Odd star shapes early evening

[Sparrow]

I don't want to hijack Ant's thread about their problems, but thought I'd post my own instead.

When I first start my session I've noticed that when looking at a bright star in live view, or at my early subs, there is like a tail off the bright stars. So far I've put this down to insufficent cool down time, therefore uneven optics, as it seems to disappear as the session progresses.

I usually allow about 30 minutes cool down before I start to get the scope on target and focussed. This is usually at dusk so it's not dark enough to image and I don't actually start taking subs for another half hour or so. the M36 image was a convenient example as unually it was taken before it was completely dark and about half hour after the scope was taken outside.

Any ideas - would starting collimation from scratch perhaps help?

Here's a crop from the cntre of my M36 image

[ollypenrice]

As I said on Ant's thread, pinched optics - the lens or mirror being distorted by the cell holding it too firmly and distorting it. This is often temperature related. The effect is often to turn stars vaguely triangular.

BTW, I don't think your point was a hijack. Quite the opposite, I think you've nailed it.

Olly

[Ant]

Yeah I think you might be onto something as well.

Thanks for chipping in and it's something that I'm looking into now - how to loosen up the lens cell.

Cheers

Ant

[Sparrow]

Thanks Olly - I'll loosen the primary and re-collimate it. Hopefully it will solve Ant's problems too.

[Sparrow]

Well I've re-collimated the scope and still have the same problem but it does seem to be worse than ever now. I'm beginning to wonder if it might be the alignment of the secondary rather than the primary causing this. Anyone got any thoughts. I did slacken the primary right off and re-collimate it (the collimation screws - I didn't do anything with the actual clips that hold the mirror in place).

I've attached a better example of the distortion from an attempt on M44 last night - aportion enlarged to show the distortion clearly from one of the subs.

[michael8554]

I think Olly was suggesting the clips holding the mirror in place are pinching, the collimation screws shouldn't normally distort the mirror support.

At the risk of inserting foot in mouth I'm going to disagree with Olly though.

To me the images suggest only poor collimation, possibly due to the weight of the camera moving the focuser tube off its normal axis?

(Now where did I leave my flak jacket...)

Michael

[Sparrow]

Thanks for that Michael. Any input greatefully appreciated.

I'm sure it's not collimation. Since my last post, I've totally started from scratch with the collimation, doing secondary and primary. The abherration was still there.

Your point about the focusser does make sense, although I did fit a brand new low profile focusser to it last year which is good quality. I will check it for any play though, as the camera slumping under gravity would potentially cause this. I resume it's posible to tighten up a two speed crayford.

I'll let you know how I get on. Unfortuantely, with so few clear nights, I'm having to wait ages between each session to test it.

Regards

John