ASA8H secondary position using Cheshire

[Nickb]

Hi all,

I have an ASA8H and recently purchases Catseye collimation tools. The Telecat XLS cheshire sight tube indicated that the secondary was not centered under the focuser tube. I moved the secondary back 5mm to center it. See pic of adjusted secondary through the Telecat XLS.

I had previously followed the ASA procedure to position the secondary using a laser (Glatter) in the focuser with the secondary rotated 90 degrees, and moving the secondary back until it intersected the laser. This placed the secondary at about the same position as it was originally (5mm forward of the focuser axis).

Does anyone know if a Cheshire can be used to position the secondary mirror?

If a cheshire will work, why does it result in a different position than the ASA procedure?

Thank you for your help!

Regards, Nick

 

[MarkS]

Hi Nick,

You have opened a thorny subject! Newtonian type astrographs are very difficult to collimate AND achieve coplanarity on the image plane and the sensor plane.

All collimation tutorials that I've seen achieve collimation of the central ray, which is generally fine for observing, but the problem is that there are any number of configurations that will give central ray collimation, BUT ONLY ONE that results in coplanarity of image and sensor planes. The faster the optics the more critical the problem is because the focus depth becomes extremely shallow. In your case it is about as critical as it gets!

I have checked this by obtaining "perfect" axial collimation on my ASA10N for a range of secondary axial positions encompassing several millimetres. Many of these alignments yield disastrously misshapen stars in the image corners  when using a 16200 class camera with an image diagonal of about 34mm.

The ASA method does get to the correct distance, but I would recommend that you ensure you get the same positioning with the mirror turned 90degrees in BOTH directions. I found this tedious but doable.

Note that when both the setting and the axial collimation are correct the image seen looking down the focuser barrel with Cat's Eye or similar tools will not exhibit concentricity. Again, the faster the optics, the more pronounced the effect is.

I am assuming here that the ASA8H behaves similarly to the ASA10N.

Hope this helps.

Mark

 

[Nickb]

Hi Mark,

This is very helpful!

That is what I suspected but seemed counterintuitive since the focuser tube is not angled towards the center of the secondary. I recently moved to a full format cmos camera and am having problems with peripheral stars. I’ll go back to the ASA procedure (with your additional mirror check) and let you know the results.

Many thanks!

Best,

Nick

[Nickb]

I was looking for the ASA8H corrector model number and found it. It is 3KORRH8.

Edited May 12, 2023 by Nickb
I found the answer - 3KORRH8