DIY Off Axis Guider

[Jerry]

Im getting my 10" LX200 ready to use for astrophotography. I plan on using my Photometrics Quantix monochrome camera. To go with this I am using an Optec Filter Wheel and I also want to use a off axis guider for guiding. An off axis guider picks off a section of sky with a small mirror or prism and sends that to another CCD camera. This camera connects to your computer and software watches a star you pick put of the field of view and sends commands to the scope to keep it on target. This is the most accurate way to guide a scope. PHD Guider is probably the software I will use.

The filter wheel is needed because the sensor on the main camera is monochrome. It is a stepper motor driven wheel that moves filters in the path of the camera. The filter wheel I am using has 5 filters, Red, Green, Blue, Luminance (Clear with IR cut filter) and Hydrogen Alpha. Later on you stack these images and get a nice color image.

The problem is that I am also using a Moonlight focuser which attaches directly to the 3" visual back of the telescope which holds the field reducer if I choose to use it. I could put the filter wheel and guider behind it but by the time I have all the adapters I am getting pretty long on my optical path. Optec also recommends keeping the filter wheel as far from the camera as possible to reduce the effects of dust. They do make an adapter that will mount it directly to the visual back but the problem is then I would have to put the OAG after the filter wheel which can't be a good idea.

So I decided to make my own Off Axis Guider. This guider will fit between the visual back adapter and the filter wheel using the Optec 2.4" taper. I also need an adapter that will go from the threaded back of the filter wheel to the 3-1/4", 16tpi visual back threads. Optec was kind enough to give me their taper and thread specs.

The advantage of this is I keep the OAG in front of the filter wheel and the focal reducer. It also keeps the weight off the focuser. Disadvantage is I am going to have to use quite a long extension tube to get the OAG in focus with the camera hanging off the back.

Fot the two adapters I started with a chunk of 1"x3.5" aluminum bar. I chopped that in to two 3.5" square pieces. At work we have a lathe with a 4 jaw scroll chuck so a drilled them out, bored the ID to 2.2" and faced on side. I brought the pieces home and used my Monarch 10EE lathe to do the rest of the work since I have a nice Buck 6 jaw for it. I turned the OD down on the pieces and machined them as needed. 3-1/4"-16 female threads and the taper on one and 2.34"-20 threads and 3-1/4-16 male on the other piece. I also turned some fine (72tpi) threads on the bore for anti reflection. The visual back threads on the scope itself are slightly over 3.25 so I ended up just cutting till the piece fit.

For the OAG I found another chunk of aluminum in my scrap pile about 3" square and about 1.125 thick. I bored that out to 2.2 as well and also made the recess of the male taper. Turned the OD down on the 10EE and cut the male Optec taper. For the optical pick up I got a couple 9mm square right angle prisms from Surplus Shed. I cut down a piece of aluminum to 1/2 x 5/8" and rounded over the edges with a 1/8" radius cutter. Drilled through with a 9mm bit and counterbored 3/8" on the camera side of the pickup most of the way through. On the pickup end I milled out the center to hold the prism and drilled and tapped the flanges 4-40 to hold the prism in place.

To make the hole in the OAG housing to receive the pickup I used my CNC mill. Simple program to cut out the hole to match the pickup dimensions. I then turned the part 90 and counterbored and drilled for a 8-32 screw to lock the pickup.

So thats is where I am as of tonight. To Do:

• Set up the 4th axis to drill for the taper locking screws
• Make up the adapter to go from the pickup to either a C mount or T2 fitting. Probably C as I can do that on my Monarch, my CNC lathe is having issues threading.
• Anodize the parts.

Here are some pics:


IMG_1221 by macona, on Flickr


IMG_1222 by macona, on Flickr


IMG_1223 by macona, on Flickr

[Jerry]

IMG_1224 by macona, on Flickr


IMG_1225 by macona, on Flickr


IMG_1230 by macona, on Flickr


IMG_1231 by macona, on Flickr

[Doug Coulter]

Man, that's lookin' sweet indeed. I would guess that a main reason not to have the guide pickup behind the filter is that it might not work well in a narrow band of light (less total light)? If one was worried that a slightly off axis filter shifted the image...maybe it'd be best to have the tracker compensate that? Ah, no - you have control over that later when doing the stacking anyway - usually you have a way to compensate a slight shift image to image, and I'd imagine by now the software isn't as primitive as the astro-art stuff I used - it might be able to do that automatically.

A trick I found good to do is what I call stochastic selection to improve images. I took a bunch of pix at each filter (actually for bright things I took movies). Then I went back and ditched images that showed a bunch of seeing blur, or in the case of dim targets, just didn't have any photons from the object of interest. The few that make it through that selection process I would then stack. You skip all the really bad ones, and the ones with nothing useful to add to the result -- this cuts sky glow as well. A cute trick. I was heading in the direction of trying to find a gen III night vision microchannel plate so I could take shorter exposures for this trick, in essence sample above Nyquist for the atmospheric smear change rate. You can use FFT's to deconvolve a simple smear or distortion, but an image that has more than one of those added in can't be fixed up well by comparison. However, the cells in the atmosphere go by at a couple hundred Hz, so you'd have to use millisecond exposures to ensure you had only one smear (or none at lucky moments) in a particular image. This would result in a bunch of very crappy looking (but clear) images to stack up later.

[Jerry]

I will probably use something like Nebulosity to stack: http://www.stark-labs.com/nebulosity.html

Usually with OAG's there is little to no image shift frame to frame. They say they get sub pixel accuracy with modern software assuming your mount has an accurate polar alignment.

[Jerry]

I got the parts back from the anodizer yesterday. They look real nice.

Got it all put together and on the scope. My CoStar auto guider camera showed up last week. Still need to make an extension tube for the guider.


IMG_1519 by macona, on Flickr


IMG_1518 by macona, on Flickr


IMG_1522 by macona, on Flickr


IMG_1526 by macona, on Flickr

[Doug Coulter]

Looks like whoever did that anodizing earned their pay quite nicely. Would've been a shame otherwise, with such fine input to his process.

[Jerry]

Pioneer Metal Finishing did the anodize. This is their "Optical Black" which is supposed to be 5x less reflective than standard black, especially in the IR range. Normal black anodizing is pretty much transparent in IR. I took some pics with my IR modified camera of some anodized parts and in the pictures you could not tell they were anodized.

Normal anodizing is a $55 minimum charge, this was $70 for the Optical Black.