Quote:
Originally Posted by Slawomir
From what I understand, a f/4.3 Newtonian can be a very capable imaging instrument 
My guess is that there is an orthogonalty issue resulting in uneven spacing and thus different correction of stars (over, good, under correction) in four corners.
Hopefully you will get it resolved quickly. |
Thanks Suavi :-) I thought there was some tilt, but I've never experienced a newt before, so wasn't sure I was interpreting the images correctly.
Quote:
Originally Posted by Shiraz
ahh- isn't it fun
very much FWIW - a few ideas that might possibly help.
1. get a laser collimator and align it precisely - as Alexander showed http://www.iceinspace.com.au/forum/s...ad.php?t=75601
2. carefully check the central spot ring on the primary and get it exactly right.
3. do a laser collimation through the coma corrector
4. check that collimation holds reasonably well by moving the scope around the sky with the laser running - if possible tighten anything that allows movement of the collimation spot. pay particular attention to the focuser and make sure that the shaft is tight enough that you do not get any rocking of the draw tube
5. if you still have corner-to-corner variability, then you have tilt, either in the camera or in the CC. make a cardboard spacer shim so that you can screw on the camera at a different rotation angle on the CC. If the tilt has rotated with the different camera position, it is in the CC, if it does not change, it is in the camera. if the corners all look the roughly the same, but are not fully corrected, check the CC spacing.
also FWIW, I had to add strengthening to the CF OTA around the focuser to stop flex and had to put new side shims on the primary to stop it from sliding too far sideways. I also added two nylon locking screws on the side of the focuser draw tube to ensure that the CC tube is locked solidly to the draw tube (it could wobble slightly when only locked at the top end). Also shimmed the camera to minimise very slight tilt in both the CC and the camera - found the rotation angle at which the two slight tilts came closest to cancelling each other. The optics still tends to drift a bit and recollimation is needed maybe every month or two - not a big price to pay for a fast scope.
cheers Ray |
Thanks for the detailed response, Ray :-) Of course I had a laser collimator that I used once to identify / correct some tilt on the Esprit...... and sold it.
The center spot is as close as I could get it - I removed the stock one and replaced it with a catseye hot spot.
The tube is a ~5mm foam core CF tube and it's not a cheap scope so hopefully will hold collimation across the sky--I'll be pretty unhappy if it doesn't, but I've also not verified it yet... needed to get it collimated first!
I'm not 100% sure on the spacing of the corrector. I tried a few different positions and this seemed to be the best, though it was about 3.5mm off the apparent spacing for this corrector on a scope of this focal length, so I dunno.
Quote:
Originally Posted by glend
Or you could buy a nice refractor. Sorry i could not resist. As Shiraz said, many things to check. Good luck.  |
You mean like the Esprit? ;-)
Quote:
Originally Posted by atalas
Hi Lee
Ok,It can be confusing so you need to take things one at a time dude.
1:center focuser (use a high quality laser)
2:center secondary to focuser
3:center main mirror to secondary
Ok basics over, now,looking at the second pic looks to me like the spacing of the coma correcter is good.....center right side of the image tells me this.
Top left and top right tell me you have tilt in the focuser.....this should be the first thing to take care of....center focuser before collimation of mirrors otherwise everything else is a compromise.
I see in the bottom right that you have field rotation.....PA dude otherwise It will confuse you.
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Cheers mate. I did 2 & 3 to the best of my abilities with the tools I had available to me. Not so sure on #1.
When you say the second pic looks like the spacing of the coma corrector is good, did you mean the first pic? The second pic has no coma corrector in place.
As for field rotation... this was taken on a near new, pier mounted AP Mach 1, polar aligned to within a couple of minutes.... periodic correction was enabled and these are both 15 second unguided exposures -- that's not field rotation caused by PA.