Correct gain setting for ASI-120MM Guide camera

tarbat

I use a ZWO Mini Guide Scope and a ZWO ASI-120MM camera, and I’m trying to work out the optimum camera settings to use in ASIair. Advice I’ve received about my harmonic mount (HEM15) is to use a 0.5s exposure, but I’m struggling to understand what Gain works best with a 0.5s exposure. How do I determine the correct gain setting to use?

artem

tarbat
From my point of view, there is actually no correct or a fix gain settings, since it all depends on your location and how dark your sky is, and also if you have/find enough stars at your location with 0.5 seconds exposure with your ZWO Mini Guide Scope and a ZWO ASI-120MM..
Where I am, and I always use the high gain in ASIAIR to find enough stars for guiding to work, since the sky here is light poluted and with short expsure and low gain, not enough stars are found, but with higher gain everything works great, even when using a filter and I am using an ZWO Mini Guide Scope and a ZWO ASI-290 Mini!
Good luck
Martin

w7ay

tarbat I’ve received about my harmonic mount (HEM15) is to use a 0.5s exposure, but I’m struggling to understand what Gain works best with a 0.5s exposure.

If you plan to use 1 second or faster autoguiding frame rates, then you must use multi-star guiding. 1 second and shorter will give pretty poor results from single star guiding because of atmospheric turbulence (seeing).
multi-star guiding works by using ensemble averages of the centroid of multiple stars to make it equivalent to using longer exposure times. Each star is affected differently by the turbulence.
If all else is equal (all stars have the same SNR), then using two stars at 0.5 sec would be equivalent to using 1 second exposure for a single star. Using 4 stars would be equivalent to using 2 second exposure, etc.
for an average night and location, you normally need 2 to 3 second exposure at least, for single star guiding. That means that if you are going to attempt 0.5 sec guiding, you will need at least 4 to 6 stars, if they are all equally bright.
since the guide stars are not all equally bright, you will need more than 10 or 12 stars to properly take advantage of multistar guiding (see my earlier posts on this forum on the brightness distribution of stars). The FOV of the guide scope must be wide enough, and the camera sensitivity must be good enough to achieve this.

While the other posters are trying to be kind, I will be more blunt -- I do not think that your guide setup is able to achieve this consistently, except for dense portions of the Milky Way. Both the ZWO guide scope and the old 120 camera are hopeless to achieve 12 good stars. On top of that, the optics of the ZWO scope is really poor, and you are likely to get uneven aberrations (which makes centroid averaging non-functional) across the field of 12 stars. You need a wide, noise free, flat field for the math of multi-star guiding to work, and ZWO glass and a small noisy camera makes a hopeless combination -- albeit that combination is good enough for single star guiding. Wide means a larger sensor, and flat means a better guide scope. Noise free means a better camera -- camera noise can be more destructive than atmospheric turbulence.

Chen

Kevin_A

With short exposures you need to use a higher gain with fast 0.5s guiding. Your mount may be good at 1s too so don’t rule that out. The 120mm mini is not fabulous compared to the 220mm mini but will get the job done. Its limitation is it is not a useful very high gain camera so high gain is 48 in asiair. With a big aperture guide scope gain 48 will give you the best SNR with very short guide exposures. If you use a tiny 120mm guide scope you may need to use a bit more gain (maybe 60) to get more visible and brighter stars. I would not rule out 1s guiding as if you got a good mount it will be ok with the longer guide exposure. I would use a minimum 50mm aperture guide scope to let in more light with a 120mm mini.

tarbat

I guess I have a lot to learn about guiding. Are there any good articles out there explaining how to optimise guiding success in ASIair?

Firstly, how do I know if I'm using multi-star guiding? Is there a setting somewhere to turn that on? All I do is follow the on-screen instructions, and it shows circles around a load of stars, with one star having a box around it and lines going through it. Is that multi-star, or single star? How do I get a box and lines going through it on multiple stars? What does single-star guiding look like on screen vs multi-star guiding?

And how do I decide what exposure length to use? I was told that for my mount, an iOptron HEM15 hybrid strain wave drive, that 0.5 seconds was the best setting. Is that correct, or should I instead use a longer exposure?

For info, I'm in a Bortle 4 zone.

w7ay

tarbat Is there a setting somewhere to turn that on?

After you tap the "start" icon (the two semicircles chasing themselves) you should see stars appearing on the guide window.

If you pick a star (by tapping on it) before starting guiding (the target-like icon), you will be guiding on that star. If you don't pick any star before starting guiding, ASIAIR will use multi star, like the screenshot that you have just posted.

I too use a couple of strain wave geared mounts (Korean) and with the current version of ASIAIR, using 0.5 second frame rates, I get 0.35" total RMS error (as long as there is no wind) by using a moderately small guide scope (Askar FMA180-pro) and an ASI178MM guide camera with ZWO gain set to 270 (27 dB). A little better with a Borg 55FL with the TS Optics 1.0x flattener) especially when seeing is not as good. I typically get 12 stars by the time I use a gain of "150," and the extra 10 dB of gain is to saturate the brightest stars and force ASIAIR to reject them (read my earlier postings on star SNR distribution why you want to ignore bright stars when doing multi-star guiding).

By the way, if your polar alignment is good, the declination graph (the red line) should show almost no error, and if you are close to the pole (e.g., near M81+M82), the RA graph (the blue one) should also quiet down a lot. If the graph does not quiet down, the error is coming from ASIAIR's multi-star centroid averaging computation from the guide frames that it is given. A good guide scope/camera set up should get it down to less than 0.2" RMS for each (declination, and for RA near the pole) -- good guide algorithms can do better, but you get what you get with ASIAIR.

Chen

tarbat

Thanks for all the help. So it looks like I am using multi-star guiding. As for the number of guide stars, is there an optimum number of stars that I’d want ASIair to use? At my next session I’ll experiment more with 1s exposures and different gains.

w7ay

tarbat As for the number of guide stars, is there an optimum number of stars that I’d want ASIair to use?

Did you even read what I posted in post number 4?! And again in my previous post? I am outa here.

Chen

tarbat

w7ay

Of course I read your posts. Sorry, but I am very new to guiding, I've only had two sessions so far to try and learn the best approach.

I'm just not sure what the minimum and/or optimum number of stars is when multi-star guiding? Is it 4 to 6 stars or more than 10 or 12? Is that a minimum number of stars needed for multi-star guiding? And if I can't get that many stars, am I better off using single-star guiding?

I've searched extensively for a guide to guiding with ASIair, but can't find anything that explains the interaction between exposure, gain, multi-star vs single-star, etc. Sorry to be so thick.

Kevin_A

tarbat you should have a minimum of 6 and 12 is great. The more equal weighted stars the better.