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Quick Andromeda and Pleiades


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Not the most original title I know, but that's how I felt at the time too.

Canon 450D all filters removed.

Sigma 120-300 f/2.8 OS at 300mm f/2.8.

Astronomik CLS-CCD.

EQ6 guided.

Processing with flats in PixInsight+PSE.

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32x2m subs.

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19x2m subs.

It wasn't looking clear, so I didn't set up early. But it did clear up and looked too good to waste, and I was running by 10pm. My previous plan was to get some extra data on the Veil, but with a new CLS-CCD filter I had to try that out. It's more suited to broader spectrum subjects, so I revisited Andromeda and also took a quick shot at Pleiades. Neither have enough data so the noise floor is relatively high. Also the Andromeda set isn't great as I had uncorrected focus shift since I didn't pre-cool the lens enough before using it. I used the modified 450D with the new filter to gain more red sensitivity, however I suspect its noise floor is higher than the unmodified 600D (with old filter) I used previously on Andromeda. Also I think the two data sets might be too different to combine to see if I can get more information out of it. Everything was covered in dew and I packed up just after midnight.

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Excellent start but I'd never shoot with the lens fully open, stopping down a bit to F4 will improve the stars... I know the temptation is to gather lots of data fast, hence the F2.8 but F4 will improve the quality of the stars.

Edited by Darth Takahashi
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Actually a stopping down test is on my "to do" list even if I haven't managed it so far... I've been looking at various "fast" refracting astrographs, around the f/4 ball park, but I need to know how this lens performs similarly at f/4 before I know if it is really worth it.

I know already at f/2.8, there is degradation as you go off centre, and there is astigmatism, coma, field curvature, or all of the above. But will it be helped "enough" at f/4 to be worth double my exposure times? And how circular are the aperture blades? Previous tests on another lens gave a starburst effect... Anyway, these are still test shots as far as I'm concerned (new filter), while I decide what subjects to really hit hard. Normally broad spectrum subjects aren't on my list due to LP.

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Hi GlasWalker,

Star shapes can also be improved using Deconvolution provided they aren't too bad but that's a technique that is easier once images have been separated into Just Stars and No Stars versions, a technique I remember you have not got on with in the past. But if it makes the difference between acceptable stars at f/2.8 and having to drop back to f/4 it might be worth a second look.

I'm looking forward to powering up the 'scope again but that will have to wait until my return home.

Bob.

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Bob, I'll keep your kit warm for you if you like :) Although I think I'd also rather keep myself warm where you are right now...

Back on topic, no I never quite got my head around star separation. In part, my previous data was too noisy making it harder, but I will revisit it again some day... I think the star bloat at the edge of frame is too much at f/2.8 for top quality imaging. This degradation applies to all details, not just stars. So I'm thinking f/4 would be interesting to see the difference. If I were to go CCD, perhaps that'll gain me enough sensitivity relative to DSLR to offset the drop in speed to f/4... and if the lens isn't up to it, I've already made initial enquiries into "fast" refracting astrographs around f/4 too... but they probably deserve a CCD before I get them!

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.

Actually, I'm back in the UK now but "up north"...

Another suggestion - my medium format Pentax 165mm f/2.8 was a good purchase. Designed for a large image circle I find it works well with my ML16803 although I'm not sure how good it might or might not be with smaller pixels. Not all MF lenses are created equal but if you can find a sharp and fast model at the required focal length and you are prepared to focus manually they can be very cost effective.

Bob.

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Bob, I've kinda resigned myself to slow down to f/4 by the time I go CCD anyway, since I really want to go 3nm filters and I know f/2.8 is really pushing that too far.

Erwin, thanks but I have the cut down PS Elements which doesn't exactly have a selective colour function. It does allow some selective colour hue/saturation adjustment but it's not quite the same.

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Not saying it can't be used, but the full speed benefit of having a f/2.8 optical system isn't going to be realised with 3nm filters, so I might as well go to f/4 anyway. And it's a lot easier to find f/4 astrographs than f/2.8 ones!

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Not saying it can't be used, but the full speed benefit of having a f/2.8 optical system isn't going to be realised with 3nm filters, so I might as well go to f/4 anyway. And it's a lot easier to find f/4 astrographs than f/2.8 ones!

Could you explain why the full speed benefit of having a f/2.8 optical system isn't going to be realised with 3nm filters, please? I'm still learning about these things.
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See http://www.astrodon.com/Orphan/astrodonfaqnarrowband/ for some indication. In short, the faster the focal ratio, the greater the range of angles the light goes through the filter. The filters are really optimised for perpendicular light, and as you go off angle, the filter passband shifts. The narrower the bandwidth of the filter, the more this matters. At around f/2.8 with a 3nm filter, you would be losing a significant amount of light. Hard to quantify exactly, but certainly it wouldn't be double the amount relative to f/4 in an ideal world. So looking at it the other way, f/4 isn't twice as slow in that case.

Currently I'm using Astronomik 12nm filters, which relatively speaking is a lot wider, and thus would work at greater speeds. But I still want to go 3nm, as that would reduce unwanted light by around 4x, putting aside other considerations for now. It's never simple is it?

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Ah right, I see. Thanks for the explanation - something to bear in mind :) Not that I can afford Astrodon filters - Baader are my limit. I've been using Astronomik 12nm filters (well the Ha is 12nm don't know about the OIII) clip filters in my widefield camera and have Baader 7, 8.5 and 8nm NB filter set on order from FLO which I shall use with my scope. Those combinations should be well away from the limit :)

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