New Way to Attach Guider

                      

 FOCUSING

FOR MOAGs, MonsterMOAGs and MegaMOAGs

 

MOAG-A and MOAG-ST

Astrodon off-axis guiders were designed to take advantage of the convenience of the helical focuser (HF) for focusing the guide camera.  We have provided several methods to achieve focus and good guide stars.  This article describes a new method to acheive focus if the HF is too tall within about 6 mm (~1/4"). For MOAG users, this will result in sharper stars with their AO-L or AO8.

As shipped, the MOAGs used for the SBIG AO8/ST and AOL/STL systems have an f/-250 plano-concave lens that extends focus further above the HF by about 10 mm.  This unfortunately magnifies the star making it larger with lower peak intensity. This does not cause a guiding problem because software programs, like MaximDL and CCDSoft, determine the centroid of the broadened star and guide on it.  However, with the moving adaptive optic, star intensities are diminished.  This article shows a way that was discovered my Mike Rice at New Mexico Skies, where the lens is removed and the guide camera is threaded onto the top of the HF without using the 1.25" nose piece.

MOAGs are designed to ship as shown on the right. The f/-250 lens is in its lens holder screwed into the top of the MOAG underneath the HF.  The 1.25" nose piece of the SBIG Remote Guide Head (RGH) is inserted into the top of the HF near focus and locked into place with the two thumbscrews.  A slight adjustment of the HF should readily achieve focus. 

The image above left shows the new shortened configuration. It turns out that the top of the HF has t-threads (42mm -0.75P); the same threads as used in most 1.25" nose pieces.  This allows the RGH, ST-402 or other guider to be screwed directly on top of the HF after removing the two thumbscrews.  Here is the procedure:

1. Loosen the 4-40 set screw or socket-head cap screw in the round (MOAG) or oval (Monster/MegaMOAG) fixture at the top of the MOAG.  This is a rotation lock that stops the HF from rotating.

2. Using the thumbscrews at the top of the HF, rotate counterclockwise, unscrew the HF and remove. Be sure that the retaining ring on the bottom of the HF comes with the HF.  It can sometimes get stuck in the top of the MOAG.  The MOAG should look like this picture after removal.

3. Grab the lens cup,  rotate counterclockwise and remove it.  Be careful of any sharp edges when removing.

4. Re-attach the HF all the way, but DO NOT tighten the 4-40 rotation lock screw.

5. Take off the thumbscrews at the top of the HF.
 
6. Remove the 1.25" nose piece from the guide camera.
 
7. If you purchased the new stop ring shown in the left photo above, screw it onto the top of the HF nearly all the way down to the bottom of the threads.
 
8. Screw the guide camera onto the HF all the way until it stops.  If you have the stop ring, unscrew the guide camera until it is parallel to the optical axis of your telescope system, and tighten the stop ring. 
 
9. If you do not have the stop ring, rotate the entire HF until the guide camera is aligned.
 
10. Tighten the 4-40 rotation-lock screw to prevent the HF from rotating. 
 
11. Repeat your proecedure to place a guide star in the guide camera and refocus.
 
 

MonsterMOAG and MegaMOAG

MonsterMOAGs and MegaMOAGs are not shipped with any lenses.  Two means are provided to reach focus with any guide camera having a 1.25" nose piece - (1) HF, and (2) a short 1.25" manual nose cup.

The standard camera-side adaptor shipped with the MonsterMOAG and MegaMOAG has 3"-24 male threads that screw directly into the newest, large color filter wheels (CFW), such as those produced by Apogee Instruments.  The CFW and camera are thus placed right next to the OAG with almost no additional backfocus. In this case the focus will likely be well inside the HF. So, the HF can be removed and replaced with the 1.25" manual nose cup bringing the guide camera down by up to 0.9", depending upon the length of its 1.25" nose piece.  Focus can be reached.

Similarly, there are adaptors for SBIG ST and STL cameras that bring them very close to the OAG. 

Finally, there are other adaptors that add 1/2 to 1" of space between the OAG and the CFW/camera, possibly allowing the HF to be used.

The procedure described above for the MOAG can be applied to the MonsterMOAG and MegaMOAG in situations where the focus is just below the top of the HF, i.e. where you can see defocused stars but cannot get to focus.