In going through the process of preparing the new RASA (Rowe-Ackerman-Schmidt Astrograph) scope for photographic use, it struck me how much is really involved in getting a scope like this ready to do it’s very best. So after going through the process I thought it might be useful to others to write down how I approached the whole thing in case it might be useful to other people getting their rigs together.
I recently posted a RASA 11-Inch Unboxing Video on my YouTube channel which you can watch here:
After sitting down and analyzing things, it occurs to me that he best way to describe this project is that it’s not really a telescope, it’s a system that involves a telescope!
The system involves the basic characteristics of:
Collecting light
Getting that light to the camera’s detector
The Equatorial mount for the scope needs to follow the sky pretty close to perfectly.
The telescope needs some support equipment in order to be at its best.
If you can get all of this working reliably at the same time you can take a pretty decent picture as long as you know what you’re doing after the fact with computer image processing.
That bit about ‘at the same time’ is really the crux of the biscuit here because like so many things that have a lot of ‘moving parts’, you can’t do any one thing exactly right that will guarantee yourself a good shot, but doing any one of those things wrong will most definitely ruin your picture.
The basic tube on the Losmandy G11 mount ready to be outfitted for astrophotography
RASA Facts
A RASA telescope is a contemporary take on an older design of scope that’s strictly for astrophotography–you couldn’t look through a RASA scope if you wanted to because there’s no place to put an eyepiece! This is not big deal because the benefits of this design for photography are tremendous! The camera actually is up at the front of the scope by the lens not in the back where you might expect.
First of all it’s eleven inches in diameter so it gathers a lot of light. Secondly it has a short focal length (620 mm) which gives it the ability to see a nice wide-field view of the sky and an a really fast f ratio of 2.2 (most scopes would be f 5 to f 10). This means in addition to gathering a lot of light it does it very quickly so you get the equivalent exposure in about half the time or even less. So you can shoot shorter shots and there’s less go go wrong (clouds, planes, wind blowing the tube), or if things are working really well you can shoot the same duration and be a lot more saturated in your image for improved signal to noise.
Let’s achieve some balance here…
Getting the entire rig balanced correctly is important. Really important. The telescope itself weighs 43 pounds, the camera and support equipment is probably 6 pounds altogether. There are 42 pounds of counterweights on the scope to counteract this so you’re talking about 90 pounds on your mount. Telescope mount motors are selected for their accuracy and robustness and really aren’t all that powerful so the telescope rig must be carefully balanced so that as it moves through it’s range of motion the motors are pushing with precision something that seems a lot lighter than it really is.
The hard part is, if you want to get 90 pounds to feel like 1/4 pound to the mount’s motors, you have to balance with everything on there, exactly where it’s going to go, wired up just like the real deal, and it has to be repeatable. When you’re out on location it takes long enough to get ready to shoot (about 2 1/2 hours from arrival to being confident enough to push the GO button on a 2 or 3 hour exposure run without spending time scratching your head about where something should go on the telescope and how you’re going to get a a wire or power to it. The good thing about a balanced scope is that it doesn’t cost anything which is terrific in a hobby where every time you sneeze it seems like you just spent $400
Listing Hard
So I made a list of the various boxes that need to be on the scope, took note of the kind of wiring associated with each and tried to figure out where each should go for the cleanest setup. By clean I mean wires not criss-crossing all over the place, one clean exit point near the center of the mount for cables to come and go to the ground or elsewhere on the mount without inducing ‘cable drag’ which can act like a variable weight. Here’s what I came up with.
Camera–located up in the front of the scope it has two wires going to it: one USB3 cable for data and a 12v power cable to drive the camera’s thermo-electric cooling system which cools the detector for better noise characteristics. The pros use liquid nitrogen! (-340°F)
Guide scope with guide camera installed: one USB2 cable for power and data
ZWO “ASI Air” WiFi enabled camera and mount control box. Almost every cable connects here.
Power supply for camera cooling: 110v.
Dew heater straps control box. 3 heater straps connect here, 12v in from ‘the outside’
Dew Shield on the front of the scope: not electrical but tricky to make stay put.
I took some time and visualized where things could go. Some things were self-evident like the guide scope is going to be on the top rail, probably close to the middle so as not to be contributing too much to scope imbalance. Also obvious was that the camera controller box should be near the front since the cables from it to the main camera were on the short side.
Some things didn’t matter so much like the power supply for camera cooling and the dew heater controller module. Before deciding where to put these I wanted to see if there was any consensus among other RASA owners about how the scope balances on their mounts so I looked at 3 or 4 YouTube videos and everybody seemed to have the scope slightly forward from halfway in the mount saddle which is reasonable because the heaviest thing on the scope is the mirror which is in the back.
So it turns out that the cable from the camera cooler power supply was too short to reach from the back so it went up next to the AIR and the dew controller went to the back so as not to overload the front.
The lower front dovetail bar of the RASA setup with the AIR (camera controller and mount guiding), and the power supply for the camera’s cooling system.
Let me be your guide
New for me on this project is using a separate scope for guiding the mount. In my other setups I use what’s known as an “off-axis guider” which is a little assembly with a 2nd camera that screws on before the camera and any filters to pick off a little bit of the light coming in and send it to the guide camera.
With the RASA the camera is up there in front of the lens in what would normally be the dead-zone associated with a secondary mirror, but the off axis guider is pretty large and the guide cam sticks way out in front of the lens blocking light and causing diffraction so I decided to move guiding to a 2nd small scope mounted on top.
The guiding telescope with guide camera installed. As it turns out I needed an extension tube for the camera to reach focus
Having a 2nd scope for guiding is both good and bad. It’ll be good because the guide cam will have a crisp wide-field view with many more suitable guide stars. That can be good if you’re shooting in an area of the sky that doesn’t have a lot of foreground stars. The view through the off axis guider can be kind of haphazard and the stars can be kinda distorted because it’s seeing at the edge of the telescopic field where, despite my best efforts, things can be less than well, stellar.
The bad part about the separate guide scope is that you have to align it (another thing to align!) so that it’s looking in exactly the same direction as the main scope so that a star that’s in the center of the camera is also centered in the guide scope. This is called being ‘orthogonal’ to each other and is important because the guide scope will mis-direct the mount while guiding which would make the guiding commands a destructive thing instead of a help to the photo.
I was able to bolt the guidescope to a little thing I’d had floating around that can just clamp on to the top dovetail bar providing me with flexibility of placement and the ability to quickly disconnect it for storage or move it to another rig without disturbing the alignment of the scope in it’s mounting rings.
Update: As it turns out the setup shown here is not really all that well suited from the standpoint of matching the camera’s pixel size to the scope’s focal length. There’s an optimal value of something called “image scale” which is said to be between 1 & 2 arc-seconds of sky per camera pixel. This setup is 3.3 arc-sec/pxl! So rather than return the scope I read on Cloudy Nights astro forum that you could just stick a barlow lens between the cam and scope which doubles the focal length which has the by-product of giving me the image scale value of 1.6 arc-sec/pxl! WooHoo! right in the middle of the sweet spot. Of course you also see less sky this way but I already know there’s no free lunch in optics so no big deal.
Time for your close-up!
As I mentioned before the camera goes at the front instead of the back as you would normally see. It’s a little bizarre looking but it’s supposed to be awesome so I’m gonna get used to it.
The red thing is the camera; I have to get two wires out there with a minimum of blockage.
Astronomical cameras are supposed to be a very specific distance from the last optical surface in the light path. What that distance actually is varies by manufacturer and camera model. In my case my target distance is 55mm. You have about 1mm fudge factor here.
In my regular setup I have the Off Axis Guider at a depth of 16.5mm + a 5mm spacer ring + 16mm filter slider + 17.5mm for the amount of distance the actual sensor is down inside the camera. 55 mm on the nose.
But with this setup I have to lose the OAG at 16.5mm, so I figured if I bought a 20mm spacer ring and took out the 5mm spacer I’d be at 54mm and I think I might have a 1mm ring around somewhere.
Dew Not
One of the problems with a scope with a beg lens right at the front is dew collection. If a night is dewy a lens like this can fog up and be unusable in 30 minutes so there was no way I was going to let that happen after spending two-plus hours getting ready. So I took a two stage approach: a dew shield to keep the dew from getting on the lens in the first place and heater strips around the outside to keep the lens warm enough that dew wouldn’t collect because of the temperature differential between the glass and the air. The dew shield has the added benefit of keeping local stray light from getting in the scope.
I also had to keep dew from forming on the guidescope so it got it’s own heater strip. I didn’t have a dew strap long enough to reach all the way around an 11-inch scope but I had one long enough for an 8-inch and and my 4-inch refractor so I put both of them on there with a little overlap which my online research said was ok to do. I had a strap short enough to go around a large eyepiece so that went on the guidescope.
Heater strap control box at the back of the scope. The first three are for the straps. No. 4 controls my electric underwear!
The dew shield presented a couple of issues, first it was rolled up inside out from Celestron. This was no big deal–I just unrolled it and let it sit in the sun for 30 min and it had ‘forgotten’ it’s old shape and was ready to be bent around the scope correctly. Well I quickly discovered that there wasn’t very much scope to grip and the thing would most certainly fall off in regular use so using some velcro I fashioned a couple “retaining pads” were I could temporarily strap the dew shield to the scope itself in a manner that should hold as long as the velcro sticky backing holds, which turns out to be the inherent weak link in velcro in cold, wet environments.
A little velcro strap to keep the dew shield from falling off as the telescope moves.
I also discovered that my data wire to the main camera was so thick it really messed with the dew shield’s fit so I found one of the flat USB3 cables that I had laying around and used that to sneak underneath the shield.
The whole Shebang
So here’s the completed setup, and finally, I’m ready to balance the scope! So you lean it over like this and loosen the declination axis and see which way it falls. Then right the telescope and slide it forward or backward a little, lean it over and try again. It’s always surprising how little you have to slide the scope in the saddle to make a big change in balance.
So here’s the final setup with everything on there.
Once you’ve got that part balanced then it’s time to adjust the counterweights on the bar for balance in the other direction and after a couple tries I ended up with this spacing for the two 21 pound weights:
Not quite sure how I can mark the bar so I can recover this position for the 2nd weight.
So now I need to mark everything so I can get back relatively close to this setup without a lot of fiddling.
This should get me pretty close.
Guidescope has to go back exactly as well so I marked that.
Well other than dress the cables a little better that’s about all I can reasonably do until I can test all this under the stars. Of course rain is predicted for tonight!
Carpe Noctem!
Bill the Sky Guy