Why I hate choosing a CCD

Jutscher (Sam) · #1 13-03-2011, 07:20 AM

Hey guys, and

So here is the deal. I decided that I want to move to a dedicated CCD for my imaging needs based on a couple of things... primarily that my current modded 500D that I have been using has plenty of nasty big dust motes that infect every pic that I take (please disregard if you see the same for sale... its fine

) and is in need of retirement.

As such, I have been and will continue to use a Takahashi TSA 120 with a FL of 900mm, at F7.5. I enjoy taking sharp, wide field images, but would like to be able to crop these on occasion for smaller objects such as galaxies etc.

I decided that I would put my old practices of impulse buying aside and do some homework to ensure that I got the best CCD to suit my scope and preferred style of imaging.

That said, I have been looking at dedicated CCD's such as SBIG's ST-8 and ATIK's 11000. Both have a 9uM pixel size, which would give me
a pixel angular size of 2.06 arc seconds. The 11000 would give me a much larger FOV (at a price) which could be later cropped. Whereas the ST-8 is far cheaper and comes with a guiding chip (not as important as I already have a guiding setup - it just needs improving/stabilisation)

If we were to assume that good seeing in the outskirts of Perth (sea level and usually hot) was roughly 2.5 arc seconds, then my understanding is that 9uM operating at 2.06 arc seconds would be okay, if sampled 1x1

My concern is, would a 9uM pixel size for this relatively short FL scope be risking unescessary undersampling if conditions allowed (does anyone know the usual seeing for outer un-light polluted WA?).

Am I better off choosing a smaller pixel to ensure that I am covered for that perfect day, and just bin for the majority of poor seeing days?

And just to make things even more complicated, pixel size aside, I would like a camera body that allows decent thermoregulation where darks can be taken in daytime if required, to preserve dark sky for light frames.

Hopefully thanks in advance, but I realise that this is a subject fraught with differing opinions.

Sam

tlgerdes (Trevor) · #2 13-03-2011, 08:39 AM

From what I understand, the "goldilocks zone" is between 1-2 arc-sec/pixel. Starting at 2 for 900mm means you wouldnt be doing yourself any favours if you shorten your focal length in the future. I would be shooting for 1.5.

gregbradley · #3 13-03-2011, 09:25 AM

My understanding is .66 arc sec/pixel was closer to ideal. Based on Nyquists sampling theory that it takes 3X higher accuracy to gain a decent sample (I may have dumbed that down but that is the basic concept).

So .66 as a target assumes then that .66 x 3 = 2 arc seconds seeing is available.

If your seeing is never that good then you can go for a higher than .66.

But 1.5 arc sec/pixel if that theory is true would mean you are expecting seeing of 4.5 arc seconds which is pretty poor seeing.

So following this idea along then the ideal setup would be in arc seconds/pixel 1/3rd of the best seeing you can expect from your location.

Also another rule is smaller pixel cameras for faster F ratios. I think because of how the above works out for this type of scope.

From what I have seen of images, those who apply the above seem to get very good results so it must be pretty close to being true.

Wodaski has a free CCD calc to make it easy to work this out for your setup and different scopes or cameras.

http://www.newastro.com/book_new/camera_app.php

This is a handy tool.

Greg.

[1ponders] (Paul) · #4 13-03-2011, 09:30 AM

Can't add much more than what Greg has said except to jump up and down and clap Ron's CCD Calculator. Wouldn't be without it.

Jutscher (Sam) · #5 13-03-2011, 03:59 PM

Thanks for the quick replies, and the calc link, a bit easier than plugging the info into SNPP6 for a FOV reference.

So, by applying the star size formula: Star Size = (Seeing * FL)/206265 = Then in average perth skies (lets say 2.5arc seconds)
Star size = (2.5 * 900)/206265 = 0.011mm = 11microns.

My understanding of the Nyquist rule was to 2x sample (correct me if wrong) therefore in seeing of 2.5 arcseconds I would need a pixel size of 5.5microns, pretty close to my current 500D at 4.7microns in terms of ideal sampling.

So I guess, I need to find a CCD with properties similar to my DSLR, weird.

gregbradley · #6 13-03-2011, 04:15 PM

I just googled Nyquist rule and its at least 2X. So that would be the minimum in order to reconstruct the signal.

I suppose that is why I have seen it expressed as 3X to allow a margin as 2X is the absolute minimum.

So with that then if you are matching a specific scope you can work it.

Your DSLR is already a bit more than 2X and it seems you are on safe ground there.

KAF8300 chip is 5.7 micron pixels so that would be one choice. That is the chip in the SBIG ST8300, FLI Microline 8300, Apogee U8300,QSI583 and Starlight Express and QHY cameras.

QHY is cheapest, SBIG ST8300 probably next cheapest. Then perhaps Starlight Express then Apogee and then FLI (the best).

Greg.

Jutscher (Sam) · #7 13-03-2011, 04:31 PM

Thanks Greg, that will give me a little bit of reading for the next couple of days

Would also be keen to hear from anyone using this scope for imaging, and what decision they made re: CCD.

Cheers

renormalised (Carl) · #8 13-03-2011, 05:32 PM

You can also add the Atik 383L + to that list of cameras that uses the 8300 chip. It's also cheaper than the QHY9 mono....$2599, at Andrews. Atik are very good cameras.

mill (Martin) · #9 13-03-2011, 06:30 PM

Quote:

Originally Posted by renormalised

You can also add the Atik 383L + to that list of cameras that uses the 8300 chip. It's also cheaper than the QHY9 mono....$2599, at Andrews. Atik are very good cameras.

Really you have to include a filter wheel and a set of LRGB filters in that price.
Oops it just went way over the price of a QHY9M that will come with filter wheel and LRGB filters and cools to -50 below ambient.

When you start to give information, you should include the missing parts.

You might have forgotten about the filter wheel and filters, if so then i am sorry.

Rigel003 (Graeme) · #10 13-03-2011, 06:41 PM

Quote:

Originally Posted by Jutscher

Would also be keen to hear from anyone using this scope for imaging, and what decision they made re: CCD.

Cheers

I have the FLI ML8300 and have just moved up to a TSA-120 from my FS-102. I'm planning on using it with the TOA35 reducer at F/5.6 most of the time for wide fields. Should be a good match. The cooling and download speed of this camera are great. It can easily get down to -30 degrees C on summer nights.

renormalised (Carl) · #11 13-03-2011, 10:47 PM

Quote:

Originally Posted by mill

Really you have to include a filter wheel and a set of LRGB filters in that price.
Oops it just went way over the price of a QHY9M that will come with filter wheel and LRGB filters and cools to -50 below ambient.

When you start to give information, you should include the missing parts.

You might have forgotten about the filter wheel and filters, if so then i am sorry.

You also have to be careful about being condescending, Martin

It's being included as part of a package deal. You won't buy a QHY9M for $2000-$2300 (less the price of the FW and filters), more likely $2500-$2800 by itself. So add those costs to your camera if you didn't buy it with the FW and filters originally. They turn out to be roughly the same cost.

It all depends on what FW/Filter combo you purchase as to what the final price will be.

Mind you, the QHY9M combo is a good buy, and the OSC version is a steal!!!

renormalised (Carl) · #12 13-03-2011, 11:02 PM

One thing I haven't seen too many of...piccies from the QHY10 and 12. The 12 looks like it could be a good buy for the people wanting a powerful OSC with plenty of pixels that isn't too heavy on the wallet.

mill (Martin) · #13 13-03-2011, 11:12 PM

Quote:

Originally Posted by renormalised

You also have to be careful about being condescending, Martin

It's being included as part of a package deal. You won't buy a QHY9M for $2000-$2300 (less the price of the FW and filters), more likely $2500-$2800 by itself. So add those costs to your camera if you didn't buy it with the FW and filters originally. They turn out to be roughly the same cost.

I am not beeing condescending at all Carl.

The QHY9M has been a package deal from the first day it has been sold and only gone cheaper.
I bought mine when it was $3000 and now it is $2800.
I am starting to think it will be a permanent deal

renormalised (Carl) · #14 13-03-2011, 11:18 PM

How reliable, mechanically, is the FW?

mill (Martin) · #15 13-03-2011, 11:24 PM

My filter wheel had only one hickup because the small screw in connection wasn't crimped on properly.
I tighened it and after that it worked perfect till now.

renormalised (Carl) · #16 13-03-2011, 11:28 PM

That's good to hear...that they're reliable.

ballaratdragons (Ken) · #17 13-03-2011, 11:43 PM

From looking at DSO images, lately the QHY range of cameras seem to give a better image, unless it is just coincidence that the fantastic images of late were taken with QHY's?

I know nothing about CCD cameras, I'm just giving my view of one who looks at DSO images a lot.

If that helps at all

I know, not a very technical bit of help, is it.

Jutscher (Sam) · #18 14-03-2011, 12:11 AM

yeah, the QHY9 M certainly does appear to be a pretty good in terms of covering the focal requirements of my scope while retaining a reasonable FOV. Equally the price is pretty good especially considering that it includes FW and RGB 2" filters.

Will probably be a hot contender

Addit - The FLI does look pretty sweet, but for the extra cost, I could fit in a set of narrow band filters, which would be a huge addition as I would love to do a little more backyard imaging

OzRob (Rob) · #19 14-03-2011, 01:10 AM

There's a SBIG ST-8300M with FW and filters in the classified section: LINK

Jutscher (Sam) · #20 14-03-2011, 11:50 PM

... anyone know of any FLI dealers in Aus, the FLI website gives:
Astro Optical Supplies - http://www.astro-optical.com.au, however I couldnt find any FLI cameras on their site.

The QHY9, although at a pretty good price, does not appear to have the build finish of other cameras of the same chip - images in the manual showed a early oxidisation of the fan plate, and the plugs looked poorly seated - I could be wrong, but why would you put these images in the manual?.

Where as the ATIK does not promise the same level of coolin (40C as compared to 50C) the actual product finish appears a lot tidier. and for a very similar price although I realise that this is minus FW and RGB.

As for the SBIG, I know this is shallow, but I think it kind of looks like a 1980's T.V set. Although I am sure that it is a perfectly capable camera for a good price, I am gonna rule it out on aesthetics.

Thoughts?