71 replies to this topic

[Random2310]

Hello,

I am interested in minor body astrometry and I was wondering if I could get some guidance on which telescope characteristics are important for this kind of observing?

I read the Guide to Minor Body Astrometry and they mention the following guidelines with respect to equipment:
 

 

"Your setup should be such that the pixel scale is no greater than 2"/pixel (preferably) or 3"/pixel (at worst)."

I also saw this excellent Youtube video posted by The Astro Imaging Channel, "Getting Started in Astrometry: Chasing Asteroids and Comets" presented by Basil Rowe. He uses a C14 EdgeHD with a 0.7x Reducer. He recommends a scope that can get to 1 arcsecond per pixel and no more than 2 arcseconds per pixel.

Other than these pixel scale characteristics, what other telescope characteristics are important for minor body astrometry?

• Is it important to have a flat field?
  • Would it be better to get an RC vs. an SCT?
  • Considering SCTs of the same aperture, is it worth the extra investment in a Celestron EdgeHD SCT compared to non-EdgeHD SCTs?
• Is it important to have a "fast" telescope with a low focal ratio?
   
• If neither getting a flat field or low focal ratio are important, would you focus on maximizing the limiting magnitude of your setup? 

 

There are tradeoffs with every telescope design and I just want try and get a sense of what scope characteristics are worth prioritizing and investing in and which ones I can ignore.

I understand that mount and camera quality and having a very accurate clock play an important role in producing good observations but I just want to focus on telescope characteristics in this thread.

Edited by Random2310, 30 November 2024 - 12:35 AM.

[Octans]

Depends on the kinds of objects you're looking to observe. For slow moving things far from the Sun, a larger aperture will be most beneficial in increasing the number objects you can observe, and you can usually get enough stars for astrometry within even a couple arcminutes of anywhere in the sky with the long exposures used for these objects. For fast moving things, especially NEOs with uncertain orbits, you'll need a wider field of view to boost your chance of capturing it within your field. A wide field is also important for observing things in twilight (especially comets) as you otherwise may not have enough visible stars to do astrometry.

[pvdv]

The Basil Rowe video is indeed excellent. In addition to the software he mentions, you may also want to have a look at the more recent TychoTracker https://www.tycho-tracker.com/ (personal opinion: Tycho Tracker is what happens when astronomy software gets written by someone who really can code )

Flat field as in calibration flat fields, yes, of course. Flat field as in "an instrument that provides a flatter field" not essential imho. I have such an instrument (see my sig) but the improvements in plate-solving and distortion coefficients have made the improved field a bit obsolete imho). What you want is get to the faint object as quickly as possible, have the least trailing  possible on fast moving objects (aperture), decent pixel scale, and enough field not to be bothered by tricky solves. 

The requirements are somewhat conflicting of course, but that depends on what you want to do. To recover uncertain objects and do precise photometry, one ideal amateur setup would be RASA36 (recovery, possible detection) and C14 (photometry).

IMHO, there is no "fast by F/D" scope for that purpose. At equivalent aperture; "fast" = poor pixel scale - aperture is what you need because that is the main factor in the numbers of photons you collect.

In a way, I have the poor man's version with the FFC and the ASA12.

I'd go EdgeHD but that is a personal opinion.

[SeymoreStars]

Excellent video, I will persue this as a background task to my spectroscopy work.

[SeymoreStars]

A question for pvdv. The filter wheel of the telescope I would use for this pursuit, has a starAnalyser200 in it. Would this be of any use or would these objects just reflect sunlight?

 

Thanks

Steve

[pvdv]

I imagine you should be able to spot differences in relatively bright asteroids composition after substracting the solar spectra but this is totally outside of my (meager) expertise.
Fortunately, I suspect our resident expert will chime in.

[Octans]

You could, in principle, determine asteroid spectral types, although without a slit, you'd be limited to only the brightest asteroids for which spectral types are generally well established (and unlike variable stars, they generally don't change very much, so a bunch of extra spectra of bright asteroids taken with small telescopes tend to not be very scientifically useful). Asteroidal spectral features are also quite broad, so that's also typically easier to do with photometric filters, which will let you go much fainter and potentially reach asteroids that haven't already been classified by professional surveys.

[SeymoreStars]

Tonight should be a good night to take measurements. What filter if any should be used?

[Octans]

If you're observing fairly high up (say, >30 degrees or so above the horizon), you can usually get away with no filter. Unfiltered will usually be best for sensitivity when the sky is very dark. When there's a bright moon or twilight or significant light pollution, sensitivity may be better with a red/IR filter. You'll also need a narrower filter (red/IR is best) if you're observing closer to the horizon due to differential refraction, which will make stars appear at slightly different altitudes depending on their color (essentially stretching them into a short, vertically aligned spectrum), in which case, the relative centroid of an asteroid in an unfiltered image relative to those stars depends on its color relative to those stars, making accurate astrometry complicated.

[SeymoreStars]

Interesting development shooting with no filter. A reflection revealed a significant amount of crud on the CDK corrective lens.

 

After removing the focuser this is the image with a headlamp shining down from the secondary mirror.

Attached Thumbnails

• 

[robin_astro]

Fortunately, I suspect our resident expert will chime in.

Somebody call ?  

 

Yes you can use the Star Analyser on asteroids (Divide by the spectrum of a solar analogue star, taking into account any difference in extinction due to air mass and you get the reflectance spectrum and can categorise them.) They are rather featureless in the visible though, more interesting further into the IR. Unless they are too fast to track though your ALPY 600 with the slit  would probably work better.  Here is a video of an interesting fast moving NEO I recorded using a Star Analyser

https://www.youtube....=KXFmDSCA8OM

and the resulting spectrum compared with an Xc type reference

 

 

I don't know much about the subject though or how useful these measurements are. (It might be cool perhaps to take a time series with the Star Analyser if the reflectance spectrum of the object changes with orientation ?)

 

Cheers

Robin

[robin_astro]

 Here is a video of an interesting fast moving NEO I recorded using a Star Analyser

https://www.youtube....=KXFmDSCA8OM

and the resulting spectrum compared with an Xc type reference

 

asteroid_cleaned.png

 

 

More about that observation on the ARAS forum

http://www.spectro-a...c.php?f=6&t=550

(Unfortunately Christian Buil's observation of the same object using a slit spectrograph mentioned there is missing, I think he embargoed it pending inclusion in a professional publication though I dont know if it was ever used)

 

Cheers

Robin

 

Cheers

Robin

[Xilman]

Hello,

I am interested in minor body astrometry and I was wondering if I could get some guidance on which telescope characteristics are important for this kind of observing?

I read the Guide to Minor Body Astrometry and they mention the following guidelines with respect to equipment:
 

I also saw this excellent Youtube video posted by The Astro Imaging Channel, "Getting Started in Astrometry: Chasing Asteroids and Comets" presented by Basil Rowe. He uses a C14 EdgeHD with a 0.7x Reducer. He recommends a scope that can get to 1 arcsecond per pixel and no more than 2 arcseconds per pixel.

Other than these pixel scale characteristics, what other telescope characteristics are important for minor body astrometry?

• Is it important to have a flat field?
  • Would it be better to get an RC vs. an SCT?
  • Considering SCTs of the same aperture, is it worth the extra investment in a Celestron EdgeHD SCT compared to non-EdgeHD SCTs?
• Is it important to have a "fast" telescope with a low focal ratio?
   
• If neither getting a flat field or low focal ratio are important, would you focus on maximizing the limiting magnitude of your setup? 

 

There are tradeoffs with every telescope design and I just want try and get a sense of what scope characteristics are worth prioritizing and investing in and which ones I can ignore.

I understand that mount and camera quality and having a very accurate clock play an important role in producing good observations but I just want to focus on telescope characteristics in this thread.

Along side the plate scale requirement, which depends quite a bit on your seeing, I would argue for aperture. Crudely put, if you can't see it you can't measure it.

 

As for plate scale, my recommendation is to go for a seeing disk at least three pixels across. If your typical seeing is 2 arcsec, you definitely need sub-arcsec pixels.

 

A major interest is asteroidal photometry and I use 0.6 or 0.9 arcsec pixels (2x or 3x binning) on a 0.4m scope. Photometry down to 18th magnitude and astrometry down to 19th is possible for fairly slow moving targets - less than 10 arcsec/hour say. Astrometry below 21.5 is possible with subs stacked on a predicted motion with a total exposure of an hour or few. The orbital motions of many objects are tolerably well known even if their position is uncertain at the 0.1 arcsec level.

 

Recommendation: if you take the images you may as well measure brightness as well as positions. Twice as many bangs for your bucks. I and a number of others here can provide further information on request.

 

Cheers,

Paul

[Random2310]

A major interest is asteroidal photometry and I use 0.6 or 0.9 arcsec pixels (2x or 3x binning) on a 0.4m scope.

Thanks Paul, I appreciate your insight. I was just curious what make and model of telescope you use?

[Xilman]

Thanks Paul, I appreciate your insight. I was just curious what make and model of telescope you use?

A 0.4m Dilworth (roughly speaking, a Cassegrain analogue with all optical elements having spherical surfaces). Careful design and positioning of achromatic transfer lenses correct all major aberrations remarkably well, at a very slight light loss and, of course, expense, complexity and difficulty of callibration. An optically flat window (i.e. an element with two more spherical surfaces, these having infinite curvature) serves the dual purpose of holding the secondary mirror in place and keeping out detritus common to a telescope located in La Palma such as dust, spiders, cobwebs, lizards and gecko droppings. It was made by Larissa Precision Optics, a small Swiss firm which specialises in custom optics.

 

More details are available at www.astropalma.com and links therein.
 

Please consider joining a team which performs asteroidal photometry with the aim of determining their rotational periods and surface properties. I contribute to a team with members with longitudes ranging from Abu Dhabi, through all of Europe, me in the Canaries, many sites in the continental US, Hawaii, New Zealand, China and, it is to be hoped, Japan in the very near future. We need only observers in Bangladesh / India / Sri Lanka / Pakistan to join us and we can observe asteroids in a night sky 24 hours a day and thereby obtain overlapping light curves.

 

This team is co-ordinated by the BAA-ARPS (British Astronomical Association - Asteroids and Remote Planets Section), a second team of which I am a member monitors exoplanet transits. Exoplanets are about as remote planets as we are likely to be able to study - right? - though  the area of study also includes dwarf planets, centaurs, TNOs, KBOs and other such members of the outer solar system zoo. https://britastro.or...tions/asteroids for more detail.

 

Actually, although not strictly speaking asteroids, many members of that zoo will make excellent targets for your astrometry work. I commend it to your attention because very few amateurs are working in that area. Not many probes are in orbit around those objects at present - zero to be precise - so any astrometry has to be from the ground (*) and this is an area where amateurs like you and me can make valuable contributions.

 

Cheers,

Paul

 

 

(*) Apart from the likes of Hubble and JWST, each of which spend all their time on other activities so they don't count for present purposes. GAIA has also done excellent work in that area, as a by-product of its primary raison d'etre almost, but it reaches its end of life early next year and measurements will still be needed going forward.

Edited by Xilman, 08 December 2024 - 05:07 PM.

[StupendousMan]

I'll second Xilman's call for members of this forum to consider joining the efforts of those who study asteroids.  There really is a lot of good work that the professionals just can't do.

[SeymoreStars]

The equipment problem has been fixed and I want to move on with contributing to the MPC.

 

Here's an image taken last night and calibrated with pixInsight, 10-60 second images.
There was a bright star off frame (middle upper) please ignore the reflection.
With no filter the CDK17 has a bright central reflection which is nearly impossible to mitigate.
Filters are available (Lum, B, V, R, I)

 

ASTAP showed the limiting magnitude = 19.49, SNR=7 aperture=7.51

 

Skies won't clear again for a few days. If the more experienced people would make some suggestions
about locating 7 numbered minor planets, of which at least one must be a Near-Earth Object, that
would be appreciated.

 

My location is the Northeast US.

 

Thanks
Steve Hoffman

 

Attached Thumbnails

• 
• 

[pvdv]

Steve, 

Have you considered Tycho Tracker? 

Here is C/2024 A2 (currently at mag 19.8 - https://astro.vanbui...nl/comet/2024A2) recovered blindly from 24 30 seconds exposures on the N12
 

 

This, from an awful uncalibrated test field I took for another thread (someone was complaining of a "hard stop" in the full frame field of his ASA N12, I suspect it was only an autostretch artefact)

 

Note: unless you are 100% sure you are applying a totally standard calibration, avoid Pixinsight for calibration.

 

[SeymoreStars]

Steve, 

Have you considered Tycho Tracker? 

Here is C/2024 A2 (currently at mag 19.8 - https://astro.vanbui...nl/comet/2024A2) recovered blindly from 24 30 seconds exposures on the N12
 

 

 

 

This, from an awful uncalibrated test field I took for another thread (someone was complaining of a "hard stop" in the full frame field of his ASA N12, I suspect it was only an autostretch artefact)

 

Note: unless you are 100% sure you are applying a totally standard calibration, avoid Pixinsight for calibration.

Thanks pvdv, I will install Tycho Tracker tonight and give it go, thanks!

 

Steve

[SeymoreStars]

pvdv do you recommend the Pro or Standard license?

[pvdv]

I have the Pro license. It is really affordable and amazingly well programmed. 
There are tons of tutorials by the author himself on youtube. 
And, in hands more competent (and with better skies) than mine it has already allowed very significant amateur discoveries.

Edit: fwiw, i think the minimum number of frames it requires is 11

Edited by pvdv, 27 December 2024 - 04:35 PM.

[StupendousMan]

If you're looking for some asteroids to observe, you might consider the ones generated on this website; they are in need of measurements to pin down their orbital parameters.

 

    https://asteroid.lowell.edu/critlists/

[SeymoreStars]

Interesting tidbit...PixInsight has the capability to render known asteroids, if you download the asteroid files from the software distribution page and add them to the Render process. 

 

This video explains the whole procedure - https://www.youtube....-iKRhtgWpw&t=3s

 

After following the instructions 7 asteroids were in the above image; here's the rendered image. Field of View (0.7° x 0.47°)

 

Note: only one asteroid had a magnitude below 19.

Attached Thumbnails

• 

Edited by SeymoreStars, 27 December 2024 - 11:11 PM.

[RedLionNJ]

 

ASTAP showed the limiting magnitude = 19.49, SNR=7 aperture=7.51

 

Skies won't clear again for a few days. If the more experienced people would make some suggestions
about locating 7 numbered minor planets, of which at least one must be a Near-Earth Object, that
would be appreciated.

 

My location is the Northeast US.

 

 

You can kill a lot of birds with a small number of stones, here.

 

It would be beneficial to discover how faint you can (reliably and repeatably) go with a strong enough SNR to get good astrometry.

"Interesting" minor bodies tend to be 18th magnitude and fainter, with the exception of the occasional body passing closer to Earth (I'm deliberately not using the term "NEO" because NEOs, despite the name, don't necessarily pass close to Earth, the only criteria to such a label being the ability to pass within 0.3 AU of Earth's orbit, not even necessarily in the same plane).

So I would generate a list of, say, ten objects in the early evening SE skies, then take three consecutive 60s exposures of each field. Make sure at least one of the objects is classified as a NEO. If possible, verify you actually managed to capture each object before moving on to the next one.

Wait a couple hours then repeat the same sequences for the same objects. They should all be nearer the meridian by this time.

Then wait another couple hours (from the start time of the second set) and capture a third set.  You should have all the data you need.

 

An alternative way to capture would be to only work in the early evenings and capture the three sequences on different dates, but not too far apart (a few days, at most).  That aspect is entirely up to you. The point is that the MPC only accepts (with a few rare exceptions) sets of at least three astrometric positions per body.

 

So when you calibrate and measure (using Tycho Tracker, for example), a report is generated for submission containing at least three measurements for each body.

You can check the accuracy of your own measurements by cross-referencing to the astrometric positions generated by JPL Horizons for the same bodies at the same times. I recommend limiting residuals to the order of no more than 0.2 arcsec.

 

Once you're happy with the accuracy of your astrometry, submit your first report to the MPC (at least three measurements per body), receive the acknowledgement back and wait to see if all the measurements are accepted and an MPC code gets issued.

 

How do you generate a list of reasonable candidates for measurement? Well, I do it with a script from within TheSkyX. You can likely do it from almost any planetarium program. Limit your criteria to an altitude/azimuth (or altitude/hour-angle, or RA/Dec) range, a magnitude range of, say, 16-20 and a time range of about a half hour each side of your ideal imaging time.

 

I speak from personal experience, although it's been a few years now since I obtained my own observatory code.  You may find some variation on all this works better for you and is still accepted by the MPC.

 

Happy astrometry!

[RedLionNJ]

Interesting tidbit...PixInsight has the capability to render known asteroids, if you download the asteroid files from the software distribution page and add them to the Render process. 

 

 

Note: only one asteroid had a magnitude below 19.

Like the other fellow above, I can't recommend PixInsight for astrometry. Right tools for the job, etc.  The #1 piece of software used to be Charon, then it was superseded by Astrometrica. These days, it's very much Tycho Tracker.