Can one do Asteroid Orbit Determination with a Seestar S50?
First of all please excuse my poor English because it’s not my native language.
I want to discuss here about some intriguing possibilities coming from the astronomical pictures we can easily take with ZWO’s Seestar S50 Smartscope. The images coming from Seestar S50 can be used also for interesting astronomical data collection and analysis.
One interesting field of study are asteroids and their orbit determination. One skilled amateur astronomer can compute asteroid’s orbit by his own means, using free software or cheap shareware.
In short I’ll tell you about some quite good achievements in asteroid’s orbit calculus I’ve recently made with Seestar S50, after I already had good results with other (bigger) scopes and CCD cameras. We need also some smart software, largely free for amateur use.
How to start in practice?
All these things start from the collection of a small series of CCD images of one or more asteroids, each group of pictures taken one or few days apart from the others. A good approach to start these experiments of orbit calculations is to choose a small group of Main Asteroid Belt, which can be easily identified into each star field we take with the scope CCD.
A good span of time to follow this group of asteroids can be at least twelve/fifteen days long, possibly avoiding the days near full Moon where its great luminosity can be an obstacle to collect good contrast-rich star field images.
It’s not necessary to take asteroid’s pictures every day: you can leave one, two or more days in between each imaging session. During one session is a good practice to collect at least two/three stacked images of the same asteroid, which reasonably can be separated of fifteen/twenty minutes each other.
In my experience during a total of 5 – 6 sessions in a 15-20 days period you can collect a list of 12-16 referenced RA/DEC measures for each asteroid that allow a good orbit calculus! See one of my data files to have an example for this.
Which asteroids can be imaged?
In my experience with the Seestar S50 I’ve found that this scope and his camera can easily follow asteroids up to magnitude 14,5 – 15,0, considering that I’ll limit single stacking sequence max 1,5 to 2 minutes, thus avoiding an excess of ‘moving’ effect for the asteroid through the reference star field (10 second was the imaging time for one single shot)
In practice this moving effect will be greatly dependant from the relative distance of the asteroid from Earth: in our simple approach I’ve been tracking asteroids belonging to the Main Asteroid Belt, which are relatively ‘slow’ in their relative movement toward the star field. Summarising these considerations I usually stack from 9 to 12 single ten seconds exposures equivalent to 90-120 seconds of total exposure for each frame I want to analyse.
If you want to consider also photometry data of the asteroid, avoid to over-expose your images, especially with brighter objects.
This is an example of a ‘raw’ image of Ceres(1) taken with Seestar S50: imaging field is 43.1’ x 76.6’, North is UP, 2.4 arcsec/pixel scale: is recommended to have Seestar S50 in EQ (Equatorial) mode
📷 Stacked_9_ceres_10.0s_IRCUT_20260121-184513_LUM_inv.jpg
It’s a Luminance of a single stack of 9×10sec. frames, b/w inverted. The labels are by Astrometrica, which can platesolve and automatically overlay the known objects on your image.
The numbers in brackets are the magnitudes: obviously you cant’t detect objects above mag. 14,5 / 15 .
But How can one find an asteroid?
If you’re not a new asteroid hunter (!), you must already know the ephemeris for the asteroid you want to track, but today almost everyone of us use a decent planetary software which can produce these ephemeris tables, or you can easily find online sites that do very well the same job. An ephemeris table for the asteroid Ceres (1), referenced at midnight U.T of each day can look like this:
-------------------
#(247) Roving observer: (1) CERES
Date (UTC) RA Dec
——————- ————-- —————-
2026 02 02 01 06 53.950 -00 28 58.41
2026 02 03 01 07 58.172 -00 19 03.83
2026 02 04 01 09 02.967 -00 09 08.24
2026 02 05 01 10 08.327 +00 00 48.32
2026 02 06 01 11 14.246 +00 10 45.79
2026 02 07 01 12 20.715 +00 20 44.12
2026 02 08 01 13 27.730 +00 30 43.27
2026 02 09 01 14 35.283 +00 40 43.17
2026 02 10 01 15 43.367 +00 50 43.77
2026 02 11 01 16 51.975 +01 00 45.02
-------------------
In the Seestar APP there isn’t a list of ‘asteroids’ and so you have to create, for each day of imaging, an appropriate “new custom object” and add it in your “My Favourites” object’s list. Please refer to the appropriate Seestar tutorial to know all the details for this operation. It’s quite simple. Remember that the RA and DEC parameters you give to new asteroid object must be derived through a planetary software and they change each day: at the scale of our imaging device and for the Main Asteroid Belt components the same AR/DEC ephemeris data usually remain valid for few hours near their reference date/time but this is enough for easily identify the asteroid in its star field during a whole nightly observing session.
So, after at least a couple of weeks you’d have a set of images with different star fields taken on different nights but containing the same asteroid, which is moving through them night after night.
Here comes the processing time….
The nice and professional software “Astrometrica” by Herbert Raab, which is a cheap shareware that gives you the possibility to test it for free, gives a great help with the automated search for the asteroid in the CCD frame and with calculus of it precise referenced position (plate-solving). It also will produce an astrometric report in ADES format, which is the right format to use for orbit determinations.
I can’t here explain in detail how to use Astrometrica: a simple ‘Copy and Paste’ of my settings for the S50 is attached here, showing the parameters and solutions I’ve successfully adopted to process Seestar S50 pictures with Astrometrica: please remember you have to change your latitude /longitude settings according to your observing position. I hope this can help to start with Astrometrica. (http://www.astrometrica.at)
📷 Astrometrica settings 01.jpg
Astrometrica can use several star catalogues for its astrometric calculations and this software has also very interesting features to easily identify asteroids/comets inside a picture. I can’t here make a summary of its use: if you want a hint to all the process of astrometry calculus and ADES report writing please ask me or read some tutorials concerning Astrometrica.
📷 Astrometrica data reduction 3.jpg
If you want to get astrometry data from your images with other free software you can give a try to ‘ASTAP’ (https://www.hnsky.org/astap.htm) which can easily ‘plate-solve’ the ‘fit’ images taken with the Seestar. Then you have to manually pick up the right time and astrometry data from the annotated picture and correctly set them into an ADES formatted file. A little more tricky but you can use it anyway.
Finally: Asteroid Orbit Determination
A good choice to calculate Orbital Elements from a given set of topocentric measures is Project Pluto’s Find_Orb software.
This nice and professional tool from Bill Gray, offered free to all the astro-amateur community (there is also an online version), takes an ADES file as input, for example with one asteroid/several measurements lines: if these measures are in a sufficient number and taken over a convenient span of time, this software computes the orbital elements directly from them, using several methods of orbit determination and performing refinements and error calculations.
There are quite good tutorials for this on his web page (https://www.projectpluto.com/).
As for Astrometrica I’ve attached a picture where are summarised some important functions of Find_Orb.
📷 Find_Orb results.jpg
Which software finally shows my orbit?
Using a free software like Celestia (https://celestiaproject.space ) you can easily draw and compare your new calculated orbit path with the one coming from official sources: see MinorPlanetCenter site for example or simply examine and retrieve the asteroid’s Keplerian elements through a common planetary software like Guide9 or C2A. Both these software use MPCORB.DAT as source of data.
In my case I’ve used Celestia by modding the “asteroids.ssc” file in the program’s data directory (remember to backup original file and use “administrator” privilege to mod this text file inside windows program directory)
In practice I duplicate an existing asteroid official dataset (e.g. Ceres) giving a fantasy name (MyCeres) and changing its progressive number so that it isn’t the same of the ‘original’ one.
Then I manually insert/substitute all the Keplerian elements (aka osculating elements….) like semimajor axis ‘a’, inclination ‘i’, eccentricity ‘e’ with those calculated through ProjectPluto FindOrb obtained by reducing data coming from astrometry taken during the Seestar S50 photo sessions.
The result is that the two orbits, the one ‘original’ from MinorPlanetCenter data and the one calculated with my data, almost perfectly superimpose, at least at this large scale: the original is the blue one and the derived one is red: the two colours lines are closely drawn, masking each other.
This result is anyway impressive, just considering the use of this small Seestar S50 and the relatively short timespan (about 16 days) in which CCD pictures were taken.
So, clear skies and good asteroid’s hunting with Seestar S50!
📷 Celestia MyCeres Orbit.jpg
Marco