6" Ritchey Chretien - awful images - HELP !!!!

[Skipper Billy]

I bought a used 6" RC and rapidly discovered that the primary mirror was loose.

I have taken it to bits - carefully tightened the mirror retainer so that it holds it 'just' and reassembled it.

I have fitted a Moonlite focuser, Astrophysics flattener/reducer set to about 75% and tried it out for the first time tonight to get some Focusmax V curves in the can.

The images are truly appalling !!!

I have carefully collimated it with a Howie Glatter laser and with an artificial star the defocused airy rings looked good but in the real world the stars from the middle heading right look reasonable but the top left is shocking !! The brighter stars have all got the right hand side missing !!

The guiding was bang on the money so I am at a bit of a loss and ready to chuck it through the nearest window !!!

Any ideas - have I seriously messed it up taking it apart - I had to remove the primary mirror but carefully reassembled it - the one thing I dont know how to check is the distance between the primary and secondary mirrors - I read that on pain of death you adjust the central bolt holding the secondary mirror as it affects the distance between the two mirrors - I haven't touched it - but surely adjusting the primary mirror will also alter the distance between the two ??? How critical is it ??

Any help most gratefully received !!! I dont have the experience to know what to adjust first or how to check what the issue is - hence my plea for help before doing anything (I am not lazy!!)  I am minutes away from forgetting about the extra focal length and remounting my trusty ED80 !!!

The attached is a single 300 second sub with luminance filter. Stretched in PI and saved as a JPEG.

The setup is an Atik 460 EX - EFW2 - Astrophysics CCDT67 reducer 

[Peter Drew]

It's quite possible that the amount of work you have had to do to correct a loose mirror plus the upgrades you mention have initially upset the imaging train. The larger than expected star images suggest either an inter mirror spacing issue or more likely a position issue with the flatttener/reducer.

[SonnyE]

Whenever you get something used, you also take home the sins of the previous party. So you might still be dealing with what someone did to make the mirror loose in the first place.

Just keep working with it and work the bugs out. Extra patience as well. You can do it. Keep sorting it out and you'll get there.

[michael8554]

Hi

I was curious, so I googled "Collimating 6" RC" and came up with this, which you've probably read:

http://www.hnsky.org/RC_collimation.htm

I've only skimmed it so far, but in step 5 it does describe how to adjust the primary to secondary distance.

If RC collimation is anything like the coarse alignment of a SCT that's been taken apart without marking the mirror positions, the best result may not be with everything mathematically perfectly square.

In other words, if an adjustment improves matters, don't worry if the laser says it's wrong !

Good luck,

Michael

[Skipper Billy]

Problem identified !!! (I think !!)

The collimation when I brought the scope indoors semed to be pretty much perfect - the focuser to primary was spot on, the focuser to secondary was spot on and the secondary just needed a minor tweak to get it spot on - nothing obvious that might cause such awful images.

Then I refitted the camera/filter wheel assembly and after tightening the three scres on the focuser I noticed that the camera assembly was loose !!

It seems that there is a small lip on the M48/T2 adapter and the focuser ring was sitting half on the lip and half off - after much trial and error I have decided that in its current format it is just not possible to securely and accurately attach the camera assembly to the focuser.

Any suggestions most welcome but I will start a new thread seeking ideas.