More Ha - this time the horse

[peter_4059 (Peter)]

Had another attempt with the Ha filter last night, this time on the Horse Head neb. Got off to a slow start with a band of clouds rolling through just as the object crossed the meridian. I managed 160 mins in 10 min subs before the next cloud band came through at which point I packed it in. On inspection of the subs I found the last 6 were spoiled by dew forming on the CCD . I can't understand why this is forming late in the imaging run?

Anyway here is is 10X10mins using the QHY8pro and 10". I'm not entirely happy with the star shape on the brighter stars either. I don't think it is a colimation issue as I carefully checked this before starting.

Comments/suggestions welcome.

Thanks flor looking.

[ozstronomer (Geoff)]

Peter

That's a great shot, shame about the dew issue. It did get worse as the night progressed so you didn't miss much.

[multiweb (Marc)]

That's a great shot Peter. Sharp as. Didn't notice the stars until you've mentioned them. Very minor IMHO. Doesn't distract at all from the main subject.

[renormalised (Carl)]

Nice shot, Peter

You might need to tighten up the focuser a little. I think that eggy look is most probably a little bit of flex in the drawtube.

[allan gould]

Good shot as usual Peter. Like the look of it and a see what you mean about the stars but its minor.

[troypiggo (Troy)]

That's a ripper mate! As Marc says, looks sharp as.

[Dennis]

A lovely image Peter. I’ve had similar looking star shapes when my auto guiding has been a bit off, ending up with bloated stars that also had “corners” to them, as if the guiding was wandering around in a tight pattern?

Cheers

Dennis

[[1ponders] (Paul)]

very nice Pete. Strange stars. Any hint of this in other shots you have taken?

[h0ughy (David)]

you have to be happy with that Peter, vast improvement over the other shot

[Ric]

Great image Peter

The curtain behind the HH has come up very nicely.

[strongmanmike (Michael)]

Looking good Peter.

It couldn't be astigmatism could it..? Astigmatism causes triangular stars. Have you over tightend the collimation or something maybe?

Maybe you already know but an easy test for astigmatism is to just move in and out of focus while looking at a medium bright star, if the star is slightly elongated or oval at all one way inside focus and then this slight elongation turns 90deg outside focus that's astigmatism.

Mike

[RobF (Rob)]

That's a corker of an image Peter, even if you did have a few frustrations. I get a bit of "heart shaped stars syndrome" if my guiding is wandering around a bit - the sum of say a 45degree "wander angle" and a roughly horizontal angle. I was thinking I might even do a sequence of shots passing through focus just to check its not focus/collimation related.

It may well be the image scale too that makes it tricky - I think you'd be imaging at around 1 arcsec/pixel too or even a bit less, which is tough considering seeing is often 2-3 arcsecs?

Do you think you'll do some colour sometime? Would love to see that.

[gregbradley]

Very nice. The star defects aren't that noticeable.

Here is a fix in Photoshop:

1. Duplicate layer.
2. select the stars - select/colour range/highlights.
3. Expand selection 3 pixels, feather it 2 pixels. Control H to hide the selection.
4. Set layer to darken mode.
5. Filter/offset
6. Play with different horizontal and vertical offsets to get the stars rounder.
7. Play with edit/fade to get the amount of correction to your taste also opacity slider of your duplicate layer.
8. Once you like it - control D to deselect and layer/flatten image.

There is also a PS plugin that does the above but its automated. Its free.

You can also use the smudge tool to nudge stars into shape. More work but it works especially on the brighter stars.

Probably best to use a curves on selected stars to reduce star size before you do the above to get a better result. Or perhaps a small amount of minimum filter first.

You can also lasso a portion of the image and feather it heavily and do the above if the elongated stars are in one section of the image only.

Works quite well. Obviously far better to sort out tracking issues but it saves wasting otherwise good data.

If you do it too hard you will distort the image and you will see that you have done something whacko.

Greg.

[peter_4059 (Peter)]

Originally Posted by ozstronomer

Peter

That's a great shot, shame about the dew issue. It did get worse as the night progressed so you didn't miss much.

Thanks Geoff, I've had the camera in a bag with silica gell for quite a few days so I'll see how that goes next clear night.

Originally Posted by multiweb

That's a great shot Peter. Sharp as. Didn't notice the stars until you've mentioned them. Very minor IMHO. Doesn't distract at all from the main subject.

Thanks Marc. It is funny how your expectations get higher with this hobby. A couple of years ago I was happy with an image with a handful of 10s subs. I guess that is what makes it interesting.

Originally Posted by renormalised

Nice shot, Peter

You might need to tighten up the focuser a little. I think that eggy look is most probably a little bit of flex in the drawtube.

I'm hoping it isn't the focuser - it is a Moonlite. I've been keeping an eye on the flexure and there has been less than 0.1 px per minute during my recent imaging sessions according to DSS live.

Originally Posted by allan gould

Good shot as usual Peter. Like the look of it and a see what you mean about the stars but its minor.

Thanks Allan

Originally Posted by troypiggo

That's a ripper mate! As Marc says, looks sharp as.

Cheers Troy - I guess you couldn't detect the green cast in this one

Originally Posted by Dennis

A lovely image Peter. I’ve had similar looking star shapes when my auto guiding has been a bit off, ending up with bloated stars that also had “corners” to them, as if the guiding was wandering around in a tight pattern?

Cheers

Dennis

Thanks Dennis. I have noticed the autoguiding hasn't looked quite as smooth as I've seen before. I've had a play with the settings and the balance. I was wondering if it was seeing related?

Originally Posted by [1ponders]

very nice Pete. Strange stars. Any hint of this in other shots you have taken?

Thanks Paul. I have seen this in some previous shots but not all the time.

Originally Posted by h0ughy

you have to be happy with that Peter, vast improvement over the other shot

Thanks Dave - I really like the Ha images but it sure is tricky to get a good one. There really is no substitute for a lot of long subs. Hopefully the weather improves soon.

Originally Posted by Ric

Great image Peter

The curtain behind the HH has come up very nicely.

Thanks Ric. I'm quite pleased with the curtain and some of the detail in the lower left.

Originally Posted by strongmanmike

Looking good Peter.

It couldn't be astigmatism could it..? Astigmatism causes triangular stars. Have you over tightend the collimation or something maybe?

Maybe you already know but an easy test for astigmatism is to just move in and out of focus while looking at a medium bright star, if the star is slightly elongated or oval at all one way inside focus and then this slight elongation turns 90deg outside focus that's astigmatism.

Mike

Mike - thanks for the tip re checking for astigmatism. I have seen this before when my mirror clips were too tight but wasn't aware of the method to check so will give this a try next clear night.

Originally Posted by RobF

That's a corker of an image Peter, even if you did have a few frustrations. I get a bit of "heart shaped stars syndrome" if my guiding is wandering around a bit - the sum of say a 45degree "wander angle" and a roughly horizontal angle. I was thinking I might even do a sequence of shots passing through focus just to check its not focus/collimation related.

It may well be the image scale too that makes it tricky - I think you'd be imaging at around 1 arcsec/pixel too or even a bit less, which is tough considering seeing is often 2-3 arcsecs?

Do you think you'll do some colour sometime? Would love to see that.

Cheers Rob. I can't remember what the image scale is but will check. I will definately give colour a try some time when the combination of moon phase, clear nights, no other committments and a weekend align.

Originally Posted by gregbradley

Very nice. The star defects aren't that noticeable.

Here is a fix in Photoshop:

1. Duplicate layer.
2. select the stars - select/colour range/highlights.
3. Expand selection 3 pixels, feather it 2 pixels. Control H to hide the selection.
4. Set layer to darken mode.
5. Filter/offset
6. Play with different horizontal and vertical offsets to get the stars rounder.
7. Play with edit/fade to get the amount of correction to your taste also opacity slider of your duplicate layer.
8. Once you like it - control D to deselect and layer/flatten image.

There is also a PS plugin that does the above but its automated. Its free.

You can also use the smudge tool to nudge stars into shape. More work but it works especially on the brighter stars.

Probably best to use a curves on selected stars to reduce star size before you do the above to get a better result. Or perhaps a small amount of minimum filter first.

You can also lasso a portion of the image and feather it heavily and do the above if the elongated stars are in one section of the image only.

Works quite well. Obviously far better to sort out tracking issues but it saves wasting otherwise good data.

If you do it too hard you will distort the image and you will see that you have done something whacko.

Greg.

Thanks for the tips Greg. I'll give those a try.

Peter