Edge 11 BackFocus Issue

I need to add an Off Axis Guider (OAG) to the Edge system for guiding; the Edge will definitely need to be guided. There is no way the ProTrack stuff will allow longer exposures at that image scale.

Called OPT to discuss the various guiders they carry. They seem positive about the new Orion thin OAG, or the Teleskop versions seem appropriate.

HOWEVER – it turns out I did not understand the fuss about the backfocus on the Edge. I may have an issue where I need to replace the Optec focuser.

The Edge has Optics built into the baffle tube to do the coma removal/flat field magic. For this stuff to work, the camera plane needs to be precisely 146 mm from the back of the SCT. The following graph from Celestron shows that at distances other than 146, one may see coma issues.

Note – the camera accounts for 56mm, including the T thread adaptor. So, the focuser/OAG combo need to be about 80mm (3.15″), allowing 10mm (0.4″) for adjustment.

Edge Coma effects

Edge Coma effects

My original approach was to set the Optec to middle range (3500), focus the stars using the Edge focus knob, then lock down the mirror lock. Measuring my positions, it turns out I have

Optec size                                                       90 mm
Drawtube position                             about 10 mm
Camera distance with T=thread adaptor    56 mm

     Total   156 mm

My camera is an APS-C format chip, 27mm diagonal. Thus, we would look at the curve for 13.5 mm radius. At 156, the coma messup is probably relatively insignificant.

Adding another 10mm for the OAG pushes the distance to 166mm. Now, maybe I start to see coma issues?

I looked at various replacement options for the Optec.


CRL low format focuser (1.85 in)                                           $345 sale price
HiRes Stepper control system with mini V2 USB control     $440
Short upper flange (0.55 in)
Short lower flange (0.55 in)
68mm-Tthread adaptor with Locking collar                         $79
    Total                                                                                   $864

This gets to about 3 inches, just about right.


Microtouch AutoFocuser                                                     $500
2008BCR   CRL low size Feathertouch focuser (1.4 in)      $365
Adaptor flange   (0.5 in)                                                        $49
    Total                                                                                 $914, seems like it might be too short?

Solution is not completely threaded – the focuser uses a 2″ nosepiece clamped.

The Plan

I think the plan may be:

1) Get the OAG

2) Run the Optec to about 2000 (saves 3mm). Maybe I could even go to 1000? Refocus the main mirror at this position. This gives a back focus of 156 + 10 – 3 = 163 mm. This is clearly not Optimal, but may be close enough.

3) If I see coma issues, then can look at replacing the Optec.

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