9 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.

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