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Offline richardcav

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Which Camera - Advice needed...
« on: 15:57:08, 02 April, 2017 »
I would like some advice from the "collective mind" please :-)

I have a Canon 1100D DSLR which is great but not cooled, so a friend was kind enough to loan me his Starlight Xpress SX-25C which took some great pics, however unfortunately it has to go back so I am looking at alterntaives (given that I dont have the 3000+ euros to buy one) and have been looking at the ZWO - ASI-1600.

I am trying to compare each of the cameras I have used and have come up with the image below to show their field of views using my scope (a SkyWatcher 1200/250).  My question is, will the ZWO will actually give me a more a better quality image (its 4656 x 3520px as opposed to the Starlight Xpress which has 3024 x 2016px) or am I missing something?  I guess I must be as there is a big difference in the price (the ZWO being much cheaper), but I cant figure out what - perhaps its just quality of components and "name" or maybe not?

The Staright Xpress says that its CCD is 1.12° x 0.75° where the ZWO is 0.84° x 0.64° so maybe thats it - it has a smaller CCD, but then one is a CCD and the other is CMOS?

I have tried to do some reading up on the difference between CCD's and CMOS and on the face of it for deep sky CCD seems better but then again why do all of the manufacturers seem to be moving away from CCD to CMOS?

Sorry for all the questions and confusion but that's how my mind feels today after looking at all this stuff!

Kind regards


Offline chris.bailey

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Re: Which Camera - Advice needed...
« Reply #1 on: 17:03:04, 02 April, 2017 »

A lot of questions. I'll try and summarise my thoughts (other opinions are available).

CCD vs CMOS - I suspect in a few years time we will all be using CMOS chips, largely because they will get better but also because CCD manufacture will decline. CMOS chips are cheaper to buy than large CCD chips and that is reflected in the end price. I have the Altair version of the ASI-1600 and have owned a SXM25C and do own a Atik 460 OSC. CMOS thrive on lots of shorter exposures. CCD's thrive on fewer longer exposures; it is hard to overcook an M25C and 15 or 20 minute exposures are the norm. For CMOS you would struggle much over 5 minutes. CMOS chips have low read noise, good sensitivity but have smallish pixels and limited well depth. They tend to exhibit amp glow though deep cooling can help. The M25C has big pixels and a huge well depth. Whilst the CMOS cameras are very capable for brighter objects, they start to show a few shortcomings on the dimmer stuff where long exposures are needed. Based on the current crop of CMOS cameras, I would choose a CCD based camera.

Image Scale - An important aspect of camera choice is image scale. On your 1200mm scope the little 3.8 micron pixels of the ASI1600 give you an image scale of 3.8 x 206 / 1200 = 0.65 "/pxl. That is somewhat oversampled and a challenge to guide (small guide deviations will show up). Conversely the nice big 7.8 micron pixels of the M25C give an image scale of 7.8 x 206 / 1200 = 1.34 "/pxl which is much more in the golden range of 1-3 and a fair bit less challenging to guide. The 4.54 micron pixels of an Atik 460 would sit in the middle.

The M25C is an ideal match to your scope but as you point out they are expensive new. they do come up second hand quite often so this is an option. Early ones (like the one I had) did not have set-point cooling but don't rule the out because of this. They are still very capable.

Hope that helps

LX200|ZS70|FSQ85|FLT110|Altair DF250RC|EQ6 Pro(Rowan Belt Mod)|ParamountMX
ATIK383L+/EFW2/OAG|Lodestar|Baader 36mm LRGBHaSIIOIII
Starlight Express SXVRH16/ONAG/FW|Lodestar X2|Baader 2" Filters
Starlight Express SXVRH814/ONAG/FW|LodestarX2|Baader 2" Filters
Lunt LS60PTBF1200|DMK41|Quark Chromo
Samyang 135mm f1.8

Offline Einari

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Re: Which Camera - Advice needed...
« Reply #2 on: 19:16:43, 02 April, 2017 »
Just today the last page of longish thread in CN forum of ASI1600 and it's a lot about debate over ccd vs cmos.
Like Chris wrote we are going the cmos way for reasons he mentioned.
And it also means a change in doing things - with cmos based cameras the norm is to take ten, hundreds or even thousands of frames to achieve same that ccd camera rarely needs more than say 50.
Also with cmos you have to take account of gain and offset.

One thing more - I'd like to recommend mono+filter wheel over colour camera.

Celestron C8, C9.25" & Hyperstar, iOptron CEM60-EC, TS 80 triplet,  Coronado PST, SXV-694,  ASI120MM/ASI174MC Cool


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