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Old 06-09-2019, 07:56 PM
Sunfish (Ray)
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LRGBHa imaging issues

Attached are two of my first LHaGB and HaRGB image rough mixes from DSS , Nebulosity and PI with a few mechanical and exposure issues, besides suffering from only 15x 2min subs for each filter.

One issue is that at 2min in RGB the background seems to vary widely from filter to filter and night to night and so each would require a different exposure. All at 139 gain, 10 brightness on ASI 1600mm uncooled through an FS102 , f5.9 reducer.

One other is major camera tilt so that the upper left of the image has elongated stars in a different orientation to the lower right . I suppose it is all those additional thread connections with OAG and Wheel. A little hard to take out in lense adjustment . I am waiting on a new adapter to improve reducer distance but an even distortion would be better..

I am wondering how to fix the tilt problem as I have no room for any other adjuster. Not keen on replacing the dovetail celestron OAG with a Zwo oag if it can not be a better tilt proposition. Any suggestions?
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  #2  
Old 07-09-2019, 08:32 AM
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Joshua Bunn (Joshua)
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Hi Ray,
Can you post an image of your imaging train with a short explanation of each part of it?
Cheers, Josh
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  #3  
Old 07-09-2019, 08:43 AM
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Atmos (Colin)
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Good threaded connections shouldn't introduce tilt but as Joshua suggests, images of your components at the moment can be good way of seeing where the weak point is.
Also, does the tilt change as you move from east to west or north to south?

As for every filter having a different background brightness, that's completely normal. Different moisture contents in the air change how bright the sky can be. It's easiest to just pick an exposure that is suitable for the best of nights (moonless and good transparency) and just use that exposure. You'll waste more time trying to get the perfect exposure for each filter every night for next to no gain in image quality.
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Old 07-09-2019, 10:47 AM
Sunfish (Ray)
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Here is the camera assembly on the reducer. From the reducer up: 72 mm to 57 borg adapter, 57 to T2 borg adapter, celestron t2 dovetail plate and OAG, celestron tall t2 dovetail plate, celestron t2 6 mm spacer, ZWO filter wheel, zwo double T adapter, asi 1600. The assembly seemed to have less tilt when I had a DSLR attached to the OAG.

Next to the assembly in the image is a cheap 72 mm to t2 adapter which can replace the borg adapters to reduce the distance to the reducer by 10mm. Very hard to find. I have ordered a fixed 36mm to T2 adapter to replace the helical focuser on the OAG which will mean I can take out the celestron spacers and reduce distance further.

One of the problems with the exposure seems to be that the rgb images need much more exposure when combining in PI. They seem very dark with backgrounds of 100 or so, but that may be something I have missed in PI in fitting the dss stacks.
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Old 07-09-2019, 10:59 AM
Sunfish (Ray)
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As to the difference in tilt I will need to go through my subs and check if it gets worse through the night. I pretty much image in the same quarter of sky I can see from a little east of vertical to west until the mount stops. I can not see South or far North so I suppose that would be side to side tilt which may equate with the problem.
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Old 07-09-2019, 01:59 PM
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The_bluester (Paul)
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My first suspect would be the OAG. Everything else I can see there looks solid. I have an Orion thin OAG and it has the same dovetail and screw style connection at one end (Admittedly worse as it only has two screws to hold it, 180 degrees apart) and it gives me no end of trouble with tilt issues.


Try loosening the screws, rotating the fittings a little at both ends (You can put them back where they are again, just as long as you move them) and see if the tilt changes direction or magnitude.


My TOAG is on the list for replacement, probably with the QHY one as it is all bolted together parts with screw fittings to adapt to whatever is either side.
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Old 07-09-2019, 02:48 PM
Sunfish (Ray)
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Attached are subs converted to tifs from the different filters plus a dslr shot minus the wheel. The second last one, Luminance was taken on a different night and it was about 10degrees East of vertical where the B, number 2, was maybe 45 degrees West of vertical, so flipped on the mount I suppose. Not a lot of difference in distortion . Very bad almost horizontal on the top left and just slight pointing to the middle on the bottom right.

The celestron dovetail has three large solid screws on a heavy dovetail plate and supported my DSLR ( minus the wheel ), although it does show some of the same uneven distortion as shown in the last image of the cat paw.
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Click for full-size image (LOBSTER R_00002s.jpg)
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Click for full-size image (LOBSTER B_00002s.jpg)
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Click for full-size image (LOBSTER G_00002s.jpg)
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Click for full-size image (LOBSTER 00003s.jpg)
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Click for full-size image (IMGP8542dslr paw.jpg)
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Old 07-09-2019, 06:17 PM
Sunfish (Ray)
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Thanks for the comments. Perhaps as you say the OAG is part of the problem amplified by the less than ideal reducer distance. When I look at an earlier dslr shot without the OAG the distortion is quite controlled.

Quote:
Originally Posted by The_bluester View Post
My first suspect would be the OAG. Everything edlse I can see there looks solid. I have an Orion thin OAG and it has the same dovetail and screw style connection at one end (Admittedly worse as it only has two screws to hold it, 180 degrees apart) and it gives me no end of trouble with tilt issues.


Try loosening the screws, rotating the fittings a little at both ends (You can put them back where they are again, just as long as you move them) and see if the tilt changes direction or magnitude.


My TOAG is on the list for replacement, probably with the QHY one as it is all bolted together parts with screw fittings to adapt to whatever is either side.
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Old 07-09-2019, 07:51 PM
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The_bluester (Paul)
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My aim at the moment is to more or less eliminate any non threaded connections as I just cant eliminate tilt issues as things are. The only one I cant see a way to be rid of is I have a rotator but it has a big brass wedge that locks into the dovetail section. Moving the rotation does not seem to change my tilt so I think it is probably OK. But it might go eventually too if I still have issues when I get a new OAG or work out a way to modify the TOAG to eliminate the wedge.
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Old 08-09-2019, 09:17 PM
Sunfish (Ray)
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Thanks for the reply.

If I check my RGB subs for a clear night in PI with settings for 12bit to match the camera, the k value is between 60 to 100 for the background to over 4000 for the bright stars. It would seem I need to double my exposure to get the 400 expected value. Or do I have the units or bit depth wrong?

Quote:
Originally Posted by Atmos View Post
Good threaded connections shouldn't introduce tilt but as Joshua suggests, images of your components at the moment can be good way of seeing where the weak point is.
Also, does the tilt change as you move from east to west or north to south?

As for every filter having a different background brightness, that's completely normal. Different moisture contents in the air change how bright the sky can be. It's easiest to just pick an exposure that is suitable for the best of nights (moonless and good transparency) and just use that exposure. You'll waste more time trying to get the perfect exposure for each filter every night for next to no gain in image quality.
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Old 27-09-2019, 05:45 PM
Sunfish (Ray)
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Clouds cleared. One night only. Thanks for all the suggestions.

Added the new adapter which reduces metal the back distance for the reducer to within a few mm of correct. OAG camera focal distance thus reduced and at a fixed point with another adapter rather than helecoid.

Assembled OAG /camera/ reducer with some rotating and firm clamping before mounting as suggested.

The result is better and not too bad for 20 minutes of each filter. So perhaps worthwhile collecting more data and trying several hours along with ordering a safer wider adapter from Joshua.

I am still unsure about length of exposure and think perhaps I could double each to 4min. Particularly red which seems very dark in all subs. The attached has some of older Ha subs (with a bit of moonglow ) to boost the red. Any more suggestions gratefully received . In the dark really. Ha.
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Old 27-09-2019, 06:00 PM
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lazjen (Chris)
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Quote:
Originally Posted by Sunfish View Post
One of the problems with the exposure seems to be that the rgb images need much more exposure when combining in PI. They seem very dark with backgrounds of 100 or so, but that may be something I have missed in PI in fitting the dss stacks.
At least as a test, I'd throw out DSS and just use PI for everything - try the batch preprocessing script. Throw in your lights and darks, etc into there (don't use masters unless you got PI to create the master).

Try it out, and autostretch the result to get a bit of an idea of what it looks like.
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Old 02-10-2019, 07:19 PM
Sunfish (Ray)
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Thanks for the reply , I think I need to upgrade properly to get PI running again and my trial copies have expired. I will do as you suggest with a little more data. I have added a little more data to the set in nebulosity with double the RGB time to 4 min over ten subs which seems to help but it is getting late in the year for the Lobster for me..

At least the stars are a little tidier now, if the colour is not there yet, and I can work with that.

Quote:
Originally Posted by lazjen View Post
At least as a test, I'd throw out DSS and just use PI for everything - try the batch preprocessing script. Throw in your lights and darks, etc into there (don't use masters unless you got PI to create the master).

Try it out, and autostretch the result to get a bit of an idea of what it looks like.
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