Collimation and vignetting

[sgazer]

following on from the recent discussion about collimation, I followed all the instructions and guides and got my 150p nicely collimated for last night. Everything turned out quite well.

However, I've been looking into the issue of vignetting. I've seen quite a few images on here (without flats) where the vignetting seems to be quite off centre, something I've not seen with my frac or SCT, but have seen on my recently acquired 150p Newt.

At first, I adjusted my collimation so that the vignetting was as central as possible in the image. I did this be moving the secondary up, down, in and out and taking a flat with the DSLR until it was more or less central, then performing the collimation to make the paper dot and cheshire's ring all line up.

However, this meant that things like the secondary reflection and shadow weren't central and the spider vanes weren't equal lengths, as in the collimation guides. So I followed the guides last night, which resulted in nice collimation, but the off centre vignetting (about 1/3 to the bottom right).

So I'm wondering if I should be following the guides to the letter and accepting the off centre vignetting, or getting my vignetting nice and central and aligning the optics to suit? Or, maybe I'm not doing it exactly right which is causing the off centre vignetting!)

Any opinions on this? (Other than using my nice maintainance free frac instead!! ).

[riklaunim]

If the laser colimator is good, aligned in his axis then colimation should be good. DSLR and off-axis vignetting may not be accurate colimation setup, as DSLR may be slightly off the optical axis (weight, DSLR sensor placement - not centered as colimator, focuser bending etc.)

[sgazer]

thanks Rik, I'm using a cheshire eyepiece and when I'm collimating/checking the vignetting, the focuser is vertical on top of the tube with the DSLR pointing straight down into it, so there shouldn't be any sag.

But the focuser could be out of square with the tube, it is adjustable, but checking it seems to be a PITA from what I've read. I was assuming that Skywatcher had at least got that bit set up right!

nb. Ironically, although it is adjustable, Skywatcher have done the unthinkable and placed one of the three adjuster screws under the focuser knob, which means you have to dismantle the focusing mechanism to adjust the screw, then rebuild the whole thing to check it! As an engineer, I'm stunned, what were they thinking, did nobody notice this?!! Although one maybe able to get away with only adjusting 2 of the scews, it's totally uneccessary to prevent access to the other. Anyway, rant over, back on topic......

My feeling is that it's better to have the the ccd evenly illuminated and then align the optics based on that, that way I think as much of the light is being captured as possible.

[sgazer]

I tried centralising the vignetting last night and it only took a few tweeks to the seconday spider position to do it (it wasn't as far out as I initially thought). Everything looks lined up as it should and 3 clips just showing from the primary. I think it's ok now.

I must say that previously I used a laser collimator on my 10" reflector which was really the only way of doing it as I couldn't reach the primary adjustment knobs, but having now got a short reflector, I find the cheshire eyepiece very good for doing both the secondary and the primary.

[SteveA]

I've been puzzling over exactly the same off center vignetting on my 250mm newt for some while now. All attempts at collimation made no difference!..and I had almost given up that there was anything I could do about it, though image calibration with flats does seem to correct OK.

Why I didn't think about adjusting the focuser I don't know, thinking about it now.. that must be the source of the problem, particularly as its a low profile version I fitted myself...(say no more!)..

Thanks for your thoughts...I'll be checking this out ASAP.

Steve

[TopHouse]

There's a tutorial on here somewhere by me about how to square up your focuser on a skywatcher newt.

goes off to find linky

http://stargazerslounge.com/equipment-discussion/99623-centering-focuser-tutorial-focuser.html

[SteveA]

Thanks for that link...I'll give that a shot when I get a chance...Also a question..do you fing that having flocked the inside of the scope it has made a difference ?..was it a worth while exercise??

Steve

[TopHouse]

Absolutely 100% worth the exercise. What i did though was I had the scope in bits to flock it and so did the full stealth mods, (link in a mo) and centered the focuser up at the same time. I was then and still am collimation OCD and after this I started precision collimating my scope with all sorts of ideas, there's a very long thread around about camcorder centering of secondary etc around somewhere too.

When i used the scope after doing all the mods, it had the same effect as when I first got my scope, the difference was really that noticeable, the moon was stunning, LOTS more contrast, after doing thall that i also got my best ever views of saturn with the banding on the planet showing. I will say at this point though that i used the proper flocking (protostar).

http://stargazerslounge.com/astro-lounge/99648-project-stealth.html

[cathalferris]

It's quite likely that the secondary does not have the correct offset. For visual work this would not be an issue, but for CCD/camera work.

See FAQ about Collimating a Newtonian telescope error type 2 there.