SharpCap Forums

Hi everyone

I mostly look at asteroids for the MPC, and I would like to maximize my detection magnitude. So far with the C14 and RASA-11 I've been pretty much winging my exposure settings at gain of 102 (unity is listed as 100) and 2 min exposures x 40. I'd like to be a little more scientific with my settings.

Here are the specifics for the Observatory:

Celestron C14 with 0.7 Focal Reducer
ZWO 533 camera (color and mono)
Celestron CGX-L Mount
per astronomy tools I am B-4, I'll be measuring that tonight to get the true sky brightness reading.

I've run the sensor analysis on my camera as set up in the observatory (including light pollution filter) and I'll be running smart histogram tonight. I'm planning on using the smart histogram to try and adjust binning and exposure with a goal of Mag 20 (a man can dream) using the color camera. While I can get 20.5 with the mono 6200 on the RASA 11, I'd like to maximize the color camera settings on the C14 and use that as a starting point for trying to bump up the detection with the 533mm.

This is my first long focal length camera so binning and smart histogram are new concepts to me.

1/ I assume that since I unchecked 'binning analysis' when I did my sensor analysis SharpCap will either try different binning on the smart histogram or I just need to cycle through and see which one works the best (1x1 gives 0.25 arc sec per pixel). I seem to get the best images binned 3x3 or 4x4 with my mark one eyeball approach, but I'd like to test it to see which is best. I'm open to suggestions.

2/ It seems to me that the smart histogram is optimized for 'stationary' objects (DSO, Tracked Planets, etc). Asteroids move against the background. Are there any tips and tricks you would recommend above the standard smart histogram settings to try and pick up faint moving objects between the brighter, stationary background stars? For example should I take the original recommendations and optimize my asteroid detection by going for a longer exposure? In other words, if I run the smart histogram on that patch of sky, it will calculate brightness and make gain and exposure recommendations for the stars- say a gain of 235 and 60 sec exposure. Should I then set it for a 90 second exposure and have it recalculate he gain? Should I accept the resulting gain or bump it up to blow out the stars a little but increase the brightness of the asteroids to make them detectable?

Thank you in advance for giving me a little guidance.

Bob

Hi Bob,

an interesting question

The first thing to note is that the calculations that are being done by the Smart Histogram function assume that you will be stacking multiple images to bring out faint details. They are calculating the optimal gain/exposure for dividing up a fixed amount of sky time (say 60 minutes) into sub-exposures that are then going to be stacked together. Now, I'm wondering if you would actually be doing stacking in your quest for faint asteroids, since they would be moving but the stack would be aligned on the fixed stars, so that the asteroid would not be reinforced as the stacked images are adding as it is shifting position...

If the asteroid is moving very slowly (just a few pixels per hour) then I guess you would stack and the smart histogram results would be valid. For faster moving targets, you are right that you might have to put a limit on the exposure time based on the movement rate of the asteroid. As a rough rule of thumb, get the FWHM of stars in the image and then divide that by 2. If the asteroid is moving more than that many pixels per frame then it will start to streak out noticeably.

On the subject of binning, the binning analysis stage of the sensor analysis tool tries to work out if the camera bins by adding pixel values (images get brighter when binning) or averaging values (images stay the same brightness but noise level drops when binning). The assumption is for averaging binning if the measurement is not run as averaging is more common. I just tested with a ZWO camera and oddly it seems to use averaging for bin 2&3 but additive for bin 4, so a single measurement at bin2 isn't accurate anyway

There is no real benefit to using binning on a CMOS camera in terms of sensitivity to faint targets - unlike on CCD where binning is done by grouping together the charge collected by mulitple pixels before reading it out, CMOS always reads the values from individual pixels and the binning is performed by averaging or adding the digital pixel values after they have been read. So, you will get identical performance by capturing without binning and then using software later to 'bin' the image by reducing the resolution by a factor of 2, 3 or 4 in each direction. Binning does not change the exposure/gain recommendations for CMOS, so work with the recommendations for bin1 images even when using higher bin levels.

Hope this helps,

Robin

Thank you for the advice, let me review it a couple of times and see what I can do with the exposure time.

Bob

One concern I have (perhaps rightly so) is that faint asteroids need longer exposure times. Am I incorrect in this assumption? Would stacking 120 30 sec exposures bring out fainter asteroids than 30 2 min exposures?

Bob

Hello Bob,

For the asteroid ... it depends how you perform the stacking.
You can stack on the stars ... then the moving asteroid will be faint (or too faint).
You can stack on the asteroid ... not easily possible with a not known asteroid, but possible from the orbit parameters.

It is similar as doing the stacking on a moving comet (tool in SharpCap).

Concerning the noise ...
On a single image, you have the background noise + the noise of the faint asteroid.
On the stack, you have a less noisy background, but with still the noise of the faint asteroid (in the case that the individual image of the asteroid are separated).

I think that for the stacking ... a strategy would be to do both stacking ... on the stars and on the asteroid.
On the "stars stack" you can perform a radiometric analysis. And on the "asteroid stack", the same radiometric analysis can be performed.
For the astrometry ... I guess you can find all the information you need on the images.

At the end, you need some signal from the asteroid on each individual images.

If the asteroid is too faint ... then you need to go home or to change the guiding of the telescope.
I mean to perform the guiding on the asteroid movement by changing the RA-rate and DEC-rate movement.

Regards,
Jean-Francois