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Asteroid observing in light pollution with color camera

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#1 martinrw

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Posted 05 December 2021 - 09:01 PM

I am a newbie and just now gathering equipment and software to do asteroid astrometry. I live in a light polluted area and need to find something to reduce effects of light pollution. I have read info on several light pollution filters but I know the filter will reduce the light from an asteroid also. What can I do?

 

Also I got a good deal from a friend on a nice astrocamera, the ZWO ASI294MC-Pro. Problem is it is a color camera and nothing I have read in Astrometrica software mentions any processing for color images. Is there some preprocessing that I must do?

 

I need any help.



#2 ted_barnes

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Posted 05 December 2021 - 09:58 PM

Hello martinrw,

 

I have been pursuing faint asteroid astrophotography as a fun sideline for several years now, and will be interested in seeing what recommendations people have for this topic. I use the same model ZWO camera you mentioned, which I have had for over 3 years. In my experience, bringing out the faintest asteroids requires that you concentrate the light in the smallest possible area, like 2x2 pixels, so you need good optics, good seeing and sharp focus. I use a 190mm Orion Mak-Newt, which performs very well, and a CGX mount. Another problem is that with siderial tracking the asteroids trail visibly if the exposure is longer than a few minutes. So, my magnitude limit to date for main belt asteroids has been about 18.3. For "fixed" stars in comparison I have reached about 20.5. I use stacks for asteroids rather than single exposures because my guiding (PHD2) with long single exposures does not give me a tight enough image. Ideally one could use PHD2 comet tracking to follow an asteroid, but again I don't yet get tight enough images with this approach. I don't use any filters, I need all the photons I can get. I also find that my best faint images are a few days from new moon.

 

We are fortunate to have excellent databases, e.g. using TheSkyX and dnloading the Lowell Observatory file astorb.dat you get positions and predicted 24 hour paths of over a million asteroids, and can select the magnitude range of interest to you. The All Sky Plate Solver is very useful in finding the fainter ones.

 

As a random example I have attached a crop from an image of asteroid 4740 Veniamina, mag 16.3, d = 1.21 AU, just outside the orbit of Mars and currently in Orion left of Betelgeuse, near Monoceros. This is a stack of 240 4-sec frames (total exposure 16 mins), with gain 400 and the usual post processing, taken about 11 PM Eastern 3 Dec 2021; the trail center is at RA 06 12 51.1, Dec +09 13 05 (J2000.0), just as expected.

 

4740_Veniamina_240x4sG400_WDS_snip_crop_edit_jpg.JPG


Edited by ted_barnes, 05 December 2021 - 10:04 PM.

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#3 martinrw

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Posted 06 December 2021 - 10:50 PM

Ted Barnes - Thank you for replying to my questions. I am glad to have found someone experienced who uses the same equipment as I do. I looked at TheSkyX and the Lowell Observatory file astorb.dat that you recommended. They look like they have a lot of benefits over the Minor Planet Center recommendations. I will soon try them out. I need to learn all about these new tools you have suggested.

In regard to the color output of our cameras (ASI294MC-P) is there any color processing you used since Astrometrica seems to only process B&W images. Does it read color files and do we pay a  penalty in light sensitivity for the color? I was thinking the magnitudes of color versus B&W may be different when compared to the reference stars in the catalogs where we are trying to make a match. Is this a problem?

I admired your image of asteroid 4740 Veniamina that you posted. You must have great equipment and experience. I know you have a great observing site as I was at ONRL several times attending meetings. Your sky is very dark. I was with the University of Dayton Research Institute for 37 years where my last couple of years we teamed with ORNL to solve a corrosion problem for the Navy at Naval Research in Virginia. 



#4 555aaa

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Posted 07 December 2021 - 12:42 AM

You really want a mono camera. You lose too much light through the beyer matrix. It is also critical that you have time stamping accurate to better than one second because your best objects now are very fast moving, so your exposure times may be only a few seconds but you might stack 50 to 100 using a moving stack approach or synthetic tracking.
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#5 Tapio

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Posted 07 December 2021 - 01:07 AM

You really want a mono camera. You lose too much light through the beyer matrix. It is also critical that you have time stamping accurate to better than one second because your best objects now are very fast moving, so your exposure times may be only a few seconds but you might stack 50 to 100 using a moving stack approach or synthetic tracking.


Don't know what objects require few seconds because they move so fast. I understand the timing if you are doing occultation observing but asteroid astrometry or photometry doesn't require it.
With an color camera can't you use binning to mitigate the effects of bayer matrix?

#6 ted_barnes

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Posted 07 December 2021 - 12:10 PM

Ted Barnes - Thank you for replying to my questions. I am glad to have found someone experienced who uses the same equipment as I do. I looked at TheSkyX and the Lowell Observatory file astorb.dat that you recommended. They look like they have a lot of benefits over the Minor Planet Center recommendations. I will soon try them out. I need to learn all about these new tools you have suggested.

In regard to the color output of our cameras (ASI294MC-P) is there any color processing you used since Astrometrica seems to only process B&W images. Does it read color files and do we pay a  penalty in light sensitivity for the color? I was thinking the magnitudes of color versus B&W may be different when compared to the reference stars in the catalogs where we are trying to make a match. Is this a problem?

I admired your image of asteroid 4740 Veniamina that you posted. You must have great equipment and experience. I know you have a great observing site as I was at ONRL several times attending meetings. Your sky is very dark. I was with the University of Dayton Research Institute for 37 years where my last couple of years we teamed with ORNL to solve a corrosion problem for the Navy at Naval Research in Virginia. 

Hello martinrw,

 

Asteroids is/are a great topic, I hope you will enjoy their pursuit as much as I do. Their qualities vary widely, and there are over a million known.

 

I mainly use a ZWO color camera because colored star and nebula images are attractive, and I post my images on fb for my old HS friends. I see other people here have recommended a mono camera for improved access to faint objects. That's an interesting suggestion. I just save the ZWO ASI294MC-P images in SharpCap as jpgs for no good reason except file size, and do extreme contrast stretching to pull out faint objects using a generic free program, "Windows Live Photo Gallery," my only excuse being that it works for me. I don't know if I lose much sensitivity that way. My impression is that sharp focus matters most.

 

I'm glad you liked 4740 Veniamina. 16th magnitude is not difficult at all. Since you mentioned astrometry, note you can plate solve these images, and pin down the start and stop locations of the trail quite accurately. Comparisons with jpl ephemerides on ssd.jpl.nasa.gov allow a nice check of results.

 

Also intriguing is pursuing outer asteroids like Jupiter Trojans (often moderately bright), and TNOs and dwarf planets, which come in starting at about 17th mag, and are very exciting.

 

My current frontier is improving my guiding, and learning how to guide on the asteroid's predicted path rather than just following field stars.

 

n.b. Skies here near Oak Ridge are not nearly as dark as when I moved here 30 yrs ago. Knoxville and its light pollution are spreading. Then we had a galaxy; now we only have a few hints of one. Still, you can work on your techniques in preparation for access to darker skies.

 

Please keep me informed of your progress!

 

Ted
 


Edited by ted_barnes, 07 December 2021 - 12:52 PM.


#7 ted_barnes

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Posted 07 December 2021 - 12:40 PM

You really want a mono camera. You lose too much light through the beyer matrix. It is also critical that you have time stamping accurate to better than one second because your best objects now are very fast moving, so your exposure times may be only a few seconds but you might stack 50 to 100 using a moving stack approach or synthetic tracking.

Hello 555aaa,

 

What model mono cameras would you suggest, and what are their advantages and limitations?

 

Can you say more about moving stacks / synthetic tracking?

 

Thanks!

Ted Barnes



#8 martinrw

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Posted 07 December 2021 - 06:56 PM

Ted Barnes - Thank you for your additional suggestions and 555aaa suggestions also. I have a lot of learning to do. 

I have some wide field poor quality color star field images and I will try extreme contrast imaging on them very soon. Thanks again.

I will try to follow posts by both of you.



#9 ted_barnes

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Posted 07 December 2021 - 08:11 PM

Ted Barnes - Thank you for your additional suggestions and 555aaa suggestions also. I have a lot of learning to do. 

I have some wide field poor quality color star field images and I will try extreme contrast imaging on them very soon. Thanks again.

I will try to follow posts by both of you.

I usually post my astroimages on fb, at www.facebook.com/ted.barnes2 with open permission.


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#10 jgraham

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Posted 07 December 2021 - 08:52 PM

I have been using color cameras for photometry of variable stars for many years with excellent results. My camera set includes the ASI294MC Pro and it works fine. Color should have no effect on astrometry unless you are close to under sampling. If the color bothers the software all you need to do is to desaturate the image. Easy peasy.


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#11 martinrw

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Posted 08 December 2021 - 02:13 PM

jgraham I thank you for your post. Do you use photoshop to desaturate an image? Did you try using a light pollution filter? I am told that a filter wastes too much light? By the way I was a member of MVAS for 3 years but then I moved south to the Mason area 5 years ago where there is a lot of light in the sky. I wish now that I could do this at John Bryan.

Clear skies to you.



#12 555aaa

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Posted 09 December 2021 - 11:21 AM

Don't know what objects require few seconds because they move so fast. I understand the timing if you are doing occultation observing but asteroid astrometry or photometry doesn't require it.
With an color camera can't you use binning to mitigate the effects of bayer matrix?

Go to this page

 

https://www.minorpla...rm_tabular.html

 

This is the list of recently discovered asteroids which need immediate follow-up astrometry so that they won't be lost.

 

Now select the objects that are brighter than say 19th magnitude, and generate ephemeris for them.

 

When I did this, I get one object moving at 41 arc seconds per minute at 17.7 mag and one at 18th mag moving at about 300 arc seconds per minute (!).  Most objects on this list are pretty fast moving, several arc seconds per minute. And since you need to get a position accurate to a fraction of an arc second, it means very short exposures.



#13 555aaa

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Posted 09 December 2021 - 11:44 AM

Hello 555aaa,

 

What model mono cameras would you suggest, and what are their advantages and limitations?

 

Can you say more about moving stacks / synthetic tracking?

 

Thanks!

Ted Barnes

The most common software package for producing research-grade astrometry for asteroids is called Astrometrica. It has a feature called "track and stack" where you load a whole series of images (your stack) and if you know the position angle (PA) and speed of the asteroid (from earlier observations), then when Astrometrica stacks the images, it offsets each one based on the image time stamp and the PA and speed you entered into the program.

 

The other popular program is called Tycho tracker. It is a synthetic tracking program, meaning it can do a blind search for moving objects by generating a large number of postulated PA and speeds, performing the stack, and then looking for objects which are stationary in the stack. This is a common method used by professional observatories also. Tycho can also stack based on a manually entered PA and speed as I understand it. It was written by a fellow here who's handle is asmcoder8088.

 

I don't have a specific camera recommendation other than it needs these criteria (IMHO)

  • mono only (because it produces a more accurate centroid than a OSC camera)
  • CMOS (because they are fast)
  • Needs precise timestamp (which is usually a SW issue).

The reason I would avoid OSC is that you are trying to get a measurement accurate to a small fraction of an arc second. If you are imaging "normal" and reasonably bright asteroids, the residual fit to your positions should be around 0.3 arc seconds, which is much less than the size of your pixels typically. A good plate solve with a polynomial fit can have a RMS error as low as 0.07 arc seconds, with a mono camera. If you are producing images with residuals as large as an arc second, they are pretty much useless.  You can check your obit fit residuals by taking a set of same-night measurements of a known asteroid and pasting them into an orbit calculating program e.g. find_orb, which will calculate a set of possible orbits, pick the best one, and then tell you what is the error of your observation set to that orbit.

 

I have some ASI ZWO cameras which are very nice but when you use the ASIimg program, at least the version I have, the image time stamp can be off by six or seven seconds which is terrible. The way I found this out was by creating a high speed clock image on my display, and then attaching a video camera lens to the astro camera and taking fast images of the computer screen. Other image capture code that uses the ZWO camera driver doesn't have this problem, apparently. But in any case you have to verify that the capture time is accurate, and also in your computer you need to absolutely be sure you have precise timekeeping, which is easy with a GPS synchronizer or at minimum a time sync service, and you probably want to double check using shortwave radio.

 

An example of what can be done from light polluted skies, check out the Northolt branch observatories team

https://www.facebook...BObservatories/

 

I also recommend the Roger Dymock book "asteroids and how to observe them" ISBN-13 9781441964380

 

You can also do asteroid light curves using MPO Canopus or AstroimageJ but that is a different discipline.


Edited by 555aaa, 09 December 2021 - 11:56 AM.

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#14 jgraham

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Posted 09 December 2021 - 04:11 PM

Yes, I would use Photoshop to desaturate a color image since that is what I have. Many utilities can convert a color image to gray scale.

I use a light pollution for general imaging, but not photometry as it clips the green filter. I have also noticed a reduction in the effectiveness of my light pollution filter as we shift over to LED street lights. I'm going to stop using it. A light pollution filter shouldn't be all that useful for astrometry.

Yep, JB is still there, though I haven't used it in 30 years. I do all of my work from my Bortle 8 backyard. It's not dark, but it's home. :)

#15 ted_barnes

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Posted 12 December 2021 - 02:12 AM

The most common software package for producing research-grade astrometry for asteroids is called Astrometrica. It has a feature called "track and stack" where you load a whole series of images (your stack) and if you know the position angle (PA) and speed of the asteroid (from earlier observations), then when Astrometrica stacks the images, it offsets each one based on the image time stamp and the PA and speed you entered into the program.

 

The other popular program is called Tycho tracker. It is a synthetic tracking program, meaning it can do a blind search for moving objects by generating a large number of postulated PA and speeds, performing the stack, and then looking for objects which are stationary in the stack. This is a common method used by professional observatories also. Tycho can also stack based on a manually entered PA and speed as I understand it. It was written by a fellow here who's handle is asmcoder8088.

 

I don't have a specific camera recommendation other than it needs these criteria (IMHO)

  • mono only (because it produces a more accurate centroid than a OSC camera)
  • CMOS (because they are fast)
  • Needs precise timestamp (which is usually a SW issue).

The reason I would avoid OSC is that you are trying to get a measurement accurate to a small fraction of an arc second. If you are imaging "normal" and reasonably bright asteroids, the residual fit to your positions should be around 0.3 arc seconds, which is much less than the size of your pixels typically. A good plate solve with a polynomial fit can have a RMS error as low as 0.07 arc seconds, with a mono camera. If you are producing images with residuals as large as an arc second, they are pretty much useless.  You can check your obit fit residuals by taking a set of same-night measurements of a known asteroid and pasting them into an orbit calculating program e.g. find_orb, which will calculate a set of possible orbits, pick the best one, and then tell you what is the error of your observation set to that orbit.

 

I have some ASI ZWO cameras which are very nice but when you use the ASIimg program, at least the version I have, the image time stamp can be off by six or seven seconds which is terrible. The way I found this out was by creating a high speed clock image on my display, and then attaching a video camera lens to the astro camera and taking fast images of the computer screen. Other image capture code that uses the ZWO camera driver doesn't have this problem, apparently. But in any case you have to verify that the capture time is accurate, and also in your computer you need to absolutely be sure you have precise timekeeping, which is easy with a GPS synchronizer or at minimum a time sync service, and you probably want to double check using shortwave radio.

 

An example of what can be done from light polluted skies, check out the Northolt branch observatories team

https://www.facebook...BObservatories/

 

I also recommend the Roger Dymock book "asteroids and how to observe them" ISBN-13 9781441964380

 

You can also do asteroid light curves using MPO Canopus or AstroimageJ but that is a different discipline.

Hello 555aaa,

 

Astrometrica's "track and stack" option sounds like my dream program. Thanks, I will look into it. I have either been stacking (w/o tracking the asteroid), so I get a trail, or using Comet Tracking on PHD2, but that requires long single exposures which have light pollution problems.

 

I recalled that I also like color because light pollution tends to produce highly colored backgrounds, which can be ignored. Asteroids are usually a recognizable flesh-colored dot.

 

Ted Barnes



#16 Zapster73491

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Posted 11 March 2022 - 12:37 AM

Giving a little bump to this topic because I wanted to point out some of the many parameters that go into to getting the best sensitivity for minor planet detection. If you want to actually have a hope of discovering an asteroid these days with modest amateur equipment (14" aperture or less), then you need to maximize target photons. That means, no Bayer masks (i.e. color cameras). On a pixel by pixel basis, a Bayer mask kills more than 65% of your signal. That's like swapping an 11" aperture for 6.5" (35% of the photon collection area). You must use a mono camera.

 

You also want your mono camera (these days, almost everyone is using CMOS rather than CCD) to be back-illuminated and cooled in order to maximize quantum efficiency while maintaining low read noise and low dark current. Back-illuminated CMOS devices can have a quantum efficiency peak over 90%, compared to front-illuminated that rarely tops 60%.

 

Another factor is image scale: you want to marry the right focal plane to your optics (specifically the focal length) and your typical seeing. Critical sampling is about 2-3 samples of your FWHM in order to get good astrometry (positional accuracy) while not smearing your signal over too many pixels and thus losing sensitivity.

 

For instruments with apertures greater than about 5 inches (the only ones with any hope of discovering new asteroids), your asteroid and star FWHM sizes will be driven by your astronomical seeing and the quality of your tracking. With a good mount and good guiding, you're ultimately limited by your seeing. For example, if the best seeing you typically experience is 1.5", then you'd want to target an image scale of 0.5" to 0.75". Since you've always got the option to bin 2x2 or even 3x3, it's perhaps better to be optimistic about your seeing and lean toward the smaller image scale.

 

Here's an example. I've got a Celestron EdgeHD 11" telescope, and for reasons that will become apparent, I'm using a 0.7x focal reducer which drops the focal length from 2800mm down to about 1960mm. I went with ZWO's ASI 294MM Pro (mono camera) because of its high QE, low noise, good well depth, and most importantly a pixel pitch I could work with. Initially, camera users did not have access to its native resolution (8288 x 5644) but only its internally binned 2x2 data (4144 x 2822). The effective pixel pitch was then 4.63 microns rather than native 2.315 microns. So my 2x2 binned pixel scale is 4.63 microns / 1960mm = 2.362 microradians, or 0.487". That's perfect for 1-1.5" seeing. (Notice that without the focal reducer, my image scale would be 0.341", i.e. 3x oversampled for 1" seeing. I'd rather have a 43% larger FOV than be that oversampled even under great seeing.) On the extremely rare days where my seeing improves to 0.5", I can go to that camera's native resolution.

 

The final puzzle piece, and the main reason for my post, is exploiting the spectral content of my target, while (if possible) suppressing the wavelengths most impacted by light pollution. Asteroids tend to be "red" in that they have higher solar reflectivity from yellow to near-IR than they do in UV to green. Since there are significant light pollution sources at some of the shorter wavelengths (e.g. high- and low-pressure sodium vapor), it certainly makes sense to filter these out if possible. Unfortunately, LED streetlights are now also a big source of light pollution, and while they peak in the blue, they also have significant spectral content out to 650nm (red), but rapidly diminishing beyond 650nm.

 

It would seem that from light-polluted imaging sites, what would be optimum is a long-pass filter that cuts out everything below, say, 630nm, but passes everything else out to at least 1000 nm (1 micron). Astronomik's CLS filter is close to doing that: it has great transmission beyond 645 nm and notches out everything from about 540nm to 630nm, which covers the worst of the sodium vapor light pollution. But it has a second transmission window from 450-540nm which lets through the peak of the LED street light spectrum, where it might be better (from a signal-to-noise ratio perspective) to chop out *everything* below 640nm if asteroids are what you're after. I get the motivation for having a transmission notch there in the CLS filter: it lets through the blue-green hydrogen beta line and the green Oxygen III lines, which you certainly care about if you're doing DSO imaging. But if I'm chasing minor planets, I don't care about those.

 

So ... is there a filter out there that cuts everything below 630nm or 640nm, but is otherwise transparent out to 1 micron?


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#17 *skyguy*

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Posted 11 March 2022 - 12:08 PM

Giving a little bump to this topic because I wanted to point out some of the many parameters that go into to getting the best sensitivity for minor planet detection. If you want to actually have a hope of discovering an asteroid these days with modest amateur equipment (14" aperture or less), then you need to maximize target photons.

 

Great information ...

 

Besides discovering and reporting new asteroids, you might also want to submit your astrometric data on known and critical asteroids to the Minor Planet Center (MPC) to help refine their orbits. So, the image scale should be between 2-3 arc-seconds/pixel and this requirement could determine your decision on what equipment to use.

 

From the MPC:

 

"the pixel scale is no greater than 2"/pixel (preferably) or 3"/pixel (at worst). In practice, your optimal pixel scale is something that you will have to determine for yourself, taking into consideration the capabilities of your telescope and CCD and the seeing at your site. If your pixel scale is much larger than the values quoted above, then the quality of the astrometry will suffer. If your pixel scale is too low for your local setup, then the signal-to-noise of the images may be low as each image is spread over a large number of pixels."

 

I use a Meade 12" LX200 @ f/6.3 (1920mm) with an old SBIG ST9-E ccd camera (20x20 micron pixels). This set up gives me an image scale of 2.15 arc-seconds/pixel and I can get down to 19+ magnitude with a single one minute exposure. This set-up has worked well for me over the years with my submissions to the MPC.

 

Good Luck getting your equipment sorted out ....




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