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Levenhuk carbon fiber 8" RC on the way!

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#26 Madratter

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Posted 20 August 2013 - 11:30 AM

I think a KAF 8300 based sensor will be somewhat over sampled at the Native f/8 focal length. Specifically you have a focal length of 1600mm. The 8300 pixel pitch is 5.4 microns. That gives an image scale of .7 arc-second per pixel.

If you have seeing good enough for 2 arc-seconds that is almost 3 pixel sampling instead of 2 pixels.

All that said, I would rather be somewhat over sampled than under sampled. And the Sony cameras tend to have even smaller pixels. For example the Sony 460 EX monochrome camera has pixels of 4.54 microns.

I have been imaging recently with my C8 at the native f/10 2032 mm focal length, so I am even more over sampled than you would be.

(Also in comparison, assuming you have the T3 and not the T3i, the pixel size of that is 5.1 microns).

#27 rflinn68

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Posted 20 August 2013 - 11:54 AM

Yes, I have the T3 (1100D). I believe it is 5.2 microns which would put me at 0.67 arcsec/pixel. What would you go with for this RC then? Also keep in mind though that I'd like something to work with my other scopes as well. I have an AT65EDQ and the AT8IN that I also enjoy imaging with. Thanks

#28 rflinn68

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Posted 20 August 2013 - 12:13 PM

The Atik-314L is 6.45 and would produce an image scale of 0.83 arcsec/pixel. Would this camera be a better choice for my RC? It would certainly "zoom" in on the smaller targets which was really why I wanted a long focal length scope. I can always continue using my T3 on the other scopes and just use the CCD on the RC.

#29 Madratter

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Posted 20 August 2013 - 12:17 PM

No one camera is going to be ideal for all of those. The AT65EDQ has a focal length of only 420mm. The AT8IN is in the middle at 800mm. The AT8RC is at 1600mm.

Any camera that is sampled at 2x for the AT65EDQ is going to be way over sampled for the AT8RC.

That said, KAF-8300 is well matched to the AT8IN if your seeing is around 2.8 arc-seconds. And for wide field with scopes like the AT65 EDQ, a lot of people are under sampled and live with it.

#30 Madratter

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Posted 20 August 2013 - 12:23 PM

The Atik-314L has not got much FOV. Personally, I would rather live with the oversampling of the 8300 (which can be mitigated by binning if you wish). Binned 2x2 the 8300 has even bigger effective pixels at 10.8 microns, and still has a larger FOV. Binned 2x2 it would be well matched for seeing of 2.8".

Of course there are other factors to consider too such as the noise of the chip, cost, etc.

#31 rflinn68

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Posted 20 August 2013 - 12:35 PM

Now I'm really confused. I got that the 8300 would be good with my AT8IN. I dont know anything about under-sampled or over-sampled or binning. :(

#32 jrcrilly

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Posted 20 August 2013 - 01:10 PM

Any camera that is sampled at 2x for the AT65EDQ is going to be way over sampled for the AT8RC.


That's one reason I switched to an 8300-based camera. When I want small pixels I have them, and when I want larger ones I can bin and still have a 2MB pixel count (larger than the 314L). It is quire versatile.

#33 Madratter

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Posted 20 August 2013 - 01:16 PM

The basic idea is this. Stars are not just points when they are imaged. Instead, they are spread out somewhat in (ideally) an image circle. According to sampling theory, you need at least 2 samples in each dimension to get the usable information that is there.

If you have less than two samples, you lose information and are under-sampled. The practical result is that your photographs will tend to have square looking stars. You can see this in many Wide Field photographs.

If you have more than two samples in each direction, you are over-sampled. The practical affect of this is that you are losing signal to noise ratio. The reason is that each of those separate pixels has noise in it and that noise tends to be higher than if a chip was made that had the larger pixels that was sampling at 2x.

Binning is a way of combining multiple pixels into one pixel. When this is done in hardware, one of the sources of noise (read noise) is minimized.

Binning has its own problems however. For example, with the 8300 chip you can get so called horizontal aliasing if a binned pixel gets saturated.

At any rate, the bottom line is that you will not find a camera that is ideal for all 3 of those telescopes. It doesn't exist.

#34 Madratter

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Posted 20 August 2013 - 01:20 PM

Any camera that is sampled at 2x for the AT65EDQ is going to be way over sampled for the AT8RC.


That's one reason I switched to an 8300-based camera. When I want small pixels I have them, and when I want larger ones I can bin and still have a 2MB pixel count (larger than the 314L). It is quire versatile.


I also have an 8300 based camera (a SBIG STF-8300m). And in practice, I like it and find the binning quite useful.

However, even at 1x1, it is under-sampled on the AT65EDQ but then so is his T3.

#35 jrcrilly

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Posted 20 August 2013 - 01:28 PM

However, even at 1x1, it is under-sampled on the AT65EDQ but then so is his T3.


Yup. Anything will be - but it's not as bad as the 9 micron pixels lots of folks use at those focal lengths. That's why my STL-11000 left. Not sufficiently versatile.

#36 rflinn68

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Posted 20 August 2013 - 01:35 PM

So what would you say is the ideal camera for each of my 3 imaging scopes. In a medium resolution class ~$2000

#37 Madratter

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Posted 20 August 2013 - 01:52 PM

I can't say I have enough knowledge to make that kind of recommendation.

What I will say is that 8300 based cameras are very popular because you get a pretty reasonable FOV at a pretty reasonable pixel size at a reasonable price.

What the Sony camp will point out is the nice low noise of their sensors.

In practice what that really means is fewer hot-pixels. I find the 8300 quite liveable in that department since the software does a good job of dealing with them as long as you have good darks, flats, and some movement between frames (either from dithering, or small amounts of flexure).

#38 HunterofPhotons

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Posted 20 August 2013 - 03:09 PM

....What the Sony camp will point out is the nice low noise of their sensors.

In practice what that really means is fewer hot-pixels.....


Are you saying that low noise sensors have fewer hot pixels?
As compared to what?
I was not aware of this correlation between 'low noise' and hot pixels and would like to learn more about it.
Can you provide a link to studies or research that establish this correlation?

dan k.

#39 Madratter

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Posted 20 August 2013 - 03:37 PM

I recall that from a thread over in the CCD forum. Someone with a huge library of calibration images and such from different cameras was making this point on the Sony chips. I don't know if that is true in general or not.

I doubt I can find the article at this point.

#40 Madratter

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Posted 20 August 2013 - 03:48 PM

I found the thread. See the statement by Kevin Nelson from QSI.

http://www.cloudynig...cd/Number/57...

#41 CounterWeight

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Posted 20 August 2013 - 06:31 PM

These days,

I'd only worry if I was undersampling by a margin large enough (as John mentions) to rob you of image quality - and you won't be doing that with the 8300M. There's plenty of reason not to worry about oversampling within reason - and if the supporting chipset allows binning pixels, there goes that worry. The 8300 chip's wild popularity among the manufacturers speaks for itself - as do the images made from them so I think you can buy with confidence.

I do think it's important to understand what the expression is taking about, detail or lack of it, and where in the grand scheme it matters. For us in general, small pixels and lots of real estate = good.

More important to me is the issue of using a 'OSC' version with it's Bayer matrix for color (and it's debayered resolution in each color plane), or the mono version where every pixel can provide resolution and detail. Also look at the aperture and filter discussions as relates to focal ratio. Also if the CCD is anti blooming or not, is microlensed, and then it's temp regulation specs.

There are reasons FLI, QSI, Apogee, SBIG, others adopted the 8300 chip, and there are differences in the exact models that were brought to market. So don't assume all the same and only different $. When I was going through it all when SBIG and possibly others initially announced I found it very helpful to spreadsheet all the offering and all the specs/options between them.

Re-reading your post (trying to be certain I'm not wandering OT) I think going 'mono' 8300 at your price point may not be possible outside a fantastic deal on the used market (and I wouldn't rule it out entirely). Adding in the cost of a filter wheel and filters puts you up and out of that bracket.

With the color or 'OSC' version I'd take a look at how they get the color and what that has to do with final resolution (arcsec/pixel and over/under sample) even after all the software bells and whistles.

When I jumped into imaging I bought an OSC and then sold it after saving some more $$ for a mono variant. I was in a hurry to get started and see what it was all about.

#42 rflinn68

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Posted 21 August 2013 - 10:10 AM

Thanks for chiming in about CCD's Jim! I understand I wont get filters and a wheel for that price. I was talking about the camera by itself. The SBIG 8300M is on sale at Astronomics for $1995 and QHY has their QHY9 mono (8300 sensor) with a filter wheel for $2099. The QHY9 looks pretty nice to me with a 2 stage TEC cooling and the SBIG is a single stage. I'm kinda leaning toward the QHY but they havent answered my email from several days ago regarding the filter wheel. It says its a 2" filter wheel and I was wondering if it will hold the 36mm filters. I also dont know how many filters it will hold. I suspect just 5 and I'd like to have an 8 position wheel. Then I could get the LRGB filters, Ha, OIII, SII and have 1 open for exposing my flats. Is this how you do it or do you expose flats with the Luminance filter?

I was able to get the RC collimated last night........sort of :( It looks near perfect on the intra-focus side but it doesnt look concentric outside of focus. What causes this? Also, stars in the bottom corners look pretty good but the top corners still look pretty bad. :(

#43 Madratter

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Posted 21 August 2013 - 10:31 AM

Personally, I expose my flats using the luminance filter.

EDIT: Ideally you would take a set of flats for each filter actually used.

As for the collimation problems and it changing between intra and extra focal positions, I don't know how that can happen without slop in the focuser. If the star is staying centered in the exact same place (i.e. no slop), I'm at a loss.

I'm also wondering if you have some camera tilt.

#44 jrcrilly

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Posted 21 August 2013 - 10:35 AM

have 1 open for exposing my flats. Is this how you do it or do you expose flats with the Luminance filter?


The really serious folks do flats for each filter. That does have the advantage of handling dust on the filters. I think most of us do flats through the L filter. When I use flats (which isn't always) I use the L filter. I keep my filters clean, though.

#45 HunterofPhotons

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Posted 21 August 2013 - 11:43 AM

I found the thread. See the statement by Kevin Nelson from QSI.

http://www.cloudynig...cd/Number/57...


Thanks for the link.
I read it and the subsequent links, but I didn't see where there was a connection between 'low noise' and the number of hot pixels.
Temperature can affect the number of hot pixels, but that was the only direct connection that was cited in a subsequent link.
It's easy to fall into the trap of assuming a correlation is evidence of a cause-and-effect. I guess I'll remain skeptical. <g>

dan k.

#46 rflinn68

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Posted 21 August 2013 - 11:43 AM

Ok, After looking at these on my desktop I can see I still have work to do but you can still clearly see the difference in how it looks inside of focus vs. outside of focus. This first one is outside of focus. I was on Arcturus and at ISO 1600 even 3.2sec over-exposed it for a test like this but when I cropped it I included the star to the left.

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#47 rflinn68

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Posted 21 August 2013 - 11:45 AM

And here it is on the inside of focus....

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#48 CounterWeight

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Posted 21 August 2013 - 12:18 PM

Argh! just lost a post?! Must have forgotten to push the button after checking it over.

What I wrote was that I agree, flatting it all depends where the offending item is. If not on the filter directly using the Lum channel seems to work just fine on both the Baader and Astrodon filters I have used.

On another tack, the SBIG and flats requires special mention because of the mechanical shutter. I have the original 'ST' version and wheel and learned quickly that I had to increase my expose times for flats very dramatically to get rid of the shutter effect - more than 2.5 seconds or ~3 seconds seems adequate. This was what drove me to getting an illuminated panel for flats.

In all my astro purchases support figures in very heavily, and SBIG is fantastic. I think it's important to mention this. Clear nights to image are a premium in cloudyopolis, and may be when the problem shows is after a long wait to image again (it can and has been months between at times) and waiting days on end for support and 'all that' just does not float my boat. Neither does getting someone who has no actual expertise or concern and urgency when I finally do get through.

SBIG has exceeded my expectations which are high. I've sent mine in twice with the filter wheel and turn around was as fast as I could hope with clear communication always from them. If I remember correctly the first visit I was out only the shipping, and they went beyond the minimum in servicing the units.

Good luck with the final tuning of the optics. Takes some patience to learn each scope's 'personality' when it comes to tweaking in the optics. When these came out there were some pretty huge threads on dialing them in, i think in the 'Cat and Cass' forum?

#49 Madratter

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Posted 21 August 2013 - 01:05 PM

I can't tell much from Arcturus other than the collimation is not perfect.

The fainter star makes that completely clear. What is also clear is that there is some astigmatism. That is why the orientation of the overall oval shape changes.

I would not jump to conclusions on that. First off, the star isn't centered (I realize this is a crop so it is probably pretty close but still).

Second that astigmatism could be caused by things like camera tilt rather than the optics. Are you seeing the same behavior visually. Also, is there a diagonal involved in any way? It is very common for diagonals to be miscollimated.

Third, I am no expert on this stuff. I am a duffer at best. I would head over to the Cat/Cas forum and post there.

It is possible that it is normal for RCs to show this kind of intra/extra focal image when off axis.

At any rate, I would be very leary of making any pronouncements of optical quality until it is collimated properly.

And again, I could be all wet. The people in Cat/Cas will be much more helpful than I could ever be, simply because some of them actually know what they are talking about.

#50 rflinn68

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Posted 21 August 2013 - 02:44 PM

Thanks guys....Madratter, yes it is cropped and no I do not use a diagonal when imaging. I would be VERY interested in this astigmatism issue!! They have a 45 day return policy and I might want to take them up on that. Its not the most expensive scope in the world but it was a lot of money for me and I want a nice scope for that kind of money. Here is the image uncropped to give you an idea of where the other star was in the frame. I know Arcturus was overexposed and thats why I cropped it and included the other star.

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