Under or over sampled?

[LewisM]

If I have a 7.4μm pixel camera mounted in an 800mm / f/8 refractor, with usual seeing of around 3 arc sec, I should achieve around 1.9 arc sec per pixel, which theoretically SHOULD be borderline undersampled (assuming rule of thumb of half seeing)

If I then run it reduced by 0.7x, this seems to make it slightly OVERSAMPLED.

So, which is the better position to be in? Run it reduced (larger FOV, faster, slightly oversampled), or native (smaller FOV, slower, slightly undersampled).

I just don't want darned blocky stars :) (I really regret selling my KAF8300 now... oy vey!)

[multiweb]

If you go with a reducer you will undersample but at 3asp you're not likely to get square stars. Either way you won't be oversampling either at 1.9asp at prime focus.

[RickS]

I get square stars at SRO (typical seeing around 1.5 arcsec) with an image scale of 2 arcsec/pixel but Drizzle makes them nicely round again ;)

The two times rule is fine for sampling a sine wave in one dimension but not accurate for 2D spatial sampling. Critical sampling is more like 1/3 to 1/4 of the seeing. See for example: http://www.stanmooreastro.com/pixel_size.htm

Cheers,
Rick.

[LewisM]

Thanks gents.

Forgot how much I like these older SBIGs...

[rmuhlack]

back in the saddle again eh? :)

[LewisM]

Saddle up Apone!

[rmuhlack]

:thumbsup:

[Slawomir]

Same as Rick, I have also found 2arcsec per pixel to be a tad too low; stars were squarish even with my HEQ5 mount. Now shooting at 1.34 arcsec per pixel and this resolution seems to produce satisfying results, and probably it's at the limits of my mount too...

[gregbradley]

I have been looking this over myself a lot recently.

Roland Christen suggests 1 arc sec/pixel is a good guide for average seeing astrophotographers.

However I have had good results using a Trius 694 (4.54micron pixels) at 2 metres and that is .45 arc sec/pixel. It gives noticeably better resolution than a 9 micron CCD on galaxies.

With the KAF8300 and the 3 high QE Sony CCDs (ICX694 6.3 mp,814 9 mp,834 12mp) with super low read noise the equations seem to lean towards the smaller pixels for most applications except perhaps long focal lengths??

1.9 arc secs/pixel I would regard as undersampling by quite a bit. I agree with Rick here and it matches with my experience with CCDs in th elast few years.

But if you routinely get bad seeing then all bets are off.

Greg.

[Paul Haese]

On my FSQ I am under sampled at 2" per pixel just like Rick at SRO and on my RC12 I am slightly over sampled at 0.76" per pixel.

It is better to be slightly over sampled (meaning the system has sub arc second coverage per pixel) than under sampled. Some of the advantages are sharpening of detail and stars are round. Although there are exceptions to this thinking. An FSQ106 with 11002 sensor being one I can think of easily. Results with a 9.0um pixel are exceptional in that example.

I agree that I think you are under sampled at 1.9".

However, 7.4 will still produce good results if everything else is working as it should.

[LewisM]

Thanks again everyone. Guess I will just have to actually use it and see.... but that requires the weather to co-operate. :(

Setup with Baader LRGB-H-a (7nM), with Astronomik CLS LP filter in front of it all. Might have to use an UV/IR cut as well in the train, but trial will tell (have written SBIG to ask if they used UV/IR cut in their front window back when this was made)

I looked back over some old subs I took with the 9um ST-8Xe, ina VERY similar setup, and those stars were borderline blocky, so 7.4 can only improve upon that.