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Old 12-12-2018, 06:14 AM
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A start on the Rosette

11X600s subs through an ED80/5nm Ha/1600MM-Pro (139/21 -15) combo. 10 minute subs really pushed my untuned Heq5 (took 14 subs in all, 11 questionable subs survived, but of these only 2 were really solid) so could probably weight the mount more in one direction and somehow tighten the RA up. Errors in PHD2 were generally kept within the 1" band. Image has been lightly processed with a some background division, deconvolution, noise reduction and stretching. Planning on 200 minutes or so in each narrowband channel (Ha, O3, S2). Thoughts always welcome, especially on getting more out of the Heq5.

Update: On discussions elsewhere the question arose “why do 600s subs with such a low read noise camera anyway”? Is there any reason to do this? An optimal sub length for scope, camera, gain, filter and sky conditions?

Astrobin: https://www.astrobin.com/full/380690/0/?nc=user
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Last edited by Benjamin; 12-12-2018 at 07:46 AM.
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Old 12-12-2018, 12:10 PM
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multiweb (Marc)
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Superb shot! Pinpoint stars and a wealth of details.
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Old 12-12-2018, 12:22 PM
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strongmanmike (Michael)
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Nice, it's a good looking object in B&W the'ol Rosette

Mike
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Old 12-12-2018, 01:03 PM
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Stonius (Markus)
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Looking spectacular so far! can't wait for the finished product.

Also, thanks for posting such a full description on the acquisition method so others (like me) can follow along.

I wonder what the exposures like on individual subs? Where did your Mean sit?

Great work!

Markus
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Old 12-12-2018, 03:16 PM
Placidus (Mike and Trish)
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Very well done indeed.

Because it's sharp but a tiny bit gritty, it's tempting to do some more exposure on it should the opportunity arise.


A great result.
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Old 12-12-2018, 07:06 PM
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Lovely. A nice Ha of the Rosette.

Greg.
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Old 12-12-2018, 08:29 PM
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Thanks for the encouraging comments. More Ha data (as well as O3 and S2) will be added as soon as the clouds clear (seems like Brisbane is covered in cloud for the next week at least). In answer to Markus I’m not entirely sure what the mean of each sub was. The statistics tool in PI says the mean is 9.7260e-03 on a single linear calibrated sub. After stretching the image the mean is 2.5957e-01. Sorry this has yet to mean anything to me in regards to processing, although I could imagine why this could be relevant to sub length? Happy to have this explained to me, or pointed along the path to further research. Just making my way through Chris Woodhouse’s “The Astrophotography Manual” which is really fascinating.
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Old 13-12-2018, 08:19 AM
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Looks great, Ben!

Quote:
Originally Posted by Benjamin View Post
In answer to Markus I’m not entirely sure what the mean of each sub was. The statistics tool in PI says the mean is 9.7260e-03 on a single linear calibrated sub.
You can set the Statistics tool to give a 16-bit (0 to 65535) read out which you might find a little more intuitive than a normalised real number.

I'm doing shorter subs (120s for NB) that I think are sky limited on my fast optical system with high gain/low read noise (gain = 200). The reason I only think rather than know this is because the ASI1600 has weird behaviour with short subs, including bias frames, and I need to revisit the calculations to allow for this.

There's a discussion here about how to figure out appropriate sub lengths: http://www.iceinspace.com.au/forum/s...d.php?t=117010

Cheers,
Rick.
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Old 13-12-2018, 10:02 AM
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I’m being lazy here but a quick search seems to suggest the target mean ADU value for a ZWO ASI1600MM-Pro should be around 850 (with a gain of 139) and this should be measured from an uncalibrated sub. So, converting to 16-bit values I have means around 1000 (value drops as the Rosette gets higher in the sky). From this it would seem I need slightly shorter Ha subs, especially when shooting nearer the horizon. The calibrated subs have a mean value of 700 to 750. My Horsehead uncalibrated subs (300s) have a mean value of around 600 in less moon affected skies and a mean value of 1300 in moon affected skies. So it would seem to be a case of really monitoring as you go with certain expectations when the moon/cloud/dust are about. The other factor in this is the stability of the mount (guided Heq5 that is setup on each imaging session) and I guess shorter subs will assist in dealing with its roughness: so while the target ADU could be kept in mind there is perhaps a point where other limitations apply.

Does all this seem correct in theory and practice?

Extra info: the mean values I’m getting are for the entire image. Should I really be getting these values from the background sky only?

Last edited by Benjamin; 13-12-2018 at 10:25 PM.
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Old 13-12-2018, 10:18 AM
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Quote:
Originally Posted by RickS View Post
There's a discussion here about how to figure out appropriate sub lengths: http://www.iceinspace.com.au/forum/s...d.php?t=117010

Cheers,
Rick.
That is an awesome thread. I'd read it before but forgotten all about it. Thanks for the reminder. Reading it again I actually have half a clue what you're talking about!
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Old 13-12-2018, 10:26 AM
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Quote:
Originally Posted by Benjamin View Post
I’m being lazy here but a quick search seems to suggest the target mean ADU value for a ZWO ASI1600MM-Pro should be around 850 (with a gain of 139) and this should be measured from an uncalibrated sub. So, converting to 16-bit values I have means around 1000.
[...]
Does all this seem correct in theory and practice?
Doing the same calcs for my 1600 at unity, I get target ADUs of 66 (1063 in 16bit).

I could very well be doing something wrong though.

Target ADU 66.451
Target ADU in 16bit 1063.216
Bias 48.495
Bias in 16bit 775.92
RN ADU 1.34
RN e 1.34
Camera Gain 1

Markus
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Old 13-12-2018, 10:34 AM
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Nice pic Ben....I have an HEQ5 as well - fitted a Rowan belt drive kit and PE dropped from 35 arc secs to 12 arc secs total. Can now do unlimited length subs - might be worth a look?
cheers
Gary
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Old 13-12-2018, 10:46 AM
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Quote:
Originally Posted by garymck View Post
Nice pic Ben....I have an HEQ5 as well - fitted a Rowan belt drive kit and PE dropped from 35 arc secs to 12 arc secs total. Can now do unlimited length subs - might be worth a look?
cheers
Gary
I’ve done the belt mod too :-) I think sometimes I just need to tighten up some bolts here and there (user error). I went and jiggled my mount this morning and found some odd play and then realize the centre bolt was way too loose (I must have left it loose after polar alignment. So get the feeling the mount may not really be so much the problem I think it is. Plus it’s Christmas and who wouldn’t want a brand new mount! However, I’ve become rather determined to make this Heq5 work!! I would happily take it to someone to do a fine tuning if I knew someone in Brisbane who does that??
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Old 26-12-2018, 08:27 AM
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Added some more Ha data (33x300s extra subs). Processed a little more sensitively, although still need to get more of a grip on Deconvolution. Might be time to get some S2 and O3. As usual comments very welcome.
Astrobin: https://www.astrobin.com/full/380690...dle&real=&mod=
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Old 26-12-2018, 09:30 AM
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Looks good, getting very smooth now.
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