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110 mm Apochromatic William Optic Zenithstar F 7,0 refractor, William FLATIII 0.8X reducer flattener (to f = 616 mm and F 5.6), CEM 70 Ioptron mount with PHD2 guiding using an 80 mm SW startravel refractor at f = 400 mm.

ZWO AS1294 MC camera, 4.63 uM pixel size cooled to -10C

Captures with Sharpcap and preprocessing plus processing in PixInsight. Both captures under moonless skies.

Image 1 29 x 110s at gain 124 under Bortle 6 conditions (Berkshire Thames Valley, U.K.)

Image 2 74 x 46s at gain 124 under Bortle 4 conditions (Devon U.K.)

So the total exposure for each image was about 55 min and the gain was the same.

The Bortle 6 frames were slightly sharper (FWHM of stacked unstretched frames at Bortle 6 was 3.65 as compared to 3.95 at Bortle 4). However the SNR of the stacked Bortle 4 frames was > 4X superior at 16.5 as compared to just 4.1 at Bortle 6.

I thought that it was nice just to see that things work out at least more or less as expected with an approximately 2 fold (theoretically 2.5 fold) improvement in SNR for each 'Bortle' point. It does illustrate what a huge gain can be had from finding darker skies. No need to label which image is which below ..


Tim

Tim

A good exercise to prove the advantage of the Bortle 4 skies over Bortle 6. I assume you lugged all the gear down to Devon then?

Dave

Hi Tim,

Very nice but I thought that the Brain would have recommended a longer per-frame exposure for Bortle 4 skies. It seems to be the other way around.

Thanks,
Brian

That's it: I'm going to move my appartement with balcony to the countryside :p
Great comparison Tim! It proofs again that gasoline is the best filter.

Menno

Hi Dave,

Yes that's right. My long-suffering wife puts up with the entire boot of our small car being taken up with astro stuff while taking short breaks to out of the way places which - like some phase-shifted version of lycanthropy - always occur around the new moon . Actually I do think that useful skills for this hobby do include 1) remembering to take absolutely everything that you need (I have been flummoxed by forgetting some small but essential component) and 2) learning to set up quickly in new places - which usually involves electrical extension cables and (my record) up to 25m of USB 3.0 cable.

Tim

Thanks Brian,

Yes indeed..why 46s and the unnecessary extra read noise ? The real answer is that I took the Bortle 4 images last year and was less far along the learning curve at that stage. The Bortle 6 was this year's image.

But from some perspectives 45s perhaps wasn't such a bad decision for that particular object? Although a great problem to have, the dark sky image was harder to process because of its increased dynamic range. Even with only 45s exposures it took some serious compression during processing to get reasonable contrast at the core while, at the same time, showing the faint outer arms.

If I am fortunate enough to get another dark sky opportunity at M33 I think that I would now go for a high dynamic range composition and do something like combine a 30s stack with say a 3 minute stack? In addition - sticking with the same set up and still sampling at 1.5 arcsec/ pixel - I would probably be tempted to switch to using the MONO camera only (resolution 30% better). ...either that or stick with the OSC and sample at 1.0 arcsec/ pixel using the Newt.

...but dark sky opportunities are all too rare..

Tim

Hi Menno,

Well perhaps - but a dramatic step indeed and only if you are after chasing faint galaxy details . Galaxies do seem to be one area that absolutely do benefit from dark skies. Fortunately there is a lot else - perhaps most of it where it less critical. In particular I am amazed how it is possible to get really good images of nebula detail even under moonlit and poor skies using HA. I would count my HA filter as possibly my best purchase to justify getting the scope out under almost any conditions

Tim