Focuser collimation on a refractor - do I have a problem?

MarkInSpace (Mark) · #1 24-03-2021, 11:14 PM

Hi -
I have been unhappy with one corner of my Astro photos having enlongated stars when I use my sky rover 115 refractor.
To investigate, I cut a circle of paper based on the lens cover, created a centre by folding and carefully blue-tacked it onto my telescope.
When I inserted my laser collimator into the back, I see the point is not at the centre. I next tried this with the reducer/flattener in place and got the same result.
Rotating the collimator did not change the location of the point as the laser itself is well collimated.
Is this a problem?
Would this explain the slightly enlongated stars in one corner of my photos?
How do I solve this problem?

MarkInSpace (Mark) · #2 24-03-2021, 11:19 PM

Here’s an example of the enlongated stars in the lower left hand corner.

Karlz (Karl) · #3 24-03-2021, 11:44 PM

If your laser isn't pointing at the centre as you say then there lies the problem and the back end, it's not square. Focuser is out of alignment somehow.

glend (Glen) · #4 25-03-2021, 12:33 AM

Yes it sounds liked your focuser maybe the problem. I suggest you rack it in and out and see if the spot on your target moves. If the focuser is misaligned you may see the spot move in a line. Many higher end focusers are adjustable. What type focuser is it? Rack and pinion or Crayford?

gregbradley · #5 25-03-2021, 09:51 AM

If the focuser is not square to the objective lens then that will cause coma like distortions.

I had that once on an APO and it was way off. It got a lot better once I squared it up as much as I could.

Greg.

MarkInSpace (Mark) · #6 25-03-2021, 10:22 AM

Hi, the point doesn’t shift as I rack out the focuser. It seems squared, but not centred.
I guess the problem is the flattener is not aligned with the centre- maybe that’s causing the issue?

MarkInSpace (Mark) · #7 25-03-2021, 10:24 AM

The telescope has a 3” rack and pinion focuser. Does that lend itself to adjustment?

Merlin66 (Ken) · #8 25-03-2021, 10:39 AM

Mark,
How is the focuser fitted to the OTA?
Screwed fitting or slip fitting with three(?) holding screws??
Edit: Looking at the images of the Sky Rover the focuser seems to be screwed into the OTA.
Makes life a bit more difficult.....
Have you tried rotating the whole focuser relative to the OTA?

MarkInSpace (Mark) · #9 26-03-2021, 02:41 PM

I’m unable to rotate the focuser without undoing screws

Is there a telescope collimation service available in Melbourne?

gregbradley · #10 26-03-2021, 06:38 PM

Quote:

Originally Posted by MarkInSpace

Hi, the point doesn’t shift as I rack out the focuser. It seems squared, but not centred.
I guess the problem is the flattener is not aligned with the centre- maybe that’s causing the issue?

I meant the focuser is not square to the tube, so if you rotate the focuser with a laser in it the laser point does not rotate around the centre of the objective lens.

It may be adjustable? Otherwise pack out your camera so it counteracts that.

Looking at your sample image it appears to be only out by a bit.

Greg.

PRejto (Peter) · #11 28-03-2021, 06:01 PM

Looking briefly at your photo I would say the stars are elongated on both the bottom left and right.

You might try rotating the camera 180 degrees. Does the distortion remain on the image in the same location or does it also rotate 180 degrees?

In your test with the laser how are you holding the laser in the focuser? A lot of clamping mechanisms can introduce some tilt for sure. The Baader click lock is quite good to use in this test. I've done this test with my TEC refractors and a few times I was convinced I had a focuser error only to try again with a new mask, different laser clamping tube and eventually prove to myself that all was actually good!

How far off from center is the laser spot and does the location of the spot relative to the normal camera position look like it might be responsible for the distortion? i.e., imagining a compass rose if the spot is off center at 180 degrees one might expect to see distortion on the photo at either 180 or 0 degrees (or both), but perhaps not at 270 or 90.

If your camera has a tilt plate as Greg said previously you can try using that to compensate for the tilt. It's not too easy to do if it means you cannot access the adjustment screws without taking the camera off. CCD Inspector can help with this....the whole thing can start to do your head it that's for sure. If you want to make this easier you might look into this device: https://www.gerdneumann.net/english/...ting-unit.html

Also, you might be surprised just how many adapters are not well made and introduce tilt and centering off set errors. If you have a dial indicator you might be surprised by what you discover!

Good luck!

Peter

MarkInSpace (Mark) · #12 29-03-2021, 02:15 PM

PRejto said: [A lot of clamping mechanisms can introduce some tilt for sure.]
Yep - I think you're right... clamping is probably the primary offender.
I had a critical look and to laser test I have to consider the camera rotating bracket tightener, the visual back the screws into that has a 3 knob clamp for the 2" adapter at the back, then there is the 2" to 1.25" adapter with a single tightening knob.

Adjusting any of these knobs affects the laser point, especially the 3 knob clamp.

I will concentrate more on camera tilt rather that collimation to see if that is the culprit.
Thanks for the insight.

PRejto (Peter) · #13 30-03-2021, 03:01 PM

Good observation!

If you want to eliminate tilt I would avoid all clamping type connections. Dovetails can be pretty good (and can allow for shims) and screw type are rock solid if made well but cannot be adjusted. With clamping types you will constantly be adjusting and wondering if things are right...and, if you need to break down the system you get to start over!

It's worth considering new adapters made by someone trustworthy, like Joshua Bunn!

Peter