NGC 7000 in Ha (with a few issues!)

[Interested]

Although I have been into this hobby for just over 18 months, what with getting the kit together and the bad weather, I don't have that much imaging time under my belt.I've probably been out for about 5 of the 10 clear nights we have had in Bristol since the middle of March. That said, last night was really clear so, stayed out until 3 am (up again for work at 6 am !).

The image shown was a combination of 6 x 600 s in Ha & 2 x 600 s in Hb (seemed a shame to waste them ) with associated darks....no flats. Scope used was the 'Baby Q' and camera was the 383L+ mono. The frames were stacked in DSS and the only processing carried out has been some noise reduction and some histogram stretching in AA5. Guiding was with PHD and the graph was pretty flat.

If you zoom in (hope the resolution is good enough!) and look at the corners of the image the stars are elongated. They are not elongated in the same direction though, they seem to form a circle. Can any of you seasoned imagers figure out what is causing the elongation?

[Dipper]

I'd be surprised if you haven't heard about coma, or comatic abberation. It's an optical issue when stars towards the edges of the image appear to form not circles but triangular shapes, or comet shapes. Coma correctors fix this issue. In layman terms, the "faster" the scope the more coma becomes apparent.

[davew]

I don't think Takahashi make Coma correctors for the FSQ 78

Some thing doesn't look quite right and the larger stars look diamond shaped. The Jpeg compression make it hard to see.

Can you post a better image ?

Dave.

[davew]

That should have read FSQ 85

Dave

Just spotted the edit button is back !! Don't miss a trick me.

[Interested]

I don't think Takahashi make Coma correctors for the FSQ 78

Some thing doesn't look quite right and the larger stars look diamond shaped. The Jpeg compression make it hard to see.

Can you post a better image ?

Dave.

Still a jpeg, but uncompressed can't seem to upload a 16 bit tif file?

Pete

[davew]

It's clearer now cheers.

At least the diamond pattern has gone !

I'm guessing, looking at it, you used the reducer. Well I hope that's it. I feel the distance between the camera and reducer is slightly out. Can you adjust it ? I've had to change the metal back distance on my scope to take account of the Ha filter.

If you haven't used the reducer I'll have another think,

Dave.

[Interested]

I should also add that I have carried out a star test and the pattern achieved was the same either side of focus - as near to perfect as I have seen. No I didn't use a reducer - the Tak has a built in flattener. I wondered if the camera chip was to big for the 85 to cope with (Kaf-8300 chip) - but I wouldn't have thought so. The imaging circle for the optics is meant to be 40 mm.

[davew]

You didn't. Damn. I'll have a think but don't hold your breath.

May be you'll have to wait for another 85 user to show up.

The FSQ 85 can use a reducer. Just not a flattener / reducer.

Dave.

[Interested]

You didn't. Damn. I'll have a think but don't hold your breath.

May be you'll have to wait for another 85 user to show up.

The FSQ 85 can use a reducer. Just not a flattener / reducer.

Dave.

Hi Dave,

Many thanks for your input. Yes your right, the reducer allows imaging at f3.8/9! Not sure if I can get the filter wheel in the optical chain If I get one though. Will have to research that as I might be getting into 'chip to reducer distance' issues!

Regards,

Pete

[davew]

I've had a think. I even tried doing a very crude ray trace pattern on a piece of paper. Stumped, unless the chip distance is altered by the filter. If you understand what I'm on about ?

If you bought it new I would now suggest sending a good quality image to the vendor.

Dave.

[Interested]

Having, talked with the supplier today I think the problem is field rotation (not experienced it before!). The image posted here is 600 s exposures, I have another image with slightly worse star trailing that was taken using 300 s exposures. I would expect a consistent error if the scope was at fault. I guess I can no longer rely on just polar alignment and will need to start doing drift alignment as well now, if I want to use longer exposures!

Guess I should repost the image for comments on quality now, only, I KNOW someone will say "I think you have a field rotation problem there!".

Regards all,

Pete