M57 experiment

[nytecam]

Last night I had a play with M57 trying for the minimum exposure to record the central star - something that can be very ellusive for visual observers. I used my 30cm SCT+ SX MX916 in 2x2 binned mode and rescaled the images below in exp from 3s - 60s. Really no contest - even the 3s exp shows the mag 15 central star and unlike visual observer seeing has little or no effect on the record.

[opticalpath]

I agree: it's surprising just how deep a short exposure can go. Some years ago I did a lot of big stacks of short exposures when my tracking wasn't good enough to go longer! But in my experience, I found that seeing conditions can make a big difference.

This is a single 12 second exposure of M57 taken with a 10" LX200 at f/10 in very good seeing conditions (well, very good for Maidenhead anyway!) This was taken back in 2005 with just a modified webcam; a large stack of these was the basis of a very pleasing final image.

Another example: In 2004, I tried many times to capture an image of the dwarf planet Varuna at around magnitude +20 with the same equipment - webcam and unguided short exposures. I failed every time until one night of very steady seeing when I finally captured it as a short streak at magnitude 20.1. I'm sure the excellent seeing made all the difference on this occasion.

Going through the individual 15 second sub-exposures, I was surprised to see some very faint stars. In this single frame, the two faint stars right of centre are the two magnitude 17.6 stars in the inset and the very faint one near centre of frame is Vmag 18.4.

Adrian

[nytecam]

Thanks Adrian - some very interesting results there. Perhaps my comments on seeing affecting imaging were a little glib - certainly it's a big issue with visual probably due to the very brief sampling rate of the eye whereas CCD is maybe x10 -x1000 a minimum sampling rate of the eye and the smoothing effect that has. Mag 15 is not a challenge for CCDs in brief exposures with even with modest apertures.

I have for the last 3yrs been using a FR on my 30cm f/10 down to f/3.6 = 1100mm fl with the Lodestar-C as prime imager in brief sub 5min exposures and my comments were fully valid and one of the reason I used this short fl to forget about seeing ! However I'm currently back to 2m fl @ f/6.3 and the M57 images taken immediately after shooting Pluto low in Sgr @ 17deg altitude where seeing via subs was poor eg soft stars so I switched to 2x2 binning on my SX MX916 cam for 'speed' to good effect. Aiming to M57 overhead at 2x2 binning was probably undersampling but ok for the experiment.

[opticalpath]

Yes, I get your point about image scale and sampling being big factors in all of this. If seeing is excellent with small PSFs, then critical sampling for maximum sensitivity will be at relatively longer focal lengths. In more typical conditions with larger FWHM, binning can get you closer to critical sampling. At the image scale I was using then, I probably HAD to wait for excellent seeing to achieve maximum sensitivity! An interesting experiment.

By the way, what reducer are you using to get down to f/3.6? I don't image any more with my LX200 but when I did, I used the 0.63x to get down to about f/5. I found the 0.33x reducer all but useless except for a ery small imaging circle. I have a Lodestar too, on a OAG. If I can bear to remove it, might be interesting to try what you're doing with yours!

Adrian

[nytecam]

I use the Meade f/3.3 FR but even with the tiny 1/2" sensor some coma in the corners but in combo it,s blazingly fast :-/