bobharmony

Back to basics - been away from the blog for a while

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by , 06-20-2017 at 12:50 AM (855 Views)
Some of you may know that I have been having issues getting the GSO coma corrector to play nicely with my setup. As a refresher, in January I installed a linear bearing two inch dual-speed focuser and picked up a spacer to get the documented 70mm of spacing between the back of the CC and the DSLR sensor. With the glut of sucky weather that has been going on around here, the first chance I had to try it out was an abbreviated session in May (abbreviated because the clouds rolled in 20 minutes after I got set up and started). The result was less than spectacular, with stars exhibiting coma, but not radially away from the center, which led to many suggestions that the imaging train was out of true somewhere.

Since then, I have readjusted the focuser to make sure it is orthogonal to the OTA, tightened the set screws that hold it to its base, and after more research found an article (or maybe Leveye found it) by the designer of the CC indicating the back spacing should be 75mm, not 70. Additionally based on info from nFA, I purchased an artificial star for testing (what the heck, I had to order another spacer anyway).

Tonight I decided I really should go back to the ultimate basics, and completely collimate the scope. I really should have done this when I put the new focuser on, but the laser collimation looked good, so I assumed things hadn't moved enough to be affected. This is the first rounding of the secondary under the focuser I have done since I bought the scope, and it was just as frustrating as I remember it. Move the center screw, don't move the center screw, figure out which way each of the secondary collimating screws actually moves the secondary while getting the mirror clips lined up... at times I reached that level of anguish where you decide the scope is so screwed up, it will never be right again... but I took a deep breath, calmed myself, and started all over again.

By the time I got to the point where the crosshairs get lined up with the central circle on the primary, it was so close that I decided to resurrect the laser collimator to finish the job, and checked it with the Cheshire once that was done. It came out perfect by both methods, and I got to blow the dust off my Cheshire eyepiece which hadn't been out of the box in a loooong time.

Whew, with all that work done, it was time to fire up the artificial star and do some coma testing. The camera and CC were hooked up and I took shots with the star at the center and at all four corners of the sensor, and sure enough they were all equally round! I thought BYEOS was saving copies for me (I had run that to use its Bahtinov mask focusing assistant), but that was not the case so you'll just have to take my word for it. If the forecast is to be believed, tomorrow night is supposed to be clear - if it is, I will be doing some real world testing!

Thanks to all who have been along for the entire journey and have provided the excellent guidance that has gotten me this far.

BTW, the definitive answer for which C6-N primary screws are the locking screws and with are the adjustment screws - the hex screws are the locking screws and the Phillips screws are the adjustment screws. I have read so many different opinions on this I had to look. The hex screws butt up against the back of the mirror, the Phillips screws go right into the bracket for the spacing control. Don't believe anything else you read about this

Bob

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