Camera based collimation help

[cuivenion]

I'm using a asi224 and a gpcam with a 12mm cs lens for collimation of the secondary mirror on my Skywatcher 130pds. The problem I'm having is that when centering the secondary under the focuser I'm getting different results when using a collimation cap/cheshire and a camera. When I center the mirror using the cap as best I can this is what I see with the camera:

As you can see with the camera its way off and it is the camera that's wrong. The only possible explanation I've come across is that " The lens needs to be precisely parallel to the CCD and both need to be precisely parallel to the focuser shoulder", however I've got now idea how I would go about doing that, would it involve buying another lens?

The problem is the same with both the asi224 and the GPCAM.

I'd like to sort out a camera based way of centering and rotating the secondary, if possible so any help would be appreciated.

[Ronclarke]

There is a collimation aid on APT which I have not used myself but know that others have and been successful. Worth a try>>

 

Cheers

Ron

[almcl]

Is the focuser absolutely square to OTA?  

It looks in the image, from the shadow on the left, as if it could be tilted.  

One method to correct that *might* be to remove the spider and secondary and then, still with camera attached, adjust the focuser tilt screws (or place shims if yours dosen't have these adjusters)  until the mark on the far side of the tube is centred on the imaging camera.  A procedure for doing this with an eyepiece is described here but it should work with a camera?

[cuivenion]

I know what you're saying but everything looks fine under the cap/cheshire so it must the camera/lens thats the problem.

[david_taurus83]

I have tried this with the ASI 120 and the supplied wide angle lens. The lens is not accurate at all. I set the 200p OTA up on the dining room table with the focuser straight up and put the camera on top. Even though it's almost a snug fit it points off to the side of the drawtube. My fix was to turn the crosshairs into circles, make the same size as the drawtube on screen and centre them up to the sides of the drawtube. Checked by rotating the camera and it was still centred to the tube. Its great for getting the secondary a nice circular shape but I always switched to the Cheshire for the fine tuning.

[cuivenion]

Yeah, I've used it to get the round shape, I was just hoping I could use it to center the secondary as well. I use a cap and a cheshire for aligning the primary mirror.

[cuivenion]

Would buying a better lens help? Or is it the camera sensor that is tilted and should I try and fix it with shims or a similer procedure.

[Skipper Billy]

I used blobs of PlayDoh (sp?) to hold the camera in place in a spare eyepiece holder and once I had got it square I glued it in place with 5 minute epoxy. Worked a treat but only for that scope ie not transferable from scope to scope.

[cuivenion]

Hi Billy, unfortunately thats not the problem. The asi224 fits in the 2" holder snugly as does the GPCAM in the 1.25". Despite this the I'm getting the error in the picture above. That was taken using the GPCAM, but the asi224 wasn't right either.

It wasn't movement in the holder that caused it, I loosened the screws and the movement wasn't that great. What I'm really wondering is do both the cams have a problem with the sensor being tilted or is it the lens.

[Merlin66]

Could be you're seeing the secondary offset....

I know you think the Cheshire is correct, but I'd trust the camera.

What do the images look like through the telescope??

 

[cuivenion]

3 hours ago, Merlin66 said:

Could be you're seeing the secondary offset....

I know you think the Cheshire is correct, but I'd trust the camera.

What do the images look like through the telescope??

 

Unfortunately I can't get any images at the moment due to constant cloud. To be honest I always thought the cap/cheshire did a good job, but centering/rounding the secondary is much easier in theory doing on a screen. If the cheshire is as far out of whack as the camera suggests I think my images would have been a lot worse.