Star testing a telescope.

[Guest]

Concentric rings when star testing a telescope  have a concentric appearance when the telescope is correctly collimated. Can the appearence of these rings show facets if there is turbulance in the air?

Edited November 23, 2020 by Guest
Spell check entered pollination instead of collimation.

[markse68]

Not sure if my scope is correctly pollinated but i did star test last night as it’s a good indicator of good seeing (which it was in spades last night!) I noticed the rings were D shaped with a flat on one side which alarmed me until i realised the ep i was using with barlow came to focus with the focuser tube protruding a little into the bore of the ota. A different ep that focused further out produced crisp round rings

Mark

[Guest]

2 minutes ago, markse68 said:

Not sure if my scope is correctly pollinated but i did star test last night as it’s a good indicator of good seeing (which it was in spades last night!) I noticed the rings were D shaped with a flat on one side which alarmed me until i realised the ep i was using with barlow came to focus with the focuser tube protruding a little into the bore of the ota. A different ep that focused further out produced crisp round rings

Mark

Pollinated. Keep bees ha.

[andrew s]

55 minutes ago, Grumpy Martian said:

Concentric rings when star testing a telescope  have a concentric appearance when the telescope is correctly collimated. Can the appearence of these rings show facets if there is turbulance in the air?

Normally,  turbulence makes them distort, shimmer, boil. Not quite sure what you mean by facets but stable linear features are more likely due to something protruding into the light path.

Regards Andrew 

[John]

If the scope has foil spacers these can cause indentations of sorts around the edge of a de-focussed star image.

 

Edited November 23, 2020 by John

[Guest]

Thankyou all for your input. I do now believe that turbulance in the sky causes this.

[Guest]

Last night's viewing was tricky due to quite a bit of turbulance in the atmosphere. Also dew on the corrector lens. Tonight I made a dewshield and fitted it. There seemed to be less turbulance in the atmosphere. The dewshield stopped dew building up. There were no problems with the star testing inside or outside the focussing. So I am happy.

I think that Maksutov's have to be understood. Patience as well. Then they will be your friend. Looking at the moon was awesome.

[Jonk]

Here’s an example of a combination of thermal instability (scope not cooled down at all) and probably poor seeing.

[Guest]

19 minutes ago, Jonk said:

Here’s an example of a combination of thermal instability (scope not cooled down at all) and probably poor seeing.

 

Looks similar to what I saw. Thanks for posting. How did you record this?. May I ask what telescope ?

Edited November 23, 2020 by Guest
Added text.

[Jonk]

This was with a zwo camera (not sure which one I used) at high gain and a fairly high fps.

[badhex]

11 hours ago, andrew s said:

Normally,  turbulence makes them distort, shimmer, boil.

+1 for shimmering/boiling. When I was having poor seeing issues on Friday with my new C5, defocusing really shows up the issue - I used to use the same trick with my old Skymax 102. Effect is just like the haze you get from a hot road but the distortions look bigger, presumably due to magnification but someone smarter than me should probably confirm that theory. 

[Guest]

I always think that eddy's of warm air circulating and moving across the cold sky. They move in front of the objects we are observing. This causing fuzziness. This is like a fast moving stream. We see the peebles at the bottom in focus. Then a swirl of circulating water passes over and the view of the peebles. They then become blurred and out of focus.

Edited November 24, 2020 by Guest
Added text.

[Jonk]

11 hours ago, Grumpy Martian said:

May I ask what telescope ?

This is an old 5 1/4” doublet that I refurbished and needed to test the cell collimation to the tube.

Firstly I put a laser into the focuser end and rotated the tube, seeing if the laser point drew a circle on a far wall, adjusted it as best I could before I pointed it at the stars.

[Peter Drew]

For most amateurs, I think star testing best serves the checking of collimation.  There are far too many variables in play to show much other than gross errors like astigmatism and severe zonal or correction issues.  A definitive  assessment calls for the controlled conditions of bench testing and examination by someone experienced in the nuances of the displayed image, many of which can have an impact on the performance.  We're not all Es Reids!           🙂

[JeremyS]

1 hour ago, Peter Drew said:

For most amateurs, I think star testing best serves the checking of collimation.  There are far too many variables in play to show much other than gross errors like astigmatism and severe zonal or correction issues.  A definitive  assessment calls for the controlled conditions of bench testing and examination by someone experienced in the nuances of the displayed image, many of which can have an impact on the performance.  We're not all Es Reids!           🙂

Thanks Peter. You just saved me a fortune on Suiter’s book

🙂

[Peter Drew]

9 minutes ago, JeremyS said:

Thanks Peter. You just saved me a fortune on Suiter’s book

🙂

Apologies to Suiter!.  I have looked at the book and to be honest, apart from the type of errors I mentioned, the majority of the finer points of reference take an experienced eye to offer a reliable diagnosis.  At the end of the day (night?) if your telescope is giving you the best images you've seen, does it count for much what the book or vendor claims?      🙂

[andrew s]

29 minutes ago, Peter Drew said:

Apologies to Suiter!.  I have looked at the book and to be honest, apart from the type of errors I mentioned, the majority of the finer points of reference take an experienced eye to offer a reliable diagnosis.  At the end of the day (night?) if your telescope is giving you the best images you've seen, does it count for much what the book or vendor claims?      🙂

I agree on testing but it is worth a read just for the myths it busts on resolution, central obstructions etc. etc. Yes in the end its what you see or photograph but that does not mean understanding of the various effects is wasted in my opinion.

Regards Andrew

[Captain Scarlet]

There’s so much more than just star testing in the book, it’s a compelling read for me.

Also the pleasure of this whole hobby for me is much more than simply the observing, the instruments and the physics are all in there.

Sure ones needs experience to interpret star tests, but experience needs to be accumulated and one necessarily has to start as a beginner.

cheers, Magnus

[JeremyS]

45 minutes ago, Peter Drew said:

Apologies to Suiter!.  I have looked at the book and to be honest, apart from the type of errors I mentioned, the majority of the finer points of reference take an experienced eye to offer a reliable diagnosis.  At the end of the day (night?) if your telescope is giving you the best images you've seen, does it count for much what the book or vendor claims?      🙂

I’ve actually got edition 1 of the excellent book, Peter. But your point about trusting the quality of the images you see is an important one. For both star resting and observing planetary detail, you need good seeing and that’s been in short supply lately. In my case, the best views of Mars were well before opposition, when it looked wonderful in the Tak TSA 120. But I’ve had consistently poor seeing post opposition. One of the main reasons for getting the Tak earlier this year was to observe Mars during this well placed opposition and it’s sobering that has now passed.

[Jonk]

So “seeing” as you lot know more than me (haha), is there anything obviously wrong with my star test vid? It was outside of focus if that helps.

[Peter Drew]

1 hour ago, Jonk said:

So “seeing” as you lot know more than me (haha), is there anything obviously wrong with my star test vid? It was outside of focus if that helps.

Putting my "poor man's Es Reid" hat on for a moment I could confidently say that it is not astigmatic, does not exhibit any mechanically induced artifacts, the extrafocal rings appear to be expanding evenly without obvious zones, the outer ring fuzziness could be due to seeing considering the rest seems to be in order. A complimentary inside focus image could add to this appraisal.  That will be 5 guineas. (How am I doing Es.)    🤣

[Jonk]

What’s a guinea? Some kind of pig?!

I only know what ‘looks’ correct, as I was only going for lens cell alignment to the tube.

[StarmanJames]

On 23/11/2020 at 02:49, Grumpy Martian said:

Concentric rings when star testing a telescope  have a concentric appearance when the telescope is correctly collimated. Can the appearence of these rings show facets if there is turbulance in the air?

 

 

Edited November 25, 2020 by StarmanJames
obsolete comment.