Lagoon and trifid complete with blue bloating

[Mike Hawtin]

Took this last night as a test of my Borg 77EDII in combination with my old QHY8. At first glance it doesn't look too bad, just in need of more subs to smooth things out but closer examination reveals less than ideal shapes on the brighter stars and the dreaded blue bloating is quite pronounced. I have to say I'm quite disappointed with these faults as this is not a cheap scope. The spacing from the reducer to the camera chip is bang on nominal, the camera is also square to the optical axis according to ccdinspector. Anyone have any thoughts?

Mike

[RAC]

They stars are perfect small and tight right to the corners though.

[Gina]

Doesn't look that bad to me - quite nice in fact It could probably do with more data to bring out the blue round the Trifid and more Ha in the Lagoon but at least you got a perfectly presentable image. I've not had any clear night skies since I got my Atik 460EX

[Mike Hawtin]

Yes, RAC, Gina, it's getting there but I'm struggling with irregular star shapes on the brighter stars which suggests to me my reducer/camera spacing is not right but I'm right on spec and making alterations of up to +/- 5mm doesn't make a noticeable difference.

Mike

[davew]

I can't say I know too much about this scope but I'm assuming it's not an APO.

Are you using a filter ? What is the star shape you're worried about ?

Dave.

I've just had a look at the image in PS. The green is near perfect. Depending on what you look at, the red and blue can be a bit off at times.

I'll mull it over !

[davew]

I've mulled

As you've got the Fits files, are the stars over saturated in any of the channels ? In other words, are you over doing the sub length ?

If not then if it were my image I'd have a go at processing channel by channel and see what happens.

As I said before, the green channel is near perfect. Sorry I can't come up with anything better right now.

Dave.

[ollypenrice]

While it's a decent image I admit that I would share your disappointment. There is some blue bloat. I've imaged this region with an FSQ85/Atik 4000 and had far tighter stars. Yes, the Tak is much more expensive but the difference seems enormous rather than subtle.

I think your green channel is under-stretched, giving rise to a magenta caste in the blue stars.

There are things you can do in processing to reduce the blue stars, though, the easiest being to run Noel's Actions 'Reduce blue haloes'. You could try a star reduction just on the blue channel, too.

The first thing I'd do, though, is lift the green channel a tad. Get the magenta out of the blues and, with a bit of luck, introduce more pale golden colours into the nice dusty Milky Way background.

Olly

[Mike Hawtin]

Dave, the star bloating shows up in shorter sub exposures too, I'm pretty sure it's a spacing and/or reducer setting issue, more fun things to fiddle with hey ho.

Olly, I agree there are ways of processing that can mitigate the bloating, I'm just more than a bit miffed that I should have to, especially since I had to dump the rubbish focuser the scope came with and buy a starlight. The thing is supposed to be an astrograph for pete's sake!

Disgruntled from Kalo Chorio.

[swag72]

Regarding your spacing and trying to optimise it, I found the following worked well, although I know that it shouldn't really be used as it won't be that accurate.

I couldn't get my spacing right with the reducer and it said that I needed 72.2mm. I had that to the nearest 1/10th of a mm and still my corners were out. I opened up Maxim (I'm sure other programmes would do it) and used the pinpoint function to solve my image. It told me I had a focal length of 332mm, when it should have been more like 328mm with the reducer. I added space and kept taking images until I reached the magic 328mm focal length in the solved images - My corners were spot on! And my spacing was 2mm over the recommended distance.

I know that this isn't massively accurate, but it may help you to get your spacing more to your satisfaction.

[ollypenrice]

Regarding your spacing and trying to optimise it, I found the following worked well, although I know that it shouldn't really be used as it won't be that accurate.

I couldn't get my spacing right with the reducer and it said that I needed 72.2mm. I had that to the nearest 1/10th of a mm and still my corners were out. I opened up Maxim (I'm sure other programmes would do it) and used the pinpoint function to solve my image. It told me I had a focal length of 332mm, when it should have been more like 328mm with the reducer. I added space and kept taking images until I reached the magic 328mm focal length in the solved images - My corners were spot on! And my spacing was 2mm over the recommended distance.

I know that this isn't massively accurate, but it may help you to get your spacing more to your satisfaction.

I didn't know that you'd done it like that but well done. I think this may help Mike but I don't know if it will solve the problem.

Mike, another idea would be to follow a great bit of advice from Peter (Psychobilly.) Focus on a star lying at one of the four intersections of the 1/3 lines on the chip. This is the 'best compromise' position with regard to field curvature. It is now part of my routine and makes for a focus which lasts longer as temp varies. It is rather an interesting technique.

I can see why you are narked, certainly. As things stand, this is not 'astrograph.'

Olly

[Mike Hawtin]

Sara, thanks for the tip, I've tried it already and it didn't work out for me, I still had odd star shapes. I think I need to go back to square one and quantify the scopes native focal length and then do another series with different spacings so that I can calculate the correct distance and then refine my camera alignment to ensure it really is orthogonal as I'm beginning to doubt the results from ccdinspector. What has thrown me today though is advice from Ted Ishikawa at Hutech to alter the reducers internal setting rather than the reducer/camera distance, wonderful, this adds another variable to the mix!

Mike

[Mike Hawtin]

Mike, another idea would be to follow a great bit of advice from Peter (Psychobilly.) Focus on a star lying at one of the four intersections of the 1/3 lines on the chip. This is the 'best compromise' position with regard to field curvature. It is now part of my routine and makes for a focus which lasts longer as temp varies. It is rather an interesting technique.

Olly, it's part of my routine as well, thanks for the thought though.

Mike

[RAC]

Does the qhy8 have a uv/ir cut filter. If full uv was getting in then that could do it?

[Mike Hawtin]

Does the qhy8 have a uv/ir cut filter. If full uv was getting in then that could do it?

Yes, the camera does have a uv/ir filter.

Oh well, on the positive side at least I now have something to do while the moon is too bright for imaging.

Mike

[billhinge]

just for reference

Here are 2 images of m20 in Ha and UV, still collecting data on the UV and others and this is just a rough edit

They were taken with a 20" remote scope, a NABG camera and Ha and UV filters - the UV certainly shows a lot of UV glare (interestingly I get the same effect when I took a pic of a UV diode using a full spectrum camera at home)

I think I recall reading many types of glass fluoresce under uv light (I used a glass called spectrosil which doesn't to convert my diy full spectrum conversion )

[Gina]

I was contemplating a Borg scope but if the problem really is the scope perhaps that's not such a good idea. I'll be interested in seeing how the SW Esprit 80ED works out for cost and quality - not that I'm in a position to consider anything expensive ATM.

[Mike Hawtin]

Gina, optics aside you really need to avoid the stock helical focuser and budget for a capable alternative if you do decide to go down the Borg route for AP.

Mike

[ollypenrice]

There may be an FSQ85 coming up second hand in the fairly near future. Not mine, alas, since mine has gone. This is the answer, trust me.

Olly

[Mike Hawtin]

Olly, I'd love to have one but being retired means I have to watch the pennies and I've spent far too much on astro gear already this year.

Mike

[Gina]

Same here!