weather through the summer months, along with some work related issues
that killed my free time, kept me from getting finished with the upgrades
and taking the scope out at all. I have done quite a bit, and think
I really have a versatile and functional setup now, so I thought I would
take some time and summarize everything for the club. Maybe someone
can get some good ideas from what I have done and improve on them.
I started this
project earlier this year with Orion’s 10” f/4.7 OTA. This is a plain
scope with very little in terms of extras. No baffles, no finder,
no primary fan, heat, etc. In short, I got the mirrors mounted in
a steel tube with a focuser and little else. I also bought a pair
of tube rings, which worked well, and already had a Celestron bar to mount
with. I already had a 9x50 finder and a hypertuned CG-5 mount.
This summer, Russell passed a note from someone in Florida that was selling
a new CGEM mount for half price, which I pounced on to replace the CG-5.
I already had a laptop and an assortment of other stuff, but really did
not have a good solution for autoguiding, so I purchased Orion’s Magnificent
Mini Deluxe Autoguiding package (the one with the helical focuser), which
came with their StarShoot Autoguider (SSAG). This is really just
a 9x50 finder, but with a helical focuser for a 1.25” eyepiece, where they
added the SSAG. Swap out the guiding camera with an eyepiece and
you have a finder, which is nice though I don’t need to use it that way.
The only problem with this setup is that it is really not good for any
focal lengths over about 1700mm, and the Orion 10” comes in at 1200mm.
This means that I am okay using the SSAG to guide at prime focus, but not
if I use a 2x Barlow. I was also able to acquire an Orion ShortTube-80
(ST-80) for more wide-field work, which Orion also sells as an optical
tube for guiding with the SSAG. I added another Celestron bar I had
in my junk box, along with a pair of tube rings, and this little scope
fits nicely on topside of the OTA’s rings. It has a finder block
for the SSAG to mount also, so I can image through the ST-80 as well.
I added a pair of threaded brass screws pointed out from the tube rings
on the 10”, which fit into holes I drilled on the ST-80 bar to mount the
two together. So, I can image through the 10” at prime focus using
just the SSAG, or I can mount the SSAG camera on the ST-80 and then mount
that to the 10” for imaging at longer focal lengths. Cool.
One thing that
bothered me when I did get the big 10” hanging on the little CG-5 mount
was the weight . I was right at the CG-5’s weight limit of @32lbs,
and the slightest little bump or breeze really moved the scope quite a
bit. I like to describe it as akin to a kite in a hurricane, and
it really felt like it, but the entire assembly mounts quite nicely and
very sturdily on the new CGEM without issue. I haven’t modified the
CGEM, except for the addition of a couple of extra weights and a custom
machined counterweight bar. I also added a PTFE bushing between the
machined face where the mount sits on the top of the tripod. This
weekend’s testing session revealed that the two faces do not move easily
when drift aligning, so this was a must-do to smooth things up. Other
than that, no modifications to the CGEM were made, other than moving the
polar scope over from the CG-5, and setting the entire thing up on a trio
of Celestron’s vibration isolation pads (which now sit on small pieces
of heavy plywood).
that we all hate is that of dew. I HATE losing an evening to dew
on my secondary. As most of you know, dew is typically not much of
an issue in a Newtonian because the primary is at the bottom end of the
tube. However, it seemed to be worse on this OTA than on my earlier
6”, but I also did not have a dew shield or any kind of anti-dew controls.
I enjoy fabricating things from scratch and experimenting with different
designs, so I thought I might try to make some resistive heaters.
I could have bought some Nichrome wire, but decided to use a pile of ½
watt power resistors I have had on my workbench for a while. These
are REALLY easy to make, and you almost can’t screw it up if you follow
basic soldering principles. First, I decided how much wattage I wanted
on everything (primary, secondary, focuser, ST-80 objective, ST-80 eyepiece,
finder objective, finder eyepiece, Telrad, SSAG objective, etc.)
These values are easy to find on the Internet, and you only need to be
approximate. I also decided to keep everything on a 12v system because
I had a few extra automotive batteries collecting dust, which included
a 104 amp-hour deep cycle marine battery. The amount of resistance
(in Ohms) required to obtain a given wattage of heat is easy to calculate.
Just square the voltage and divide by the wattage you want, which results
in the number of ohms required. For example, if you want 5 watts
at 12v, then you simply divide 5 into 144 (12 squared) to get 28.8 ohms.
So, you only need to solder enough resistors in series (end-to-end) to
make about 30 ohms. You must be careful using ½ watt resisters
though. To get 5 watts using ½ watt resistors means that you
must use at least 10 (15 would be better) equal-sized resistors or you’ll
burn them up. Another way is to wire them in parallel so that a common
hot wire (+12v) runs down all of one end, and a common ground wire (the
negative side) runs down the other. Then, to get 5 watts, one only
needs to figure out how many resistors you want to use and then calculate
the heat produced by each one. I wanted to keep each resistor to
less than a third of a watt (as a safety margin), so to get 5 watts, I
simply divided 5 by 1/3 to get 15 resistors. A single resistor of
about 432 ohms produces about 1/3 of a watt at about 12v, so that was easy
to do. Be careful, though. A fully-charged lead-acid battery
actually puts out 12.75v, so your wattage will be about 13% higher and
you don’t want to burn up the resistors by driving them right to ½
watt each. Once you get the resistors soldered together, just sandwich
the assembly between two layers of electrical tape, making sure your wires
are not so close that they might touch and short the whole thing out.
Then sandwich the taped strips between two layers of hobby foam (I used
3mm black foam, but any close-cell foam will work – Hobby Lobby sells in
many colors). Solder however much slightly heavier wire you need
to the hot and ground side, then solder the male side of an RCA connector
set to the other end of that, and DONE. One resistive heater.
Use an ohmmeter to check the resistance, then double check your calculations
just to be sure. Make one heater strip for each thing you want to
warm up and you are finished. If you want to wrap the heater strip
around something, you can attach Velcro strips or whatever you want to
use (I used some old Velcro wire ties). Be careful not to put too
much heat around anything plastic, as it will obviously melt.
the heaters’ output is a different story. Simply wiring them to a
switch is a problem because you are either heating up your optics more
than is needed, or you are not heating them at all. This will then
require constant attention (with an infrared thermometer) and switching
to get the right amount of heat. If you heat up your optics too much,
especially the primary and secondary glass, the excess heat WILL change
their optical shape. This is especially true for astrophotography
applications, which is my passion, so I needed something to control 12v
that was cheap, durable, and easy to use. I found a solution on eBay
that is both affordable and perfectly suited to the task, but which will
also allow me to control three separate channels. The solution is
a 12v PWM LED strip-lighting controller (see eBay item number 170858664352).
PWM stands for Pulse Width Modulation and basically means that there is
a microcircuit that opens and closes a solid state switch very rapidly.
The switch controls the application of 12v to whatever is on the output
side of the circuit. Think of it like this. Imagine that you
wanted 6v applied to your heaters because 12v was too much and 0v was too
little. 6v is 50% of 12v, right? In a given period of time
(1 second for example), the PWM circuit would simply turn on the 12v 50%
of the time, and turn it off for 50% of the time. The average voltage
is 12v, which is perfect for things such as resistance heaters, DC electric
motors, and even color LED dimming. If you want only 9v, then the
circuit would be energized for 75% of the time, and off for the other 25%.
Perfect!!! About $12 and a few days later, I had one 3-channel controller
(36 watts total output per channel – a TON of heat!!). I divided
up the heaters I made into 3 groups that made sense at the time, and connected
them each group to one channel of the controller. I also needed to
add a few 12v switches to control power to a few things, so I decided to
mount everything in an old aluminum hard drive enclosure that I had in
my junk box.
that I wanted to do, which all astrophotographers should think about, was
to get the battery out of my Canon DSLR. Some of the internal heat
within the camera comes from the battery itself (especially during long
exposures at high ISO settings), so why not relocate the battery out of
the camera completely? I found a 110v power adapter for the camera
on eBay (similar to #270979950899), then cut off the 110v-to-7.5v transformer.
I then bought a 12v-7.5v buck transformer (small – eBay #360736667420),
connected it to the part that inserts into the battery slot in the camera,
then wired the other end to an RCA connector. Perfect, now no battery
in the camera and it will run on 12v, just like everything else.
I did the same thing to the 3v (2 AA battery) power source in my Telrad
(the 12v-to-3v transformer weighs less than the batteries – see eBay #121134160042),
and then routed all cables to lighted switches in the new power box I made.
I then added a 12v PC cooling fan (40CFM) to the primary mirror cell (the
factory mount was a perfect fit), then wired that up as well. I thought
it prudent to add a fuse to the input side of the power box and will add
an output fuse bank later. I added 12v input wires, with battery
clips, put the whole thing together, and then bundled the wires together
with lightweight polystyrene cable wrap. I also wrapped in a couple
of USB cables for the camera and the DSLR. I hung the new wiring
harness to the OTA with Velcro cable ties. I used female RCA adapters
for quick disconnects, with the center pin positive. Always use female
RCA to reduce the chance of accidental short circuits, and make the center
pin positive so that you don’t accidentally reverse the polarity on any
12v accessories you might later add. The entire harness adds very
little added weight to the mount and keeps the cables from moving around
much during astrophotography sessions. I can control the heat levels
on everything, switch all non-heater accessories on/off at will, and everything
runs a LONG time on a single 12v battery. I did discover that the
deep cycle marine battery I had been using was actually a factory defect,
so AutoZone replaced it at no charge. I think the bad battery was
what blew up my 500W inverter in September, which I also replaced with
a new 800W unit by Black and Decker. Now, I have 12v straight to
the power box, the CGEM, and the inverter, which supplies 110v to my Dell
laptop and a powered USB hub.
The Dell laptop
is a dual-processor Windows 7 box, but is nothing fancy. I did load
a few pieces of software though. I connected the CGEM mount keypad
to the laptop through a USB serial adapter, which allows me to slew the
mount using Stellarium. Everything connects to the laptop via USB,
which includes the mouse, CGEM mount, SSAG, Canon DSLR, and a new TEMPerHUM
device that records temperature and humidity. There were not enough
USB ports, so I added a powered USB 2.0 hub that I had in my junk box and
moved everything to that except for the Canon and the autoguider.
This means my mouse, the mount’s serial input, and the TEMPerHUM all share
a single USB port on the laptop and the two camera sensors don’t have to
compete for USB bandwidth. I installed copies of copy of AstroPhotographyTool
(APT) that I wanted to try out, but also wanted to try BackYardEOS (BYE),
which Wes and Phil introduced me to a month ago. I had loaded a trial
copy of BYE on the laptop, along with Stellarium, PHD (which came with
the SSAG), and a couple of other things. I use Stellarium to identify
which targets I want to image and then to slew to them without having to
touch the mount. I then use Bye to control the imaging session, which
includes dithering the mount during guiding and such. I must admit
though that APT has a much better drift alignment tool than the simple
reticle assist provided in BYE. I used APT to drift align, which
is something that normally takes me an hour or so to do properly without
software assistance. Using APT however, I can do it in about 10 minutes
or so and can even do it as soon as I can see a few stars. Nice,
cheap, and easy to set up. Simply set up the mount, do a quick polar
align with the polar scope, then do a two-star align (no calibration stars).
Once that is done, slew to a star near the equator and meridian and run
the polar align routine in the CGEM software (takes about a minute).
Then, while still pointed at the same star, use APT’s align routine to
adjust azimuth, slew along the equator to a star on either the east or
west horizon, and run APT’s align routine to adjust altitude. Once
that is done, slew back to the meridian and run them both again.
That’s it. Drift alignment is completed. I then slew back to
the home position (OTA pointed almost at Polaris), cycle the mount’s power,
and then go through a two-star align with a few calibration stars.
Done, aligned, and ready to accept input. Of course, I also focus
using a Bahtinov mask that I had cut by a guy in Great Britain, and as
soon as I can, I align the finder, Telrad, and whatever else I have mounted
to the OTA while it is pointed at a bright star. One can also align
the accessories in the daytime using anything in the distance, and there
is a nice distant light several miles to the east, straight down Cliff
Hill’s runway, which is perfect for aligning just as it is getting dark.
that the contrast with this OTA was not that good from the factory, and
a quick inspection soon revealed why. The interior of the scope was
not black at all, but was a very dull flat green color. It was also
not baffled, which led to internal reflections that also hurt the contrast.
Baffles are merely rings that are affixed to the inside of the OTA that
are used to block internal reflections by shadowing the primary and the
focusing tube. The interior of the focusing tube was nice, clean,
shiny aluminum. Darn. It should have been jet black and flat.
This would be a simple fix, but as I told my wife, it is like digging a
ditch. Not really a complicated process, but it really takes a lot
of effort. First, I used the online tool “Newt for the Web” (http://stellafane.org/tm/newt-web/newt-web.html)
to calculate the baffle sizes and locations for the OTA. To use this
tool, one will need a micrometer that measures in fractions of an inch,
and a good precision ruler. Simply enter the values in the online
tool as you measure them, press calculate, and then switch to the “Dimensions”
tab. Here, you will find a very precise measurement of the inner
diameter of each baffle, along with where in the OTA it should be fixed.
Easy. Just remember to measure twice and verify everything over and
over until you are sure your measurements are correct. Cutting the
baffles, however, is not so easy. As it turned out, cutting a perfect
circle is a real pain in the fanny, unless one is cutting thin paper (useless
for this purpose). I found a plasticized paper often used for matting
artwork, and had a local framing shop cut the baffle rings with their computerized
mat cutter. Total cost to have the baffles cut (including the material)
was less than $30. To affix them to the inside of the OTA required
a complete disassembly, then using the precision ruler to place one side
of a baffle where it needed to be. I then dabbed a small bit of hot
glue to hold it, then set the other side of that baffle, and repeated at
+/- 90 degrees. After each baffle was set, I ran a bead of hot glue
around both sides to permanently fix it in place and then moved to the
next baffle. The three narrowest baffles, however, are at the focuser
end of the OTA and were too thin to do this way. Rather, I used black
foam rubber adhesive door-sealing tape, which just happened to be the right
thickness. A dab of super glue here and there makes sure the adhesive
doesn’t come loose. After everything was in place, I used an entire
can of flat black Krylon on the inside of the tube to REALLY darken it.
Krylon has a waterproofing effect and should also seal the baffles against
any moisture that might land on them. Finally, I bought a sheet of
black velvet from Hobby Lobby and cut a few lengths to line the inside
wall of the OTA opposite the focuser. This stuff is as black as can
be had for less than a buck (technically, $1.09 with tax). I used
the remnant pieces to line the inside of the focuser tube and a couple
of other things. While I was painting, I also removed and painted
the focuser tube inside and out. The inside of the OTA is so dark
now that one must use a flashlight to see much, even in a well-lit room.
This was a cheap and HUGE improvement!!!
I noted during
this process that the focuser centerline was only 7.5” from the tube opening.
Ideally, it should be 1-1.5 times the diameter of the OTA, which is just
over 12”. Hmmm. Most folks don’t use a dew shield with a Newtonian,
but it seemed as an easy solution to reduce dew, so I made one from a 12”
Sonotube. Sonotubes are the yellow cardboard tubes (like a paper
towel tube, only bigger) and are used as concrete forms for setting foundation
piers. I found a damaged one at Lowes (crushed on one end) and was
able to secure it from them for about half what they normally cost (I think
I paid about $5 or so). I cut a piece to length, then painted the
inner and outer face with Krylon to seal it up some against moisture and
turn it very black. After the paint was dry, I lined the inside face
with 2mm black hobby foam so that it would friction fit against the aluminum
ring on the open end of the OTA. I needed a way to keep it aligned
with the OTA, so I replaced the short ¼” screws that held the end
ring to the OTA with 1” screws of the same thread size. The replacement
screws are pointed outward, so they stick out a bit. I then put nuts
on the end, just for safety to keep them from working loose and falling
down the tube onto the primary, and cut equal-depth slots on the dew shield.
The entire things fits on the end of the OTA and the screws seat into the
slots the same way all around to keep the shield lined up and in place.
Works great and is reasonably light weight. I am not really happy
with this however, and might break down and buy a plastic dew shield made
for this OTA from Orion, which weighs about a third as much.
I had quite
a bit of left over 2mm and 3mm foam sheeting, so I made a few dew shields
for the smaller tubes on the finder and SSAG. I also used the leftovers
to insulate outer facing sides of the heaters so they would not lose as
much heat to the air. I also added a neat Telrad dew shield, from
a design I found online. One thing I noted was that the primary end
of the OTA was totally open and one could simply light up the inside of
the OTA (including the focuser at the other end) with a flashlight.
Hmmm. I found this to be interesting, and studied on it for a while.
I ended up cutting a sheet of black foam-core sandwich board (similar to
CoroPlast, but not corrugated), and set it in place in the back end of
the tube. I cut a square hole in it for the fan blades to rotate
in, then mounted it over the fan with the fan’s mount screws holding it
in place. I then hot glued a strip of 3mm foam around the perimeter
to help isolate any fan vibrations. To remove it, I merely remove the fan
mount screws and the entire thing comes off in my hand. One added
benefit is that it acts as a shroud to direct the air entering the fan
up and around the primary itself, rather than simply bouncing air against
the primary and back out to the atmosphere. This should improve the
efficiency of the entire fan assembly considerably.
As I said before,
I use an unmodified Canon T2i DSLR for imaging, but I did load Magic Lantern
on it. This allows me to take very long exposures repeatedly and
automatically without having to touch the camera (the Canon is limited
to 10ea x30sec exposure runs from the factory). Using BYE negates
the need for Magic Lantern, but it does have some cool functions to use
if one is not using a laptop to control the camera, so I mention it here.
had only to put it all together and line everything up before heading to
the field on Saturday. To keep the center of the OTA’s mass as close
as possible to the axis of rotation, I mounted the OTA in it’s rings such
that the focuser drawtube is pointed directly back at the axis. I
then mount the camera such that the hot shoe is pointed at the open end
of the OTA. This does three things. First, it orients north
in the same position in the image every time I set it up in the field,
which means I can image over multiple nights and not have as many alignment
problems during post-processing. Second, it makes the X and the Y
coordinates of the image correspond to the DEC and RA axes during APT’s
drift alignment process and makes that much easier as well. Third,
it moves the center of mass of the camera inwards so less counterweight
is required. Orienting the SSAG sensor in a similar manner makes
autoguiding more efficient, because RA and DEC move along one of the two
sensor axes and the algorithms don’t have to work as hard (mathematically)
to determine how much to adjust during guiding.
With all of
these mods done, I was ready to go out and give it a try to see how it
was going to work. Our October 12th stargaze at the Tuskegee Airmen
National Historic Site was postponed due to the government shutdown, so
I headed over to Cliff Hill’s place where we usually meet for our new moon
stargaze. I arrived about sunset and it did not take me long
to set everything up and balanced. Everything worked almost as I
had hoped, with only a couple of minor exceptions. I did note one
bad connector, which supplied voltage to the secondary heater (which I
had wrapped around the secondary’s aluminum support post). I fixed
that a few days later. I also noted that I forgot to align the mount
for the finder scope with the OTA during reassembly, which I corrected
on the spot. I tried out AstroPhotographyTool (APT) for the first
time, and used it to focus and drift-align. I will continue to use
APT rather than BYE for aligning, because it simply works better and is
more automated. This was also the first time I tried to use my new
TEMPerHUM device, which interfaced automatically with both BYE and APT
(both add the data to the pic files!). This device (paid about $20
new on eBay) revealed that the temperature stayed only a few degrees above
the dew point. I ran a couple of short imaging sequences with BYE,
which worked flawlessly. My new heaters worked well everywhere except
for on the Telrad, which I had inadvertently unplugged. After plugging
it back in, the dew was gone very quickly. The dew was not really
all that bad, as observing sessions go, because of the cloud cover, so
it will take a few more observing sessions before I am confident that this
will work as I want it to. I checked the focus using the FWHM function
of BYE, with the tube fan both on and off. There was no difference,
which means that I might not use any primary heat at all and may simply
leave the fan on 100% of the time, which is less likely to change the optical
characteristics of the mirror’s shape. Autoguiding worked great,
though I had to tinker around with the PHD settings until I found what
worked, but the real problem seemed to be that I needed to let it recalibrate
each time I slewed very far across the sky before autoguiding. All
new hardware and modifications seemed to work well, though I have a few
very minor modifications to make to a couple of things before next new
moon stargaze(mostly just tweaks to mounting and such, nothing substantial).
The skies were VERY cloudy with only broken patches to look through before
midnight (and nothing from then until 1am), so there was not chance for
any serious astrophotography, though I did get a few shots at M42, M33,
M1, and a few other things. All-in-all, a very successful testing
and tuning evening.
of the images, I noted something in some test images I made of Deneb.
The diffraction spikes from the secondary’s spider looked normal, but there
was a weird fan pattern between them and additional, but less pronounced,
set of spikes at 45 degrees from the spider’s spikes. It turns out
that the primary’s retention clips are very large and invade much more
of the primary mirror’s surface than is necessary. I did a little
research and talked to a few people more experienced than I, and discovered
that the 10” mirror’s edge right at the bevel is likely slightly mis-formed.
I came across many others that have had the same problem and corrected
it with an additional baffle right at the mirror’s edge, but cut in such
a way as to cover the outer 5mm of mirror surface. This “mask” baffle,
along with a modification to the mirror’s clips to bring them out some,
results in NO clips being visible to the secondary. I also noted
that the mirror clips are steel, but sit on rubber L-clamps that were clamping
down on the mirror face. The underside of the mirror sits on four
cork buttons right near the middle 1/3 of the mirror. I believe that
the downward pressure of the clips on the edge, coupled with the upward
pressure from the underside cork, as well as a probable turned down few
mm or so at the edge of the mirror face, was deforming the mirror considerably
and causing the strange diffraction pattern. The modifications should
have taken the stress off the mirror but has reduced my aperture from 254mm
to 244mm (which increases my focal ratio from f/4.7 to f/4.92). I
would rather have 244mm of well-shaped surface than 254mm of surface that
includes a turned-down edge. The increase to the focal ratio should
also help reduce a slight bit of coma that my GSO coma corrector is not
adjusting for. I also found out during this process that the GSO
coma corrector requires 75mm between the last lens face and the sensor’s
imaging face, but I only had 61mm, so I added a 13mm extension tube at
the end of the T-ring. I’ll try another test on Deneb soon, but suspect
that things are MUCH better.
to all those that provided input and advice. Hopefully, these details
can help someone else.
Newtonian and imaging gear
Hope to see
everyone at the meeting,