Star Test Interpretation

[heliumstar]

I did a star test yesterday and encountered something interesting that I can't explain. Maybe someone here who is more versed in star testing and optics theory can explain. First I did a star test on Altair and saw a series of rings of equal brightness and all aligned. Pretty much textbook star test both inside and outside focus. But then later in the night I did another one on Aldebaran and observed a different thing. Please see the image. Ignore any image irregularities as this was a minute Inkscape job. The gap between the two last rings was wider at the bottom than at the top. I observed the same the other day so it's not a one off thing.

[CraigT82]

What was the scope? If rings bunched to one side I’d say collimation, or star wasn’t on the optical axis. 

[heliumstar]

5 minutes ago, CraigT82 said:

What was the scope? If rings bunched to one side I’d say collimation, or star wasn’t on the optical axis. 

The scope is APM-LZOS 115/805.

Collimation was my first thought as well but not sure how to explain Altair having concentric rings. Also, the issue was only really discernible on the last ring. I'll do some more tests and if repeated will send it to collimation. Wondering who in the UK offers collimation service. I know Es Reid is the person to go but have no idea how to contact him.

[vlaiv]

1 minute ago, heliumstar said:

Collimation was my first thought as well but not sure how to explain Altair having concentric rings

Could be number of things.

What was the position of each respective star? If Altair was high in the sky and Aldebaran lower - then it might be loose lens in a cell that moves when you move the scope. Can you hear any appreciable rattle or any sound as you handle the scope?

Another thing that can happen is motion due to temperature. Since this happened later in the night - could it have been much colder at that time? Thermal contraction can also shift something out of alignment.

[Captain Scarlet]

Thermal gradient inside tube suggests itself as possibility to me too. Try rotating the tube a bit when you see it and see if the narrow bit shifts gradually to top position again

Edited September 15 by Captain Scarlet

[heliumstar]

6 minutes ago, vlaiv said:

Could be number of things.

What was the position of each respective star? If Altair was high in the sky and Aldebaran lower - then it might be loose lens in a cell that moves when you move the scope. Can you hear any appreciable rattle or any sound as you handle the scope?

Another thing that can happen is motion due to temperature. Since this happened later in the night - could it have been much colder at that time? Thermal contraction can also shift something out of alignment.

 

Indeed, Altair was much higher in the sky and yes it was a few degrees colder later in the evening (actually morning ). No rattle that I can hear though.

 

1 minute ago, Captain Scarlet said:

Thermal gradient inside tube suggests itself is possibility me too. Try rotating the tube a bit when you see it and see if the narrow bit shifts gradually to top position again

Will do! Thanks!

[Steve Ward]

It's a tiny bit off , could just be a small focuser sag when in a different pointing direction/orientation.

[heliumstar]

3 minutes ago, Steve Ward said:

It's a tiny bit off , could just be a small focuser sag when in a different pointing direction/orientation.

Ha! This might well be it. Thinking back I had the focuser 'shrink' a few times and had to screw it to eliminate movement. Not really sure how to explain this but when I was holding it I did a vertical movement and it moved a little bit until I screwed it in (it's rotatable).

[CraigT82]

7 minutes ago, Steve Ward said:

It's a tiny bit off , could just be a small focuser sag when in a different pointing direction/orientation.

This is a good shout, probably most likely culprit? 

[John]

You can test for focuser alignment with the optical axis of the scope in a refractor with a laser collimator (a collimated one). Pop the laser collimator in the drawtube (no diagonal) and see where the beam exits the front element of the objective - it should be in the centre.

It's the first stage in refractor collimation. If the focuser proves to be OK and aligned with the optical axis of the objective, a cheshire eyepiece can be used to check the tilt of the objective. With a triplet objective though, de-centering of the central objective element can also be the cause of mis-collimation. Collimation of a triplet objective and, if needed, re-centering the elements, is a job for a professional though, as far as I'm concerned.

 

 

 

Edited September 15 by John

[Coco]

try again and tighten the pinch bolt on the draw tube, to see if it occurs.