Monday, July 21, 2014

Replacing my OAG with a guidescope

I decided to replace my OAG with a guidescope.
  1. When I started using a non-parfocal filter (the light pollution filter), the guide camera got slightly out of focus when switching to a the non-parfocal filter. The star being out of focus wasn't too much of a problem, but if it was a somewhat dim start, the SNR went down really badly which caused guiding issues.
  2. When switching imaging gear (flattener vs. focal reducer, different cameras) I have to refocus the OAG.
At the same time, I was concerned about overall weight of the scope (that I have to carry inside and outside all the time). I asked around for recommendations, and many recommended 50mm scopes - especially the Borg scopes.

I also asked for mounting this guidescope and Astro-Physics recommended to mount the scope directly in the TAO-130 scope (it has a small mounting platform at the end). I ordered the Astro-Physics mount and the scope from Hutech.

When the Borg scope came, I was surprised that it came in pieces and had to be put together:

But assembly was pretty quick:

The next surprise came when I wanted to put the scope into the Astro-Physics bracket. The bracket has an extra ring at the end that secures the draw tube of the focuser to make sure that the focuser is tight. Took me a while to notice that I can remove the thin ring at the end that prevents the draw tube from slipping out:

With this removed, I could pull out the drawtube and put the focuser into the ring of the Astro-Physics bracket:

And finally put the draw tube back in, screw the holder ring back in place, and screw the whole scope together:

The scope has an adapter that allows to screw a lodestar guider into the back (which should reduce sagging and slack):

Astro-Physics recommends to mount short guidescopes onto the mounting plate of the TOA-130 scope (instead of mounting it on top of the scope on a separate plate):

The mounting kits consists of 2 parts: a mounting plate that goes on the scope plate (left) and an adapter for the guidescope bracket (right):

Assembly was fairly easy, first attach the mounting plate:

Then the adapter on top:
One minor challenge with the adapter was to screw it in tightly. The screw shafts in the adapter are quite small and most wrenches didn't fit in there.

And finally, the scope in the bracket on top:

First night out, I started with focusing the guidescope. But to my surprise I couldn't see any stars! Checked everything (optics, cap off??...) And then I noticed pretty large stars when the focuser was moved all the way in. But I couldn't move it in far enough to get focus. Luckily, I had the black extension ring on the scope (see above). After removing that, I could easily get focus. Phew!!

To get perfect focus, I used a small bahtinov mask that I once bought for my camera lenses. It was a bit tricky to slowly move the focuser in and out and then not change anything when I fastend the screws. It helped to focus on a start that's closer to the horizon, so that the focuser doesn't just move out from its own weight. But finally I got this:


And next was to recalibrate PHD2. I first tried to calculate what accuracy I need for my images.

My two cameras have 4.54 micron (SX H694) and 7.4 micron (FLI ML 16070).

With my two setups (flattener - 1000mm focal length or reducer - 763mm focal length), I get the following pixel widths:

(1000mm focal length)
(763mm focal length)
SX H6940.94 arcsec / pixel1.23 arcsec / pixel
FLI ML 160701.53 arcsec / pixel2 arcsec / pixel

The lodestar has a pixel size of 8.3 microns, together with the 50mm focal length of the borg scope this results in a pixel width of 34.239 arcsec / pixel.

    SX H694        FLI ML 16070    
Pixel width [arcsec / pixel]
Maximum error in guider [pixels]

That's quite some accuracy!!!

I started with the default PHD2 parameters which of course weren't great. I started with entering focal length and pixel size to get the right calibration parameters.

Next, I set the minimum move to 0.03 pixels. I then played with the hysteresis and aggressiveness to smooth out the curve and avoid over- or undershooting. Finally, I used PHDLab to understand if I correct unnecessarily (when the movement is still within the allowed range) or don't correct enough.

During this process, I tightened all screws of the guidescope (mounting plate, bracket holder, bracket screws, scope screws, focuser screws). Finally I had everything nice and tight and could easily stay below 0.5 arcsecs - even in my bad seeing conditions!