Topics

FW: M51 from Fri night

Robert Pitt
 

Howdy all:
This is what I have been trying to do on clear nights. I photographed this one on Friday from my backyard observatory (with some help with narrow band filters!).
Regards,
Bob
[Robert Pitt]

Don Waters
 

Nice Bob.

Please give some details on equipment and exposure.

Don Waters

Robert Pitt
 

Thanks Don. This was a stack of the best 75% of 20 exposures of 3 min each (need to work on my polar alignment for longer exposures and tried for 60 subs all together, but it was getting too low for the last sets so just used the first ones here). The camera was a ZWO ASI 294 (color) with a Triad Quad filter (4 narrow band 4 or 5 nm each of Ha, Hb, O3, and S2 on one filter). From my backyard observatory setup that I have been fiddling with for 20+ years now. Currently has a ME mount and a Hyperion 12.5-inch f/8 astrograph (the last scope and mount in the dome was the MI250 and C14 currently in Chandler roll-off).

Regards,

Bob

 

 

Robert Pitt, Ph.D., P.E., BCEE, D.WRE

Emeritus Cudworth Professor of Urban Water Systems

Department of Civil, Construction, and Environmental Engineering

The University of Alabama

Tuscaloosa, AL 35487

Rpitt at eng dot ua dot edu

http://unix.eng.ua.edu/~rpitt/

 

 

 

From: main@bas-astro.groups.io <main@bas-astro.groups.io> On Behalf Of Don Waters
Sent: Friday, July 03, 2020 08:26 AM
To: main@bas-astro.groups.io
Subject: Re: [bas-astro] FW: M51 from Fri night

 

Nice Bob.

Please give some details on equipment and exposure.

Don Waters

Mark Copper
 

Hi Bob,

Do your 3 minute subs saturate the pixels at the galaxy centers?

Thanks for sharing your work.

Mark

Aaron Williams
 

Nice Bob! Pulled a lot of detail out of that image

-Aaron

On Fri, Jul 3, 2020 at 2:16 PM Mark Copper <mlcopper@...> wrote:
Hi Bob,

Do your 3 minute subs saturate the pixels at the galaxy centers?

Thanks for sharing your work.

Mark

Robert Pitt
 

Hello Mark:

Probably. I was more concerned about detail in the arms. My plan was to take some shorter versions for the cores to try to blend, but it was too low by then…

Regards,

Bob

 

From: main@bas-astro.groups.io <main@bas-astro.groups.io> On Behalf Of Mark Copper
Sent: Friday, July 03, 2020 02:16 PM
To: main@bas-astro.groups.io
Subject: Re: [bas-astro] FW: M51 from Fri night

 

Hi Bob,

Do your 3 minute subs saturate the pixels at the galaxy centers?

Thanks for sharing your work.

Mark

Robert Pitt
 

I forgot to mention that I also used bin 2 as the native pixel sizes are quite small for the 294 for my focal length. Also used dark and flat calibration images.

 

From: main@bas-astro.groups.io <main@bas-astro.groups.io> On Behalf Of Robert Pitt
Sent: Friday, July 03, 2020 12:06 PM
To: main@bas-astro.groups.io
Subject: Re: [bas-astro] FW: M51 from Fri night

 

Thanks Don. This was a stack of the best 75% of 20 exposures of 3 min each (need to work on my polar alignment for longer exposures and tried for 60 subs all together, but it was getting too low for the last sets so just used the first ones here). The camera was a ZWO ASI 294 (color) with a Triad Quad filter (4 narrow band 4 or 5 nm each of Ha, Hb, O3, and S2 on one filter). From my backyard observatory setup that I have been fiddling with for 20+ years now. Currently has a ME mount and a Hyperion 12.5-inch f/8 astrograph (the last scope and mount in the dome was the MI250 and C14 currently in Chandler roll-off).

Regards,

Bob

 

 

Robert Pitt, Ph.D., P.E., BCEE, D.WRE

Emeritus Cudworth Professor of Urban Water Systems

Department of Civil, Construction, and Environmental Engineering

The University of Alabama

Tuscaloosa, AL 35487

Rpitt at eng dot ua dot edu

http://unix.eng.ua.edu/~rpitt/

 

 

 

From: main@bas-astro.groups.io <main@bas-astro.groups.io> On Behalf Of Don Waters
Sent: Friday, July 03, 2020 08:26 AM
To: main@bas-astro.groups.io
Subject: Re: [bas-astro] FW: M51 from Fri night

 

Nice Bob.

Please give some details on equipment and exposure.

Don Waters

Mark Copper
 

I understand, what with all the sophisticated astrophotography processing techniques, that this is quite naive. 

But to my mind a principal effect of stacking is to multiply the dynamic range of the camera. So ideally one times subs so that pixels of interest are generally not saturated. Then, after adding the subs together one has a lot of local detail, say 18 bits per pixel with 16 subs from a 14 bit camera. Unfortunately all that information is erased when the photo is gets projected linearly back to an 8 bit representation, essentially dividing each value by 2^10 and throwing away the fractional part. So that's where basic light curves come in: with a convex curve (think square root), one expands the lower range, and with a concave curve (think square) one expands the upper range. With a lazy "Z" curve, one does a little of each while compressing the middle values.

But that's all ignoring noise. And probably 10 other important things that I have yet to learn. Which is why I'm so interested in what you're doing.

Robert Pitt
 

My use of stacking is to reduce noise while obtaining a sufficient signal to noise ratio. Generally, 4 subs reduce noise to half, 8 subs to 1/3, 32 subs to 1/8, etc. Unfortunately, combining subs is less effective than a longer image for the equivalent time (in general, although there are practical limits, especially due to light pollution; 6 min subs seem to be optimal in many cases, but could be longer for narrow band). This is an interesting presenting on subs: https://www.youtube.com/watch?v=EMdEhQD2WxY. As many subs as possible is always best, if sky is good (I had to throw away 2/3 of my subs as M51 got too low and in the crud; fewer subs when M51 high were better). More subs builds the signal (which increases the dynamic range), but don’t want each to blow the highlights for each sub. Sky glow is usually the limiting factor in urban settings, not object brightness of DSOs.

Regards,

Bob

 

From: main@bas-astro.groups.io <main@bas-astro.groups.io> On Behalf Of Mark Copper
Sent: Friday, July 03, 2020 06:19 PM
To: main@bas-astro.groups.io
Subject: Re: [bas-astro] FW: M51 from Fri night

 

I understand, what with all the sophisticated astrophotography processing techniques, that this is quite naive. 

But to my mind a principal effect of stacking is to multiply the dynamic range of the camera. So ideally one times subs so that pixels of interest are generally not saturated. Then, after adding the subs together one has a lot of local detail, say 18 bits per pixel with 16 subs from a 14 bit camera. Unfortunately all that information is erased when the photo is gets projected linearly back to an 8 bit representation, essentially dividing each value by 2^10 and throwing away the fractional part. So that's where basic light curves come in: with a convex curve (think square root), one expands the lower range, and with a concave curve (think square) one expands the upper range. With a lazy "Z" curve, one does a little of each while compressing the middle values.

But that's all ignoring noise. And probably 10 other important things that I have yet to learn. Which is why I'm so interested in what you're doing.

Don Waters
 

Thank you, Bob.
 with 
I am a rank beginner, so some naïve questions please.
1. I have been using my Canon T3i to get started with my Meade 10" on a wedge - with obvious limitations on guiding accuracy. So far, I have managed 90 sec subs with round stars with a total guiding error under 2" RMS. I am currently running some PEC analysis on the Meade mount, and preliminarily, it looks like 1.5-2" RMS will be the max possible. I am thinking of adding a real camera. Do you recommended the ASI294? (I have a ASI290 for guiding).
3. I'm  a little confused on using a color camera with filters. I thought one must use a monochrome. No?
4. I have just received a Celestron 9.25" and Hyperstar mounted on CG-5 GEM, so I should be able to really reduce guiding issues.

Thanks,
Don

Robert Pitt
 

Hello Don:

Sounds like you are set for optical tubes and mounts. The 10/10 and 9.25/10 have similar focal lengths as my Hyperion. The ASI294 results in 0.38 and 0.41 are-sec/pixel which would be oversampling, while bin2 and bin3 could be used for about 0.8 and 1.2 arc-sec/pixel, but with reduced overall fields. A free program, CCDCalc (http://www.newastro.com/book_new/camera_app.html) allows you to calculate the resolutions and fields of different scopes and cameras (and shows field overlaid on actual images). I use the 294 with a Triad Quad filter that was designed for color cameras with 4 narrow bands in one. There are others (usually 2 and 3 bands at wider widths that are much less costly)) that may also work well. You are correct in that a mono camera is most efficient for normal narrow band filters (however, be aware that you will have wavelength offsets at the extreme f ratio of your hyperstar and may need the Baadar special set for ultra fast f ratios). You may also need a round camera for the hyperstar (although I have used a Sony mirrorless with my 9.25 hyperstar setup a couple of years ago at OkieTex with reasonable success). WZO only offers 3 monochrome cameras (1600, 183, and 6200). Differences in costs relate to sizes of the sensor. I also have the 1600 and have used it on my RASA 11/2.2, with good sampling at about 1.5 arc-sec/pixel, but would be grossly oversampling on my Hyperion without binning. The 183 is a bit more expensive, while the new 6200 is quite expensive as it is 60 mb full frame (and still rare).

 

There are many discussions on sampling on Cloudy Nights that can get quite technical, but in general, folks suggest 1 to 2 arcsec/pixel (again CCDCalc will calculate this for you). In reality, atmospheric seeing will have a big effect (we can get periods of excellent seeing down here). Oversampling is no big deal and may result in somewhat aesthetically better images, especially during periods of exceptional seeing. Undersamplng (very large pixels as in some older CCD astro cameras) can be overcome thru drizzling during exposure (available in Maxim and PixInsight). Large chips with small pixels and many sub exposures (as in using noncooled DSLR cameras or in exotic astro cameras) do put a large burden on your computer while stacking. I wouldn’t worry too much about exact matching pixel sizes and optimal efficiency as you are not using a large research scope on top of a volcano in Hawaii!

 

You may want to continue using the Canon on your setups for a while. So what if it is somewhat less efficient? You could even put a Ha filter in the adapter to see how it works. Starizona sells camera adapters for the hyperstar with filter drawers at the proper lengths; you can contact them re the T3i and your hyperstar.

Have fun!

Regards, and stay safe

Bob

 

From: main@bas-astro.groups.io <main@bas-astro.groups.io> On Behalf Of Don Waters
Sent: Saturday, July 04, 2020 08:43 AM
To: main@bas-astro.groups.io
Subject: Re: [bas-astro] FW: M51 from Fri night

 

Thank you, Bob.
 with 
I am a rank beginner, so some naïve questions please.
1. I have been using my Canon T3i to get started with my Meade 10" on a wedge - with obvious limitations on guiding accuracy. So far, I have managed 90 sec subs with round stars with a total guiding error under 2" RMS. I am currently running some PEC analysis on the Meade mount, and preliminarily, it looks like 1.5-2" RMS will be the max possible. I am thinking of adding a real camera. Do you recommended the ASI294? (I have a ASI290 for guiding).
3. I'm  a little confused on using a color camera with filters. I thought one must use a monochrome. No?
4. I have just received a Celestron 9.25" and Hyperstar mounted on CG-5 GEM, so I should be able to really reduce guiding issues.

Thanks,
Don

Don Waters
 

Thanks, Bob,

A well written and clear explanation.

I do plan to stay with the T3i for a while until I get the hang of it, I see that a cylindrical camera will be best for the f2 - especially for using a focusing mask.

And you are right about not worrying about image quality too much. I am not as concerned about nice pictures as I am in learning the technical issues involved. If I learn good technique (and spend enough $), I'm sure good images will follow.

Robert Pitt
 

You’re welcome Don. I misspoke a bit when I said that binning results in reduced fields; it doesn’t, just reduced resolution and smaller file size. The field stays the same. We are hopefully still in the golden age of astrophotography, with astro cameras getting better and less expensive with time, so no need to rush to a purchase until you are ready (do as I say, no as I do!).

Regards,

Bob

 

From: main@bas-astro.groups.io <main@bas-astro.groups.io> On Behalf Of Don Waters
Sent: Sunday, July 05, 2020 09:07 AM
To: main@bas-astro.groups.io
Subject: Re: [bas-astro] FW: M51 from Fri night

 

Thanks, Bob,

A well written and clear explanation.

I do plan to stay with the T3i for a while until I get the hang of it, I see that a cylindrical camera will be best for the f2 - especially for using a focusing mask.

And you are right about not worrying about image quality too much. I am not as concerned about nice pictures as I am in learning the technical issues involved. If I learn good technique (and spend enough $), I'm sure good images will follow.