This is coma - not rotation

[Mac]

I would be aware, although i havent had another chance to test it yet,
Just check the collimation with an eye piece before you set off on an
adjusting session, see previous thread for my troubles.

Might save you a lot of time and effort.

Still once i get the H9 on and check everything is square, the i'll be using the
CCD Inspector to check everything.

Fingers crossed, it wont be that far out.

It might be intersting to check the software against know photos.

I.E ones from APOD or even some of the globular clusters that people have taken.

Even ones from the hubble might be interesting.

might try this one when i get home from work.

http://hubblesite.org/gallery/album/star_collection/pr2006017c/.

Mac.

Dear Nasa did you know the collimation is out on the Hubble. 

[MarkS]

Mac,

I've noted the point about making sure the CCD is square-on - not sure how to check it though.  I'll collimate with the eyepiece first and then see what CCDInspector reports.

However, I've run CCD Inspector on some of my old images and the results tally very well with visual inspection of the same images:
1) For my older images, where the sharpest stars at in the centre of the CCD,  it reports good collimation.  These images do not show the weird coma/rotation problem at the edges.
2) For my recent images, the sharpest stars are a long way off centre and the coma/rotation problem appears at the edges.  It reports bad collimation for these images.

In both cases CCD Inspector correctly identifies the area showing the sharpest stars when running the curvature analysis.


Mark

[MarkS]

I played around with collimation of my C11 last night and have finally(!) overcome my collimation phobia. 

Before I began, CCDInspector was reporting a collimation error of  20arcsec (whatever that means) and the area of sharpest stars was a long way from the image center.  The error was also obvious in the eyepiece - lack of concentricity on a defocused star.  I adjusted for this as best I could through the eyepiece and then put the camera back on.  CCDInspector now reported an error of 7arcsec and the area of sharpest stars was fairly close to the image centre.  A few more iterations (tiny adjustment to a screw followed by a 2min sub) brought this down to 2 arcsec.  A final imperceptable adjustment of one of the adjustment screws brought this down to 1arcsec and the area of sharpest stars was now bang in the centre of the image.

I'm now confident enough to perform a quick tweak before any every imaging session (to compensate for orientaion of scope, camera etc.)

So what was the effect on the coma/rotation problem?  Well, when using the focal reducer, the corners of the image (outside the main illuminated disc) still showed coma but they now looked like colons (the punctuation type!) and not lines.  The colons were now identical in all four corners of the image. The appearance is like a stack of two frames where a tiny frame rotation has taken place between frames but, of course, this "rotation" appears only outside the main illuminated disc.

By the way, with my collimation fixed, the Bahtinov mask worked more precisely than ever!

Mark

[RobertM]

That's very interesting.  So you used CCDInspector for the first iteration and to confirm what you could do by eye.  Will you both be using CCDInspector or eyeball for future tweeks ?   Also make you wonder whether a mask could be designed to help with collimation !

[Mike]

Have you purchased CCD Inspector Mark or are you using the trial?

[MarkS]

Robert,

I will definitely use CCDInspector to do the pre-imaging tweak because it allows me to make the final tiny adjustment that would be undectable using an eyepiece (undetectable to me at least!) but which is detectable on the image.  The curvature map it produces also helps to get the focus absolutely spot on.

So my intended pre-imaging sequence will be:
1) Polar align mount
2) Do a 1 or 3 star alignment with the Goto handset
3) Do a Bahtinov mask focusing of camera on a bright star
4) Slew to the the target area of sky
5) Get guiding running
6) Get target nicely framed using test images
7) Run CCDInspector on those test images to make final collimation tweaks and to get focus spot on

I like the idea of a collimation mask - I wonder if such a thing already exists?

Mike - I'm still using the trial version but this will be a definite purchase for me

Mark

[JohnP]

What's the cost...?

[RobertM]

The best I can detect is about a 16th of a turn so getting it better than that would be great.  Not to mention having to recenter on a bright star each iteration and my arm not being long enough to adjust and see at the same time !

Best part of a £100 though...