Bloated stars

[Johnboy]

Hi folks,

I tried my first venture into imaging last night and thought I'd have a go at M42. I am using a CPC 1100 telescope (fork mounted and unwedged) with a 6.3 FR and a Brightstar Mammut camera. I have read that I cannot go over about 30 seconds exposures without a wedge due to rotation, so I thought I'd try 20 x 6 seconds for a starter. I have attached the stacked, unprocessed image. Can anyone please tell me why the stars seem bloated? They look overexposed to me but at 6 seconds I think this seems unlikely. The software manual says the offset and gain settings should stay at their default values so they were not changed. I have read that an IR Block filter may help. Would this be the case?

Any help please for a novice but keen imager would be greatly appreciated!

Thank you.

John

orion 1st.TIF

[DirkSteele]

Field rotation will still manifest itself even if you take lots of short exposures and stack them, so whether you take a single exposure of 120 seconds, or 20 x 6 seconds, the impact is the same. The rotation effect comes about in an Alt Az mounted scope (such as your CPC1100) because the scope has to track using 2 axis of motion to follow an object across the sky. All objects in the field will appear to rotate over the course of a night round a central point in the field. As such only a star at the centre of the image will will show pinpoint detail. The rest will start to show a trail. The only way to avoid this (other than to purchase a field derotator - Meade use to sell one for the LX200 SCT scopes, I am not sure if they still do) is to have an accurate polar alignment.

I should add, that I do not think that is the cause of the bloated stars in the case, but something to mindful of as you pursue imaging.

[dph1nm]

Just looks like the seeing wasn't very good to me.

NigelM

[Tinker1947]

You may need a filter IR type........

[Johnboy]

Thanks for the replies and all the help folks.

Would this be the kind of filter: http://www.firstlightoptics.com/uv-ir-filters/baader-uvir-cut-filter.html

Also looks like a wedge is the next thing on the list...

Thanks again

John

[Tinker1947]

Yes that's the type i use and it seems to work......

[ollypenrice]

Field rotation will still manifest itself even if you take lots of short exposures and stack them, so whether you take a single exposure of 120 seconds, or 20 x 6 seconds, the impact is the same. The rotation effect comes about in an Alt Az mounted scope (such as your CPC1100) because the scope has to track using 2 axis of motion to follow an object across the sky. All objects in the field will appear to rotate over the course of a night round a central point in the field. As such only a star at the centre of the image will will show pinpoint detail. The rest will start to show a trail. The only way to avoid this (other than to purchase a field derotator - Meade use to sell one for the LX200 SCT scopes, I am not sure if they still do) is to have an accurate polar alignment.

I should add, that I do not think that is the cause of the bloated stars in the case, but something to mindful of as you pursue imaging.

I don't agree that multi short exposures won't beat single longer ones when in Alt Az. If the stacking software handles rotation as well as alignment in X and Y then the short, relatively untrailed images will be rotated to sit on top of each other. You'll then crop the borders, which will not be aligned. Chopping up the total exposure time and de-rotating the subs retrospectively will work well, though polar alignment is the key. I never got on with wedges, though.

Olly

[Johnboy]

Maybe instead of buying a Celestron wedge for £360 I might be better off saving up a bit more and buying something like this: http://www.firstlightoptics.com/reflectors/skywatcher-explorer-130p-ds-eq3-pro-goto.html

It is much smaller aperture then my CPC 1100 but possibly easier in the long run.

Cheers

John

[DirkSteele]

I don't agree that multi short exposures won't beat single longer ones when in Alt Az. If the stacking software handles rotation as well as alignment in X and Y then the short, relatively untrailed images will be rotated to sit on top of each other. You'll then crop the borders, which will not be aligned. Chopping up the total exposure time and de-rotating the subs retrospectively will work well, though polar alignment is the key. I never got on with wedges, though.

I confess I have never attempted to use software to de-rotate images for stacking. I have heard bad things about the success of such endeavours, hence did not mention it. Is that not the case? If so, I am interested in giving it a go.

Any recommendations of software up to the task?

[Freddie]

DSS would do the job fine.