I've been struggling with my stars ... any advice?

Equipment Artifacts
11 replies•989 views
Andy Wray avatar
Andy Wray avatar
I have really struggled with my star roundness and also lots of diffraction spikes recently.  Below is my best effort to resolve it so far.  I think I am still seeing diffraction spikes and I'm guessing they are from one of my mirror clips.  

What do you think is causing the issue?

Engaging
Daniel Renner avatar
Daniel Renner avatar
Starspikes comes from your spider vanes, holding yout secondary mirror. if you dont want them you have to buy another type of telescope. And the star roundness dipends on your how good your guiding is, and also the colimation. What does your RMS say in phd2?
Andy Wray avatar
Andy Wray avatar
Daniel Renner:
Starspikes comes from your spider vanes, holding yout secondary mirror. if you dont want them you have to buy another type of telescope. And the star roundness dipends on your how good your guiding is, and also the colimation. What does your RMS say in phd2?

I'm fine with the spider vane spikes; it's the other fuzz around stars that bothers me.  My PHD2 RMS is typically between 0.44 and 0.5 arc secs RMS, although RA is usually about 50% worse than DEC.
Well written
Wolfgang Promper avatar
Wolfgang Promper avatar
To me this looks like a slightly turned down edge of the primary mirror.
Usually a simple  cardboard mask  that covers the edge should solve this.
Well written Concise
Tim Hawkes avatar
Tim Hawkes avatar
Hi Andy, looks like just a little bit of mirror pinching to me. Just loosen off the clips slightly I would say. I had exactly the same with a PDS200. Tim
Helpful Concise
wittinobi avatar
wittinobi avatar
Tim Hawkes:
Hi Andy, looks like just a little bit of mirror pinching to me. Just loosen off the clips slightly I would say. I had exactly the same with a PDS200. Tim

...agree.
you have to clarify the following...

https://www.astrobin.com/forum/c/equipment-forums/sky-watcher-explorer-200pds/skywatcher-200pds-cant-remove-primary-mirror/
Andy Wray avatar
Andy Wray avatar
Tim Hawkes:
Hi Andy, looks like just a little bit of mirror pinching to me. Just loosen off the clips slightly I would say. I had exactly the same with a PDS200. Tim

I think you may be right Tim.  Unfortunately I've tried four times now to remove the primary mirror and it is jammed tight in the tube.  However much force I apply it just won't budge
Well written
Andy Wray avatar
Andy Wray avatar
Wolfgang Promper:
To me this looks like a slightly turned down edge of the primary mirror.
Usually a simple  cardboard mask  that covers the edge should solve this.

Could it actually be the edge of the secondary?  Using a Cheshire today I noticed that the edge of the secondary mirror was visible and the opposite edge of the mirror was outside the field of view of the focusser tube.  i.e. the secondary was 3 or 4 mm too far down the main OTA axis.  I guess over the years of fiddling with the secondary I had ended up inching it down the tube quite a bit.  I also noticed that the angle of the secondary was not that well aligned (i.e. only one of the 3 primary mirror clips was visible; now all 3 are only just visible).

I guess what I'm also saying is that until today I didn't really know what I was doing with secondary mirror alignment;  it now all seems so obvious.

I don't have clear skies right now, but will take some test flats to see how even the illumination looks now (it was quite lopsided before).

FWIW:  using my laser collimator to align the secondary to the primary centre spot was much easier than before and aligning the primary using it has ended up with a primary that is much more even (perpendicular) in the tube.
Helpful Insightful Engaging
Mike Dobres avatar
Mike Dobres avatar
Hi Andy

You can take some of the guess work out by doing a collimation check with an artificial star and looking at the interference rings on the inside of focus and the outside of focus.. Issues such as  pinched mirrors, misaligned secondary, and coma are typically identified with this method. In my case I was able to identify heat currents in the tube, as a problem. Fixed by removing the shroud and using a fan.  This is exactly what I was talking about in my response to your use of a laser collimator. That is, adding a star test, even using and artificial star, is an invaluable tool to checking the alignment and performance of a newtonian

There's stacks out there on the web on this. Here's a very useful link to get you started.  Here's a useful link
http://www.loptics.com/articles/starshape/starshape.html


The artificial star I use is the Geoptik https://www.astroshop.eu/artificial-stars/geoptik-new-artificial-star-for-collimation-and-star-test/p,5976

For your 200  ap X 1000 fl scope you only need 17 meters to collimate.  1 km to perform a true start test.  Though, I've found I can detect major defects at twice the collimation distance Neat thing is this testing can be done on a cloudy night

Hope this helps!

Mike
Helpful Supportive
Andy Wray avatar
Andy Wray avatar
I'm clearly not quite there with my secondary alignment, but this is better than I have achieved before.  I will keep trying.  I believe it is actually pretty good, but we all want perfect, don't we?

This is what I had before I adjusted the secondary:


and this is what I have now:


and here is what the flat actually looks like:
Andy Wray avatar
Andy Wray avatar
Mike Dobres:
For your 200  ap X 1000 fl scope you only need 17 meters to collimate.


My garden isn't that big
Mike Dobres avatar
Mike Dobres avatar
Andy Wray:
Mike Dobres:
For your 200  ap X 1000 fl scope you only need 17 meters to collimate.


My garden isn't that big

Some folk do it in the street
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