Jump to content

Still getting some rotation - What could it be?

swag72

By swag72
in Imaging - Discussion

Recommended Posts

swag72

swag72

Posted
Posted

Trying out the modded 1000D tonight, so please excuse the redness of the two images!! Straight from the camera, here's a 2 minute guided sub. Guided using the finderscope and a QHY5, using PHD. It seems that I am getting some kind of rotation on the images still - Primarily at the top of the image, where I am seeing long stars.

My PA is good and has been checked using PHD - There is no drift. My guiding graph is flat and the osc index is showing at about 0.35. The camera has been screwed into the scope using the reducer / flattener.

I would like to iron out these issues ideally, but not sure where to look now.

The first is the full image and the second is a crop from the top left corner.

Would really appreciate some ideas on this. I hope that someone will look and have some ideas. I can not see any rotation at the bottom of the picture, so perhaps it's some kind of movement in the setup?

post-18339-133877535265_thumb.jpg

post-18339-133877535272_thumb.jpg

Link to comment
Share on other sites
  • Replies 71
  • Created
  • Last Reply
Whippy

Whippy

Posted
Posted

To me, there's no rotation (and there shouldn't be on a 2 minute sub :) ). If you've got elongated stars in 1 or 2 corners then that normally indicates that the camera isn't sitting square to the focal plane. This could either be the camera isn't sitting straight in the drawtube of the focuser or the drawtube of the focuser itself isn't straight.

HTH

Tony..

Link to comment
Share on other sites
swag72

swag72

Posted
  • Author
Posted

The stars look elongated across the top of the image. The camera is square into the focus tube as it screws into it with the flattener. How do I sort out the focus tube then? Or at least eliminate it as a problem. Thanks for taking the time to answer.

Link to comment
Share on other sites
ollypenrice

ollypenrice

Posted
Posted

Like Tony I think this is non-orthogonality. The symptoms are pretty classic. Alas, the solution may take a bit of finding.

If you are using screw-in attacments it probably isn't there and there isn't much you can do if it is.

On a commercial DSLR the chip is unlikely to be misplaced though QHY cameras have a bad name in this area.

That leaves the focus assembly. The whole back end of the telescope (everything after the back of the main cylindrical tube) could be misplaced within the tube. Or the drawtube could be sagging in its carrier. Fellow WO owners often know all about this. This can be adjusted using the little allen key contolled tension pads on which it rides. I don't know your specific telescope but they are much of a muchness.

What else? Could one of your filters be misaligned? The clip filter? I don't use DSLRs or LP filters so I don't know about how they fit.

From the sig this a SW ED120? Is this instrument expected to give a flat field across a DSLR chip without flattener-reducer? I very much doubt it. Do you have one in place? If you do, is the chip distance correct? I know you would expect a problem in this are to appear equally in the four corners but in reality it isn't like that. With a wrongly spaced chip I used to get one bad corner, one less bad and two OK.

Have you star tested the telescope, out of interest? Find a medium bright star near the zenith, go for about 100X, centre it, and gently look just inside and just outside of focus. You should see a set of concentric light-dark rings, but truly concentric. Now test again at the edge of the field. They will be less good but how much less?

Best,

Olly

Link to comment
Share on other sites
swag72

swag72

Posted
  • Author
Posted

Thanks for all of that Olly - Just a couple of answers to your comments.

Screw in attachments being used throughout with a DSLR. This issue I think I have on all the DSLR's I have used, so I am satisfied that it is not a problem with the chip in the DSLR.

I have had to tighten the focus assembly a few times as with the weight of the DSLR it was losing focus when pointing high into the sky. I can relook at this, but not sure how I could tell whether it was slightly out of the square.

Didn't use any filters on this image, so that's out of the equation.

The field flattener was in use on the SW 120ED after a prior thread where I was suffering from something similar without using it.

I've not star tested the scope - Is that collimation? I was expected this scope to be fairly stable and not need any. With the star test you mention, a couple of questions. The zenith is pointing straight up yes? This is done with EP's and not the camera? How do I achieve 100X magnification?

So, some advice on where to start testing would be good!! For starters, is there a way to test if the focuser is square in the tube?

Link to comment
Share on other sites
Merlin66

Merlin66

Posted
Posted

Do you have a laser collimator or can borrow one?

If you place a screen over the objective ( white paper is OK) and mark the exact centre of the objective on it...then the collimator should land it's dot smack on the mark. Rack the focuser in and out and see if the spot moves....if it does then something needs to be tighten up.

( the spot can usually be brought onto the mark by loosening the three screws holding the focuser in the tube...moving the focuser around and then retightening the screws.)

Link to comment
Share on other sites
swag72

swag72

Posted
  • Author
Posted

I don't have a laser collimator, neither do I know anyone out here in Spain that may have one. Is there anyway I can test the focuser without one?

Am I right in thinking that this isn't generally an issue with a refractor?

Link to comment
Share on other sites
Psychobilly

Psychobilly

Posted
Posted

Download a 30 day trial of CCDinspector and run some test images through it...

It will give you a pretty good ideas whats going on... how flat the field is how far out the "colimation" is etc...

If in doubt screen grab the displays from CCD Inspector and post them in a thread for people to commment on...

if your "organised" you can get a lot sorted in 30 days especially if you have a run of clear skies at the same time...

Can you upload a full res single frame somewhere?

Billy...

Link to comment
Share on other sites
samtheeagle

samtheeagle

Posted
Posted

I've used a laser to check this out in the past... Use a compass to mark out a circle on paper, to match the end of your OTA. The pin point will mark the centre. Cut the circle out and tape it onto the end of the OTA. Insert laser into focuser and turn on. Does the laser hit the pin point? Basically what Merlin66 said :)

CCD Inspector is also very good :o

Link to comment
Share on other sites
swag72

swag72

Posted
  • Author
Posted

Looking at the CCD Inspectore features and reading the bumpf, there is nothing there about using a DSLR. Surely I need to be using the camera that I use for imaging and not a different one? For example, Olly said that the QHY cameras have had issues in the past. How would I be able to determine where the problems lies if I use the QHY5? Does that make sense?

Link to comment
Share on other sites
swag72

swag72

Posted
  • Author
Posted

What would I need for connection purposes? So far I have only connected the QHY5 to the PC via USB as I've used the ST4 port on the mount.

Oh boy, this is getting worse by the minute. Hubby has just said that there is no way we are fiddling with the optics. Oh well.

Link to comment
Share on other sites
samtheeagle

samtheeagle

Posted
Posted

Lol, take a breath!

From what I remember of CCD Inspector you can load up any image you have, so the ones you supplied at the start of this thread will do. Then you can get it to analyse the field for flatness etc. I asked a very similar question myself a while back, I'll see if I can dig it out...

EDIT: http://stargazerslounge.com/imaging-discussion/90038-anyone-identify-imaging-issue.html

Link to comment
Share on other sites
ollypenrice

ollypenrice

Posted
Posted

Don't worry, no optical fiddling, just optical checking.

For a classical star test you need about 100X; That means fit an EP which gives about that. The magnification is the focal length of the scope (900 in your case?) over the focal length of the EP. So 8 to 12mm or the nearest you have. It is a visual test and very exacting because the observed effects arise from the wavelength of light which is rather short! It would just be nice to know all was well there.

I have never used CCD inspector but Billy will take you through that.

Olly

Link to comment
Share on other sites
Paxo

Paxo

Posted
Posted

CCD inspector analyses the images you already have. It looks at the roundness of the stars right across the image and can assess whether there is any tilt of the chip compared to the optical train and an assessment of the colimation of the instrument.

It also calculates a "false flat" from your images which is a very quick and easy way to see whether the centre of your image circle strikes the centre of the chip, my old FLT110 was off centre on occassions so, like yourself, showed problems in two corners only. Give it a try, it can only help and give you more information.

Steve

Link to comment
Share on other sites
swag72

swag72

Posted
  • Author
Posted

Thanks Peter. I have downloaded CCD Inspector and have put that image through it as the original raw file. I have ended up with the following.

I hope that someone can look at these 2 images and tell me what they are saying.

My understanding on the bottom 3D curvature image is that I should be aiming for the dark area (which is good focus) in the middle of the 'page' - So how do I adjust for this?

post-18339-133877535556_thumb.jpg

post-18339-133877535563_thumb.jpg

Link to comment
Share on other sites

Archived

This topic is now archived and is closed to further replies.

Go to topic listing

  • Recently Browsing   0 members

    • No registered users viewing this page.
×
×
  • Create New...

Important Information

We have placed cookies on your device to help make this website better. You can adjust your cookie settings, otherwise we'll assume you're okay to continue. By using this site, you agree to our Terms of Use.