sensible, attainable, pixel scales.

[rubecula]

On 29/06/2017 at 16:03, ollypenrice said:

If you're going to stick with your DSLR you have to be realistic about two things, focal length above all and then focal ratio. DSLRs have small pixels which cannot be made into larger 'virtual' ones by binning 2X2. This means that only shorter focal lengths will give sensible, attainable, pixel scales. You can experiment here http://www.12dstring.me.uk/fovcalc.php with scope-camera combinations and see what arcsec per pixel values they give you. 1"PP requires very good autoguiding to make the theoretical resolution turn into real resolution in the image. 2"PP is very nice. Detail is attractive and the system is semi tolerant! 3"PP is getting significantly easier and we still get lots of image publications and the odd prize down at 3.5"PP, which is very tolerant.

Also DSLRs are uncooled and so cannot just rack up longer and longer exposures. They need a fast F ratio to thrive. 

Refractors are easier by far but a small imaging Newt will be both inexpensive and highly effective with your DSLR. 

Olly

Forgive my continuing ignorance. I have an 8" Edge HD with focal reducer and a Canon 550D which gives 0.62"PP.  What is the actual result of such unsensible (sic) unattainable pixel scales? 

Does that blur / defocus images or am I just losing the potential resolution of the scope by using the Canon 550D?

Is there any inexpensive way of helping the situation either in capture or processing?

My next plan is to get a better mount this year and maybe a CCD next (can only save up slowly on a pension).

Thanks

 

 

[ollypenrice]

0.62"PP isn't necessarily unattainable but it will be very difficult, requiring excellent seeing and autoguiding. Yves Van den Broek and I imaged at 0.66"PP for two years here with a 14 inch Dall Kirkham and SXVH36. It's very hard to know to what extent we were realizing the theoretical resolution of the system. We were helped by having a Mesu 200 mount reliably guiding to about half an arcsecond and sometimes better. Full exploitation of the optics and camera would need about 0.3" RMS. Our original plan was to work regularly in Bin 2 but the camera wouldn't bin successfully.

Begin by converting all your units into arcseconds, for sanity's sake. So what is your guiding RMS in arcseconds? This will need to be about half your imaging pixel scale for you to reach the system's full potential.

What if you don't reach it? It's not the end of the world. Your theoretical resolution will be blurred down to an extent determined by your guiding error and by the seeing. This could be a little or a lot. The quotation you mention above was from a thread about unguided long focal length imaging and in that case the blurring of detail would almost certainly be very severe.

The only thing over which you have any real control is the autoguiding so the things to prioritize would include

- using an off axis guider to combat flexure and mirror flop

- fine tuning as much backlash out of the mount as possible

- fine tuning the balance and probably running a tad heavy on the east

- fine tuning your guide parameters, remembering not to be fooled by the apparent benefit of very short guide subs. (Very short subs keep the guide star image in the same place on the chip but overlook the fact that the real position of the guide star is being pushed around by the seeing.) You need to follow the guide star, not the seeing, so longer subs are likely to be better.

Note that I don't put fine tuning of polar alignment in there because I don't think it necessarily helps. If you have a little Dec backlash it can be beneficial to run the guide corrections in only one direction, the direction needed to correct for PA. This avoids potentially recurrent overshooting and oscillation. One of our worm and wheel mounts responds very well to this.

If your guiding errors are random you'll get round stars.

Olly

Edit: Oh yes, one thing other thing you can control is focus. Fine pixel scales require fine focus so check this carefully and regularly. I normally use FWHM but on the Dall Kirkham it was far too bouncy to work so we used a Bahtinov mask.

[alacant]

Hi. Some hands-on from from guiding a dslr on a 6" f8 on an eq6. Try DSS' superpixel which bins each Bayer block of 4 pixels into 1. You may also be able to improve your mount too by cleaning, regreasing and taking up backlash. Take images well after nightfall. Wait until targets are at their highest. Always abide by what the guide software is telling you about seeing e.g. PHD2 has the guiding assistant. Despite the theoretical impossibility of what we are doing; try it. You may just be pleasantly surprised! HTH.

[rubecula]

Thanks Olly and alacant, that's just what I needed.  I've already adopted a couple of your recommendations (OAG, east heavy) but you've both given me plenty more to work on.