Odd star shapes

[MickyWay]

Seeing wouldn't have such a marked and predictable effect.....what you get with bad seeing is star shapes that aren't round, but are different on every sub.

Rob

I'm not convinced that it isn't due to seeing. After discounting flexure I did some more research and found this article by Philip Perkins, http://www.astrocruise.com/articles/selong/selong.htm which, more or less, describes the problem I have. Whether this solves Blinky's problem is another matter.

[FraserClarke]

Seeing (atmospheric turbulence) has timescales of milliseconds->seconds at most. Over a long exposure of few seconds or more, this averages out and gives you a gaussian shape.

The effect you're describing here is atmospheric dispersion, which could cause those image shapes if the observations were taken at low altitude. But it will change slowly over time as the object's altitude (and azimuth) changes. So if the same effect is present in observations at different altitudes, it rules out dispersion. You'd also expect to see a colour shift across the images (blue at the 'bottom', red at the 'top' - measured from the horizon).

It's a bit hard to tell from the Cocoon image, but it seems that maybe the aberration is worse in the bottom-left/top-right corners than the other two?? If so, you might be seeing a bit of astigmatism from a slightly tilted focal plane?? Did you check the focus across the whole chip, or just at one point?

Then again, I'd expect something like astigmatism to be symmetric, which it doesn't quite agree with what your seeing...

Slightly pinched optics on the primary is probably a pretty good bet then. Check the mirror is clear of the clips, and not stuck to any of the collimation points.

[blinky]

OK, it's a nice day, so...... I am gonna remove the mirror and loosen (slightly) the mirror clips then recollimate.

One odd thing I noticed last time was the mirror cell has 3 screws to adjust (and 3 lock screws) but when I had the scope collimated, one of the 3 screws was quite a bit further out than the other 2 - does that make sense? Almost like the mirror would be tilted???

[terrym2002uk]

OK, it's a nice day, so...... I am gonna remove the mirror and loosen (slightly) the mirror clips then recollimate.

One odd thing I noticed last time was the mirror cell has 3 screws to adjust (and 3 lock screws) but when I had the scope collimated, one of the 3 screws was quite a bit further out than the other 2 - does that make sense? Almost like the mirror would be tilted???

Do you mean that when the lock screws are down and just resting on the cell plate, that one is further out than the rest? If so that would imply that your mirror tilt is biased in a particular direction. ie not level.

They are normally fairly level with each other, so that might imply that your secondary is not correctly aligned, which might explain your odd star shapes. Perhaps the light cone is not hitting the sensor square causing the eggyness??

Worth checking.

[Edit - Are the 3 secondary screws the same length, ie use a mirror to see if they are all screwed out about the same distance to the secondary holder - dental mirror is handy for this!]

[blinky]

Well I've not had a clear night since but on searching around today I came accross astro baby's collimation guide, at the bottom is an image of a defocused star in a tube with tube currents - it looks very like what I was seeing in my image only the stars were in focus - what do you guys think?

It was a really hot day when I was imaging and the obsy was pretty warm so maybe this is the casue?

[RobH]

That certainly looks like it could be the culprit.

[JoLo]

Did you check your pinched optics? Overtightening the collilmation screws can easily do this...does it appear visually as well? Now when i remove the mirror for whatever reason, i only tighten the bolts enough to hold it without movement, have not had a problem since.

[blinky]

I have not had a clear night to check but I did have a look and the mirror clips were on tight, tighter than I would like but this is to try and hold collimation. I have now (see my other thread) siliconed the mirror to the mirror cell and have loosened off the mirror clips a little and it still holds so fingers crossed for the next clear night (I believe there is one due around October?)

[JoLo]

I'm bettin' that might be it, hopefully you will find out before October! First time I cleaned my mirror and put it back in with the clips too tight, I thought i had ruined my baby! Backed off the clips a bit and all was right with the world.

[blinky]

Think this looks better? TBH I never even collimated tonight, just went with the collimation from the other day!

This is a 100% crop of Altair - I think my silicone has done the trick:)

[blinky]

Did a star test the other night and collimation was bang on BUT when looking at the diffraction rings they were a little flattened at one side - a little like a D shape - any ideas?

[JimStan]

Just remember blinky, " 'better' is the enemy of 'good' " ! ! ! That focused star looks really good to me. I would try some more pictures and see if there is any significant factor causing your images to be less than perfect. If not, I would leave well enough alone !

Just my personal opinion!

Jim S.