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Equipment Discussions >> Electronically Assisted Astronomy

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Chris A
Pooh-Bah


Reged: 02/03/07

Loc: Toronto, Canada
Re: Which camera? new [Re: nytecam]
      #5689116 - 02/19/13 03:38 PM Attachment (7 downloads)

Nice capture Maurice! Here is a live capture from NSN when I was showing my audience that a Mallincam is capable of capturing the Hickson 50 from a large city under mag 4 skies.

The image captured was a single 90 sec exposure using my MX and C9.25 @ f5.8 and no filters. Image was capture off the screen using Screen Shot Pilot and was only labelled with no post processing.

http://www.flickr.com/photos/47296963@N08/7061889393/in/photostream

Chris A
Astrogate


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Dwight J
scholastic sledgehammer
*****

Reged: 05/14/09

Loc: Lethbridge, Alberta, Canada
Re: Which camera? new [Re: mpgxsvcd]
      #5689209 - 02/19/13 04:22 PM

Hmmm. Interesting problem. Have you tried reducing ISO and if so, does it reduce or eliminate the sky glow? I believe ISO and gain are similar and if I have my gain at 4 or higher with the VSS I certainly encounter LP fog sooner. My DSLR images at lower ISO show less. Your local light pollution could be much worse than most too and trying to filter it out can interfere with exp times as you would have to expose longer and with more subs. I can do 5 min subs from my yard @ ISO 800 but 1600 is a washout. That is using a Lumicon Deepsky filter. I notice that the new street lamps we have here are white so it is hard to filter them out.

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mpgxsvcd
Carpal Tunnel


Reged: 12/21/11

Loc: Raleigh, North Carolina
Re: Which camera? new [Re: Dwight J]
      #5689235 - 02/19/13 04:35 PM

I am not sure if my light pollution is better or worse than anywhere else. I haven't really tested it at another light polluted place.

Again the only way to judge this is to stick these cameras on the same scope with the same sky and use the same filters and reducers. Simply put. The camera that displays the object best in the least amount of time is the winner. I think that camera will be the Mallincam but perhaps not by the margin some would expect.

Edited by mpgxsvcd (02/19/13 04:38 PM)


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Stew57
Carpal Tunnel
*****

Reged: 05/03/09

Loc: Silsbee Texas
Re: Which camera? new [Re: mpgxsvcd]
      #5689293 - 02/19/13 05:07 PM

The reason I am conntemplating picking up a GH3 to compliment the mallincams if for the FOV. I would like to see a Universe compared to the GH3.

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mclewis1
Thread Killer
*****

Reged: 02/25/06

Loc: New Brunswick, Canada
Re: Which camera? new [Re: mpgxsvcd]
      #5689328 - 02/19/13 05:24 PM

Quote:

However, I am not entirely sure what amp glow looks like? Can you demonstrate it or define it?



Travis,

Have a look at Chris's great Hickson 50 image, amp glow is slightly visible in the upper left corner.


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GlennLeDrew
Postmaster
*****

Reged: 06/18/08

Loc: Ottawa, Ontario, Canada
Re: Which camera? new [Re: mpgxsvcd]
      #5689329 - 02/19/13 05:24 PM

Chip size has absolutely nothing to do with recorded sky glow. Just because a chip is large and therefore takes in more sky does not mean the surface brightness increases. No matter the chip size, the surface brightness per unit area is identical.

If size had an impact on sky glow, then when looking up at the night sky one could visually darken skyglow by merely peering through a toilet paper tube. A smaller portion of your retina is now being illuminated by the sky. But does this darken the sky (and, by inference, allow fainter stars to be seen)? Only insofar as it might eliminate other annoying light sources. But fundamentally, there will be no change. And so it it with detectors.

---

The detection of compact sources such as the galaxies in Hickson 50 is affected to some degree by the same factors impacting on point sources. These galaxies have reasonably high central surface brightness, for which sky glow must be quite bad to completely dominate.

Many galaxies have a peak core brightness of at least 18 magnitudes per square arcsecond, and often enough brighter still. And some even a quite bad sky such as the 18 MPSAS found at full Moon, or over a large metropolis, will have an 18 MPSAS object appearing twice as bright as the sky (object and sky brightness add together). All that's requires is sufficient *focal length* to allow the tiny galaxy to at least fill one bunch of the 4 pixels making up the Bayer group, or one pixel on a monochrome camera. Of course, a larger image filling more pixels results in something resembling more than a point-like, dim star. Once the focal length requirement is met, more aperture so as to result in a faster f/ratio and hence higher surface brightness affords a shorter exposure duration.

There is no sky glow reduction occurring in the Mallincam, or any such device. The laws of optics and the normal acquisition of photons is all that's required to obtain images of faint galaxies under even notable sky glow (provided object surface brightness is high enough to result in sufficient contrast.)


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mattflastro
Vendor - Astrovideo Systems


Reged: 07/31/09

Loc: Brevard County , FL
Re: Which camera? new [Re: mpgxsvcd]
      #5689361 - 02/19/13 05:42 PM

Quote:

I am not sure if my light pollution is better or worse than anywhere else. I haven't really tested it at another light polluted place.

Again the only way to judge this is to stick these cameras on the same scope with the same sky and use the same filters and reducers. Simply put. The camera that displays the object best in the least amount of time is the winner. I think that camera will be the Mallincam but perhaps not by the margin some would expect.



Let's assume a simple hypothetical experiment to figure out what's going on.
- assume we have 2 identical cameras. Same sensitivity, same camera noise, same well depth , etc.
- use one and the same telescope with each cam to image one and the same faint target. We select the target to be at the threshold of detectability for the given scope and cameras. Let's say a signal to noise of 3 . Obviously both cams will see the target equally well (poorly but detectable).
- now we change the conditions when camera #1 is used, we use a x2 barlow . Let's think what happens to the signal, noise and signal to noise . Without the barlow, there was a certain number of photons illuminating one pixel of the camera. When the barlow is inserted, the same number fo photons is spread over 4 times the area. That means 4 pixels are receiving in total the number of photons that a single pixel was receiving without the barlow.
Each pixel receives 4 times fewer photons when using the barlow.
But because the signal to noise was 3 , now it is 3/4 .
The reality is that such a signal can NOT be seen or detected in a single frame, whereas without the barlow, it could be seen (albeit poorly).
- Question: because the camera is used at the wrong image scale, which results in the target signal being lowered below the noise floor and the target being no longer detectable , is the camera suddenly BAD ? Or just used poorly?
- now let' use the other camera and insert a .5x focal reducer . Suddenly the camera will see a 4x increase in the signal to noise . Image will be much cleaner than without the reducer.
- looking at both cameras, is one worse than the other ? Or is just the optical system not configured to use the camera(s) properly ?
Judging by the results, the same camera with a barlow has 16 times worse signal to noise and can only detect targets that are 16 times brighter than the same camera with the .5x reducer.
-now let's say we didn't use barlows and reducers with 2 identical cameras.
Let's say we had cameras with identical performance but different size pixels.
The effect is exactly the same .
Conclusion is the same , you need to match the pixel scale , as in having both cameras see the same number of sky arcsec/pixel. Otherwise, the camera with the fewer arcsec/pixel will see a lower signal thru the same scope from the same target. This will reduce the signal to possibly under the noise floor.
If you're comparing 2 cameras one with 14 square micron pixels and another with 85 square micron pixels , you need to give the pixels the same signal .
That means each camera needs to see the same image scale (arcsec/pixel) .
Since the 2 camera pixels are vastly different in size, you need to adapt each to the optical system by using adequate reducers or barlows to achieve the same pixel scale .
The above applies if you want to compare camera performance with respect to signal to noise and detectability threshold .
The requirement to use the same scope is fair.
Having the same filters is possibly/maybe fair , depending on the cameras spectal response differences .
The requirement to not adapt the cameras to the scope is not .


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mattflastro
Vendor - Astrovideo Systems


Reged: 07/31/09

Loc: Brevard County , FL
Re: Which camera? new [Re: mattflastro]
      #5689443 - 02/19/13 06:30 PM

In all the above images, the Hicksons were at the threshold of detectability . Just for fun I measured the max brightness in each photo compared to the surrounding background and the ratio is less than 2 to 1 (with the exception of a referenced image taken with a 25' dob and a Canon, which was 3:1) .
So if you do the slightest thing to reduce signal to noise a mere 30% further they simply vanish under the noise floor. No matter how long you integrate you will only see more and more skyglow and noise.
In your shooting conditions, with a pixel scale that yields pixel illumination reduced to 10%, there's no way to see them , unless you stack about 100 frames with your current setup and exposure time, or you use a .33 reducer.


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Dwight J
scholastic sledgehammer
*****

Reged: 05/14/09

Loc: Lethbridge, Alberta, Canada
Re: Which camera? new [Re: GlennLeDrew]
      #5689454 - 02/19/13 06:37 PM

Well Glen, my eyes must be deceiving me. I see objects with better contrast by increasing magnification, effectively seeing less sky. A smaller chip works the same way - the light is spread out so each pixel receives less. If I imaged a section of sky at F5 and then at F10 with the same exposure, the sky in the F5 image will be brighter.

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mattflastro
Vendor - Astrovideo Systems


Reged: 07/31/09

Loc: Brevard County , FL
Re: Which camera? new [Re: Dwight J]
      #5689515 - 02/19/13 07:09 PM

Quote:

Well Glen, my eyes must be deceiving me. I see objects with better contrast by increasing magnification, effectively seeing less sky. A smaller chip works the same way - the light is spread out so each pixel receives less. If I imaged a section of sky at F5 and then at F10 with the same exposure, the sky in the F5 image will be brighter.



The sky will be brighter in the F5 per unit of area. However, noise and signal to noise are per pixel, and if pixel sizes are different....


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GlennLeDrew
Postmaster
*****

Reged: 06/18/08

Loc: Ottawa, Ontario, Canada
Re: Which camera? new [Re: mattflastro]
      #5689888 - 02/19/13 10:39 PM

Dwight,
You're conflating what you observe in the visual, afocal configuration with what happens when placing a sensor at the focus.

At the eyepiece, increasing magnification shrinks the exit pupil, which darkens the sky glow, and any other extended objects. For stars, the contrast is definitely improved, for a point source--unless so magnified that it is seen as the Airy disk--retains brightness while the sky dims. Extended targets like galaxies *seem* to improve in contrast only because they are now seen as larger, allowing more detail to be seen.

In short, the eyepiece view of the sky is dimmed because of the reduced exit pupil, *not* the reduced area of sky seen. A small scope at a given exit pupil will show a larger piece of sky than a big scope at the same exit pupil. Yet the sky in both cases has identical brightness.

And what occurs at the eyepiece is not directly applicable to imaging.


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