View Full Version here: : Autoguiding on an asteroids motion
20-09-2010, 11:50 PM
So I want to take spectra of asteroids - but asteroids move. So I need to autoguide my scope - on a bright star - but have the guide star 'track' on the asteroids motion (make sense?)
The Asteroid itself will be too faint for my external guider/scope combination to directly guide on it.
Anyone know a piece of software that can do this? I know there are commands to send 'guiding signals to the scope to move it to match the asteroid but I'm looking for something that will do this same thing to the guide star tracking system.
My scope has to be guided at all times as it has 120 arcsec PE (yes a 14" LX200GPS Meade scope has this much bad tracking). I can get PE to fix it down to around 5 arcsec for a specific RA position but if I move the scope the PE shoots back up to 20 or 30 arcsec. Still not good enough to image without a guider.
21-09-2010, 01:45 AM
Being a man of many talents and soon to be one more, I'm sure you would be able to work out an asteroids offsets and direction. I believe that maximDL can handle these (in X and Y coordinates). I think it then guides and offsets the guide star to take into account the movement of the asteroid on the image. This would leave the asteroid as a point and the stars 'trailed'.
If the asteroid is so faint, it would require many resets as the guide star will drift accross and out of your guide field several times during the one spectral integration to build sufficient sig/noise ratio. Difficult, but I'm sure it can be done.
21-09-2010, 06:52 AM
Since Spectroscopy will be a manual operation I will have the option of choosing a guide star in an appropriate location on the guide camera (it's got an ST7 sized FOV and the motion should give me at least a couple of hours before I need to reset. I imagine then I would take a series of 'short' exposures and stack them. As a spectra, I imagine the stack would be a straight out overlay with no alignment required?
Guide star offsets - I'll have to look deeper into the manual for MaxIm.
21-09-2010, 10:18 AM
I have never used auto-guiding, but would it be possible to have a separate guide scope with the guider viewing the same object?
21-09-2010, 11:04 AM
For really bright objects - if my guider were aligned with the main scope - then yes. But since my guide scope is only 4" and it's not aligned with the main scopes FOV it's not an option.
21-09-2010, 11:25 AM
While typing this my mind went through 3 plausible solutions but the final more plausible would be to consider splitting the primary image on your imaging scope and use one as a guide.
This would though be based on whether or not the image can be split and whether the comet can be bright enough for the guide software to see it. I do know that there is some software that can split windows images into 2 streams. I do not know how effective it is but should be possible.
My 2 bobs worth
TheSky can set the tracking rate to follow an asteroid.
I would have thought that a guide scope should be able to pick up an asteroid and guide off it with a very long integration time.
Otherwise you might be able to work out a way to manually calculate the 2 necessary adjust numbers and key these into TheSky.
21-09-2010, 03:24 PM
Found this in the Maxim help file.
21-09-2010, 10:52 PM
Yes, the Tracking Offset function in MaxIm is the key. George from Cyanogen pointed out that there is also a third party app that builds the tracking offset file for you based on entered motion data. Just need a clear night to test it now.
22-09-2010, 01:39 AM
I don't think the problem will be the guiding off sets ( AA4 and other programs can guide on comets...maybe even PHD if the hourly motion is not too much)
The problem will be getting a bright enough image to guide ON.
As you say the guide scope may not be enough - I think you are considering objects around the 14-16 mag???
The split mirror idea (modified Vixen with a 80/20 split) costs about 0.5 mag to the spectrum and up to 2mags on the guide image so if you can see and guide on stars 2 mag fainter than the target object it could offer a solution. Otherwise an off axis guider with "off-set" guiding would be the way to go.
22-09-2010, 09:32 AM
What magnitude asteroids do you plan to try to get spectrums from?
22-09-2010, 11:57 AM
Guiding will be done on an actual star - not the asteroid. Tracking Offsets are then applied to that star and it's position in the guiding 'frame' is shifted by the offset - thus the scope tracks on the asteroid but guides on a star.
Although it would be nice to get mag 16 asteroids I'm really after those in the 12-14 range as these represent the binaries that I am nabbing (H<12, at opposition) and I am really only after enough signal to determine a taxonomic classification.
22-09-2010, 12:21 PM
With my 200mm scope I can get a reasonable spectrum with the slitless SA grating down to about mag 13 with ~1 hour of total exposure. With my new spectrograph I haven't been able to test yet due to cloud but I doubt I will be able to measure anything below mag 10.
A bigger scope would help a lot but would need to be substantially bigger to measure mag 16 spectrum. I think in the 1m range.
22-09-2010, 12:35 PM
Would 'slitless' effectively mean opening up the slip to allow all the asteroids light through. Is this possible on the L200? BUT would there be enough detail in the resulting spectra to do a taxonomic classification? I still have a lot more to learn :)
22-09-2010, 03:00 PM
When the slit width is greater than the target star size the spectroscope effectively becomes "Slitless"
The Spectra-L200 adjustable slit can be set from 15micron to 500 micron.
22-09-2010, 06:08 PM
Yes, but the problem is still the dispersion. With my setup the SA gives ~30A/pixel and the Spectra L200 gives 1.6A/pixel. This means the same light is spread out over a much greater area. Maybe using the L200 with a lower res grating like a 200l/mm rather than the 600l/mm that it comes with would be a reasonable compromise.
22-09-2010, 06:13 PM
You're correct about the dispersion factor....
I've already suggested a 300 l/mm grating to David.
The results can clearly be seen when the numbers are plugged into the SimSpecV3.2a_L200 spreadsheet.
22-09-2010, 10:28 PM
Yes, things look potentially promising based on the results from the spreadsheet.
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