Quote:
Originally Posted by graham.hobart
1/ considering the article Paul (Haese) wrote about reflections in his RC 8- is that what I am suffering from in the top middle of the shot? Maybe from Spica off camera?
If so should flocking the baffle sort it as in Pauls fix (sorry Paul to namecheck you but you seem to be the guy with most experience a la GSO RC's)
|
If you point the scope somewhere else nearby and the streak changes direction or goes away, then it's an off-axis reflection making its way onto your CCD. Flocking or more baffling is the only solution, however it may not be worth the trouble to get rid of it if it only affects a small percentage of your images (ones near really bright stars).
Quote:
Originally Posted by graham.hobart
2/ there seems to be a general fogginess in the picture which is maybe not all the gradient. Taking away the fact that there was a Moon and I have light pollution, and it was a DSLR whose EXIF was getting up to 30'c after a while - is is there still likely to be any improvements I will be getting when my feather touch focuser gets here- i.e is there an element of poor focus that would be improved with more rigorous technique (too small Bahnitov mask, crud focuser, etc)So if you imagine that frame flat corrected with say, focus max running a CCD - do yo think some of that fuzz would go or should I move house or try narrow band?!!
|
The flat field problem will not be cured by the focuser/mask/FocusMax or the like. The left to right gradient might be the moon (flats won't cure), and the black rectangle at the bottom is probably from the OAG (flats will cure). You need to take flats for the center glow, period. However there is a chance that even flats will not totally cure the center bit. You may have sky glow reflections off the anodized aluminum baffles/adapters creating a hot spot in the center of your image, the cure for that unfortunately is flocking again. But hold off until you have good flats to see if it's really a problem. Remember flats are an essential part of the image process, they remove gradients caused by the combination of sky glow + optics vignetting, and smooth out the uneven pixel response inherent in any CCD. The moon gradients you can only process out, or don't do broadband imaging near the moon!
Quote:
Originally Posted by graham.hobart
I mean some of the stars seem to be in focus - which brings me to the next point. I have borrowed a Tak scope to collimate but haven't tried yet as I saw no point till my focuser turns up, but looking at the stars in the corners is there a way to predict or have an idea about the direction of collimation by looking at the stars in each corner? I have included the four corners in magnification and the ones in the bottom left are not that bad really.
|
Short answer, no! I always guess wrong if trying to work it out in my head ahead of time. Turned the knob, see what happens and keep going or go back...iteration is the only way.
The problem with RCs is you have 6 degrees of freedom to worry about (secondary tip/tilt, lateral/vertical and primary tip/tilt), not counting the secondary spacing, camera orthogonality and actual darned focus position! Yikes it's a wonder anyone gets an RC to work!
But it can be done with methodical processes and patience. The Tak scope is the best (only?) way to start, get everything centered then goto star testing and learn the how the image reacts to turning each knob.
From what I can tell your image show off-axis astigmatism (the oval stars rotating 90deg in different corners), that you can't get rid of without a corrector lens assembly, or cropping the image, but you can minimize it with good collimation. And focal plane tilt is evident (bottom-left corner showing disk like stars = out of focus). This can be caused by imaging train crookedness, or simple miscollimation (e.g. curved field more curved on one side). So I think the message is: collimate!
Best of luck,
EB