[Shiraz (Ray)]

Hi

This idea arose in discussions with PRejto and was also recently raised in a post by Justin Tilbrook. The basic idea is that a form of lucky imaging can be applied to DSOs – you don’t need to image fast enough to freeze the seeing, but it can help if you image with short enough subs to enable you to deal with those occasions when seeing fluctuates over periods of minutes.

To illustrate with real data, the upper graph in the attached image shows the HFR for a sequence of 56 x 3 minute subs. (HFR is related to FWHM and these data correspond to FWHM of about 2-2.5 arc sec). The seeing varied fairly smoothly, but there were occasional bursts where it was worse. The lower data shows the same HFR data, but with each value being the average of a block of 7 shorter subs (ie this data represents 21 minute subs). Should I decide that I want to keep data with an HFR of say 1.2 pixels or less to maximise the resolution, I can keep 31 of the 56 short subs, but only 3 of the 8 longer subs (and one of those is marginal). Thus, I can keep > 40% more of the data if I use shorter subs – or alternatively I will get worse resolution if I choose to keep more of the longer subs. The basic problem with the longer subs is that they bundle good and bad data together and there is no way to weed out the bad without losing some of the good.

Of course, this method is not for everyone. You will need a sensitive system with low read noise to keep noise under control, but if you have a camera with high QE and low read noise, it might be worth seeing if you can get better resolution by using short subs. Even if you don’t get huge increases in resolution, you might get significantly better SNR, since you could possibly end up keeping much more data. And of course, it doesn’t cost a cent.

Thanks for reading. Regards Ray