Off-axis Guiding

I’ve wanted to try off-axis guiding ever since I noticed differential flexure becoming really serious in long exposures and in drift of my image center over the course of a run. Off-axis guiders work by using a small prism or mirror to pick off a bit of light coming through the main imaging optics, but outside the field of view of the main imaging camera. This gets sent to the guide camera. I finally decided to get an Orion Thin Off-axis Guider (TOAG) and give this a try. I got an open-box item on Amazon for even less than the relatively modest price these normally carry. I had inferred from everything I’ve read that using an off-axis guider was difficult to set up, but most people who use them wouldn’t go back to using a separate guiding scope. 

It has been difficult to set up; it’s hard to get simultaneous focus on both the imager and the guider, and it is certainly more difficult to find suitable guide stars (although so far there has always been one in the field of view, even if it may not have been as bright or as nicely focused as I would like). On the plus side, I saw the complete elimination of differential flexure, which has plagued me forever. The image doesn’t move around over time, to the extent that hot pixels that would normally have been eliminated by my “auto dither” due to flexure are not removed. Standard deviation stacking in Nebulosity mostly works to eliminate these hot pixels, but similar routines in Deep Sky Stacker seem to more completely take care of this. Maybe I’ll also need to go back to deliberate dithering, which I have not done in several years.

But first, I’ve been having all kinds of problems with connections to my cameras and now the mount failing mid-run. Camera issues I’ve had before, but never issues with the mount connection. I thought maybe this is a result of the MacOS Sierra upgrade I recently did, and sure enough when I checked my power settings I didn’t have things set up to prevent the computer from going to sleep when on the power adapter. I also went into the VMWare/Windows 10 power management settings and turned off power saving for USB devices. Apparently something might have worked, because I got a complete run last night with nothing shutting down.

Last night’s run was also noteworthy in that it was the first try to use my Orion ST80 scope with a focal reducer with the TOAG. I originally set this up with the AT8IN scope and found I didn’t have enough back focus to put the focal reducer on the guide camera (i.e., the focal position with the reducer in place is closer to the mirror than it is possible to get with the TOAG installed. So I measured the distances and found it was possible to put a reducer on the imaging camera and leave the guide camera with no reduction. I tried it and found I was able to reach focus on both cameras simultaneously with that arrangement.  I shot M33 this way, but didn’t get enough subframes to have a really good image because of the disconnection issues mentioned above. All things considered, it isn't too bad.

This is a first-light image of M33 taken using the AT8IN with a focal reducer and an Orion Thin Off-axis Guider without a reducer. The session failed quite early because communications with the mount and both cameras failed. I think now that the computer controlling the session went to sleep. I got quite different colors using Deep Sky Stacker’s 1.5σ κ-σ stacking than I got with Nebulosity 1.5σ standard deviation stacking (this is the Nebulosity version; DSS’ version was more yellow in the core). I guess it’s not terrible given how little data it represents, less than an hour’s total integration. I think the TOAG is helping me get smaller, rounder stars; it certainly keeps the image center from drifting over time.

Date: 3 Nov 2016

Subject: M33, Triangulum Pinwheel Galaxy

Scope: AT8IN +Antares 0.5x focal reducer

Filter: Baader Planetarium Fringe Killer

Mount: EQ-6 (EQMOD)+PEC

Guiding: Orion TOAG +DSI Ic+PHD 2.6.2.4 (Win 10 ASCOM)

Camera: DSI IIc, no chiller CCD ~10 °C

Acquisition: Sequence Generator Pro 2.5.2.4 with Nebulosity 4.1.1 ASCOM camera driver (Win 10)

Exposure: 11x300 s (RGB)

Stacking: Neb 4.1.1, bad pixel map, bias included, 33 flats, match histograms, trans+rot align, Nebulosity 1.5σ standard deviation stacking.

Processing: StarTools 1.4.316: Crop; Wipe; Develop: 50.67%; HDR:Optimize; Color:Scientific, 363% saturation; Deconvolute: 2.5 pix; Track: Smoothness 75%, 4.5 pix; Magic, shrink 1 pix. Photoshop CS6 + Astronomy Tools: a little Levels to darken background; Increase star color; Astroframe.

For this approach to be viable, it has to work on the ST80 as well; I can't hunt for proper position of the TOAG pickoff prism every time I change scopes. I found I could remove the 2” nosepiece from the TOAG assembly and replace it with the T-thread adapter for the ST80, which also allows me to set whatever rotation angle I want. This gave me plenty of back focus, so it looks like I’ll be able to move the camera, with the focal reducer inserted, back and forth between the AT8IN and the ST80 without having to reset the focal distance by moving the pickoff prism. This looked good enough that I pulled the ST80 off the top of the AT8IN (where it has resided for nearly all of the last year) and just put the ST80 by itself on the mount. This will also save weight for the AT8IN, I’m hoping enough that I can either get rid of the extension bar or use 1 weight instead of 2, but I haven’t tried that yet. I decided to use the Pleiades as my test subject because that target would take advantage of the wide field of view this setup provides. I had a terrible time getting plate solves to work, but pleasantly once I decided to use Sequence Generator Pro's blind solve failover I got a successful plate solve and was able to find and center. Unfortunately the rotation angle I used wasn’t optimum (I managed to leave out Pleione), but at least I got a complete run. This image needed liberal application of the filter tool in StarTools to get rid of nasty purple fringes around most of the stars (because the ST80 optics are far from apochromatic!).

I decided to use the Pleiades as my test subject for use of the ST80+FR+TOAG combination because that target would take advantage of the wide field of view this setup provides.  

Date: 6 Nov 2016

Subject: M45, The Pleiades

Scope: Orion ST80+Antares 0.5x focal reducer

Filter: Baader Planetarium Fringe Killer

Mount: EQ-6 (EQMOD)+PEC

Guiding: Orion TOAG +DSI Ic+PHD 2.6.2.4 (Win 10 ASCOM)

Camera: DSI IIc, no chiller CCD ~10 °C

Acquisition: Sequence Generator Pro 2.5.2.4 with Nebulosity 4.1.1 ASCOM camera driver (Win 10)

Exposure: 55x300 s (RGB)

Stacking: Neb 4.1.1, bad pixel map, bias included, 33 flats, match histograms, trans+rot align, Nebulosity 1.5σ standard deviation stack.

Processing: StarTools 1.4.316: Crop; Wipe; Develop: 72.65%; HDR:Optimize; Color:Scientific, 200% saturation, red bias reduce 1.09; Deconvolute: 2.2 pix; Track: Smoothness 75%, 4.5 pix; Filter: Fringe Killer (repeated); Magic, shrink 1 pix. Photoshop CS6 + Astronomy Tools: a little clone stamp and smudge here and there; Increase star color; Astroframe.