Collimation issue or something else?

[E_ri_k]

Hey again. Ever since the RC10 arrived a new issue arises. This is only a single frame, 60 seconds or so, but why are the stars not round in the corners, especially the top left, where there are some banana looking things!

I know RC's are very sensitive to collimation errors, could that be it? Or any other ideas?

Erik.

[Paul Haese]

That is field curvature. You can get rid of this by using either a reducer or a field flattener.

[E_ri_k]

Thanks Paul. Do you know why the curvature not symmetrical in all the corners of the image, and worst mainly in the top left?

Erik.

[RickS]

Are you using a large sensor camera, Erik? I didn't find the field curvature too offensive with a KAF8300 sensor (22.5mm diagonal). It's much worse with my STL11K camera (43.3mm diagonal).

[E_ri_k]

Hey Rick, I'm using a QHY10. It's a fairly large sensor I guess, APS size. Not sure on the diagonal. I cant remember how to calculate the dimensions of the chip based on pixel area....

Erik

[allan gould]

Your collimation is still off as essentially all stars are showing elongation in that 60 second shot. It's exacerbated in the top left as that is where you have to bring the collimation back.

[E_ri_k]

Thanks Allan. I'm still working on spot on collimation. I was digging up some old IIS threads and found one on FF for the GSO RC's, which lead me to this site:
http://www.teleskop-express.de/shop/product_info.php/language/en/info/p4006_TS-2--Korrektor-fuer-Ritchey-Chr-tiens-ohne-Verkuerzung.html

Would I be right in assuming that I need to get the collimation spot on before addressing other issues like the filed curvature?

Erik

[allan gould]

I use a qhy10 with my GSO RC 10" and don't have any odd stars with that chip. The only flattener that I use now after testing about 6-7 of them is an astrophysics CCDT67 and that works beautifully. David Cortner has examples of the use of this fr down to f5 and his shots are great. I use it at 101mm spacing as suggested and have perfectly round stars when I need a bit larger field of view. I did a post on IIS about this somewhere and should be easy enough to find.
I wouldn't throw a fr into the equation until you have your collimation spot on as that will give you two variables and guarantee failure.
Allan

[Moon]

The shape of the stars in the corners is not symmetrical, so the above statement is not helpful.

Put a bright defocused star in the exact centre and post a picture for us to look at.

James

[E_ri_k]

I agree Allan, I want to get the collimation good before anything else, otherwise I'll be running in circles. I had a quick read about the CCDT67. Is it a FR and FF in one?

James, I will post a picture tonight if the weather is clear, should be.

Erik

[tlgerdes]

The asymmetrical stars could be the result of image plane tilt, especially if you are using the stock focuser.

Download ccd inspector and get it to analyze your subs.

[E_ri_k]

I have a brand new moonlight focuser and Orion OAG inbetween. unless there is a bit of sag comming from the OAG. I might try without it and see what happens.
Erik

[allan gould]

The above is good advice. The truth is you don't really know where the problem lies. I wouldn't touch the rear of the scope and only make adjustments to the secondary. However having said that, the problem As Trevor states could well be with your focuser. I assume you are still using the stock focuser that came with the scope? Although they are an improvement they can still be sloppy and have an inherent tilt that will drive you mad in trying to get collimation. My first advice is to get a decent focuser as the scope warrants it. Either a MoonLight or Feathertouch - I recommend the former as I've had great service from mine. Then follow Dave Cortners procedure and then Al's collimation aid. This will get you very well collimated and it's free. By all means if you want a Tak scope get it but the eye is a better collimator and the Tak scope just amplifies what the eye can see. The hardest scope I had to collimate was a Vixen VC200L and that was done all by eye and is the best way to do it. But it's only advice and free too so do what you feel is right.
See this site for information re the ccdt67 and it's bigger brother.
http://www.astro-physics.com/tech_support/accessories/photo/Telecompresssor-techdata.pdf
Allan

[Paul Haese]

Just to make things plain boys. I was answering the original question about the stars in the corners. Please in future read the original post.

Just to make things even more clear. Using CCD inspector is not indicative of collimation issues and can be frequently wrong. Several things could be going on here, including guide errors, flexure and tilt. All of which can be solved but the question asked why the stars were elongated in the corners. To which I gave the correct answer of a field flattener or reducer.

[Moon]

I don't see how buying a new focuser is going to help collimate the scope if the primary mirror is not collimated properly!
Anyhow step one ( as has already been pointed out) is to check the secondary collimation. A centred defocused star should be concentric.
Step 2 is to focus properly and take a 30 sec exposure and check the stars in the corners. Make sure the camera is attached properly; you are polar aligned accurately and tracking smoothly. If all is not well then I'm afraid you need to collimate the primary. It's not hard, no need to be scared.
James

Edit
And as Paul correctly has pointed out, you will be left with some residual field curvature - but it should be symmetrical, which it's not in that image.

[tlgerdes]

Paul, I was trying to indicate the use of ccdis for inspection of curvature and tilt, not so much for collimation. Yes an FR is needed, but the asymmetric star elongation should be investigated as well. Ccdis is good at mapping that.

It could just be that the camera wasn't seated squarely during testing.

[E_ri_k]

Allan. I am not using the stock focuser as I did read up on their "quality" I have a 2.5" moonlight focuser which is very rigid.

Paul, thank you for your answer with the field curvature. It did answer my question but then a new one arose!

I realise there are ALOT of contributing factors here, and I am far from am expert in addressing them all. I will get the secondary collimation spot on first and then work down the list:Primary collimation,flexture,tracking, PE and guiding.

I have not touched the rear of the scope and wont unles I get to the stage that I have to. But again looking through the tube it looks VERY concentric.

When I have clear skies Ill post a defocused star image for you guys too look at. Visually it looks pretty good but then again I could be wrong.


I appreciate all the advice and feedback.
Erik

[Barrykgerdes]

I am not an expert in this field but I have looked at the original shot and there is more than one problem.

Firstly the elongation of the stars radially out from the centre will be due to trying to focus a spherical image onto a plane surface. This is more noticable on large area viewing planes. This can be corrected by what we refer to as a FR or FF. However the degree of flattening and linearity depends on matching the FR/FF to the rest of the optics. Before this can be properly achieved you will need to get the rest of the optics properly collimated..

If you are using an OAG (as stated) and a moonlight focuser (as stated) the camera will be well behind the rear plane of the main mirror. To reach primary focus the main mirror will be well forward in its housing. This will put the corrector plate and secondary mirror at the limit of its best position. This means that the standard FR/FF may not be optimum for the set up.

Now in regard to focal plane tilt. Trevor bought a moonlight focuser that was in fact faulty (it was replaced without bother) but the spacer block was wedge shaped and the focuser distorted to try to get around this. The amount of wedge shape was only a few thou but was easily measureable with a micrometer. In the current case the OAG + FR/FF + Focuser will be quite a large overhang and could certainly aggravate any tendency to sag and tilt the focal plane.

So the procedure should be to get the collimation correct with all the attachments. Ensure that the focal plane at the chip is orthogonal to the optical path and then try to correct the field flattening.

Barry

PS all this may take some time to get right and will need some good viewing nights (often a rareity in certain parts of the country)

BG

[E_ri_k]

Thank you Barry. That seams like a logical procedure to follow. I have ordered some stuff to aid me in collimation.

I was going to try to get a few test images tonight. I had a look outside before leaving work and it was clear, but it went completely cloudy as soon as I got home......typical.

I have a focused image of Hadar from last night also, don't know if you can tell anything from it. The deformed stars are still worst in the top left, but not as bad as my first image.

Erik

[brian nordstrom]

:) I see the elongation ( bananas :lol: I like that , Erik .) in the photo but I also see some vignetting , quite bad , have a look at the focuser / draw tube , 1/2 your problems might be there , these may be out of allignement and in the light path ? .
Brian.

[E_ri_k]

It was the best way I could describe them in that first image! I will have a look at the drawtube in the morning. Without the risk of sounding stupid, but is there a good way to check this during that day, other than looking for slop or sag in the focuser? Maybe take some flats and examine them?

Erik

[brian nordstrom]

:) If you have a clear blue sky , just point the scope to the zenith , with your longest focal length eyepiece , 32-50mm , what ever you have ( the longer the better ) and look at the blue image , it should be evenly blue right across the field of view .:thumbsup:.
look carefully at the edges for a darker blue you have vignetting and if the darkness is uneven side to side you have something in the light path :eyepop: .. banana city :rofl: makes me laugh .
Brian.

[E_ri_k]

You'll never believe it, it cleared up for half an hour......

So here are 3 images of the unfocused star. I am guessing collimation is out by that centre spot. It shows up ALOT more clearly when imaged rather than visually.

Another interesting thing. I took a unguided 30 second exposure into a random patch of sky, WITHOUT the OAG in place. Round stars except for that top left corner where they are only slightly oval. Pretty happy with that image, not as bad as the other night which was a guided image. Maybe there was a guiding issue which exaggerated something to make that corner look really bad??

Erik

[Barrykgerdes]

Hi Erik

The first picture looks reasonable. The diffraction spikes on that star show you are close to it but you will need to do a full star test collimation to look at the diffraction rings at high magnification. Then tackle the other problems. The vignetting could be due to the long path through the OAG and focuser.

Trevor had a similar trouble with elongation in a part of the photo and it was found to be a slightly tilted focal focal plane as I said earlier.

Barry

[E_ri_k]

So I got Al's collimation aid and used it on one of my images. That centre spot is obviously off centre. Is this going to be a secondary adjustment or primary?

I thought if I adjust the secondary it will mess up the "Doughnut" shape? Or am I wrong?

Erik

[Barrykgerdes]

Hi Erik

That is only the first step in collimation. The real part is done when you make fine adjustments to get a nice symetrical pattern of airie rings.

I have attached to zip files containing Thierry Le Gaults directions for collimation. I think this is the best procedure I have seen.

The originals are available from his web site.

Barry