[Shiraz (Ray)]

Hi greg

thanks very much for the comprehensive and thoughtful response.

I have tried to answer your questions below. Regards Ray

The spreadsheet is only a screen capture JPG – are you still unable to see it?

By the way what is an RCC1? A corrector of some brand?

RCC1 is the Rowe coma corrector – it got a bad writeup on IIS and does not seem to be sold here in Aus. However, it is designed by Dave Rowe who I think also designed the CDK series of scopes, so it is probably pretty good – no spec available though, so will need to test to make sure it does not introduce SA (like the MPCC, which probably adds about 0.7wave SA). SA would completely negate my underlying assumptions and my coma corrector will need to maintain diffraction limited performance over much of the relatively small field

I have imaged galaxies extensively with BRC250 (10 inch) and Microline 8300.
I currently use CDK17 and Proline 16803. I have used the ML8300 on the CDK17 for galaxy imaging and it did not work well. The smaller pixels at 3 metres focal length merely meant far less resolution and more effect of the seeing. It was interesting to see just how much it cost in detail in galaxy images. A lot. But then you are not looking at 3000mm focal lengths. The Proline 16803 however is a beautiful match.

My understanding is that the 8300 is best suited to focal lengths around 1m and it would not suit the CDK17 at all.

Clearly a higher QE camera will outperform a lower QE camera but there are other factors that enter in. The 694 has very small well capacity so overflowing pixels especially on fast systems will mean stars will bloat easily. There are not many images on the net from a 694 but the few that are there have really large stars in them. There is a galaxy image on the Starlight Express site and another and both have distractingly badly oversized stars. So I don't know if this is operator error (poor focus, lousy scope) or shocking seeing (the small pixels will get kicked around more) or small wells of the chip? More images will make it clearer as it gets used. It does have great potential as a camera sensor. I am not sure how you got the 694 was $1200 more? Which 8300 camera were you comparing that to?(most are considerably more expensive than Starlight Express 694).

My understanding is that the anti-blooming mechanism just drains off charge when the wells fill up, so it doesn’t bloat the stars. The blooming you see is more likely to be due to a combination of the underlying PSFs of the smaller scopes that are often used with small pixel CCDs and the seeing.
The QHY9, Atik8300 and H18 are all considerably cheaper than the icx-694 cameras that have made it to market – this is an expensive chip.

I would disagree with your analysis that 10 inch is optimum. That does not match my experience at all. My CDK17 leaves my older BRC250 for dead for galaxies. All the very best galaxy images are from 20 inch RCOS scopes.

I don’t think 10inch is optimum – just that it will resolve as well as anything in my 2 arc sec seeing. It will not collect as many photons as a big scope, but a high QE detector can help make up for that.
the average RCOS20 inch scope is probably very well sited at high altitude and carried on a PME, so it will have a much better chance of high resolution than I will have. My scope will live in my backyard at sea level on a EQ8 (maybe). I would expect to be able to match the resolution of a 20 inch scope that is sited alongside, unless the seeing is very much better than average, but I do not expect to match a 20 inch scope at a high altitude site in the US desert.

Are you sure a 10 inch F4 is only slightly slower than a 12 inch F3.8? I find that hard to believe as it has something like 30% more light so why only less than 10% faster speed? There must be a calculation or assumption error there.

No error. The difference is that the 12 has a longer focal length than the 10 inch, so the number of photons that get into a pixel is reduced because the pixel has smaller angular size. In fact, the sensitivity of a 10 inch f4 is exactly the same as that of a 12 inch f4 – it is only because Mike’s scope has a lower f ratio that it is somewhat more sensitive than a 10 inch f4. The tradeoff though is that Mike’s system will be able to resolve more detail in very high quality seeing.

Its a good idea to quantify this but I think you still have to realise this is theory and it would be easy to overlook something and at the end of the day looking at others images and seeing what they used if its an image you like is based on the actual not the theoretical and therefore more likely to succeed. I think the conclusion should be more about which focal length is optimum for your area and you get the largest aperture you can afford to match that. There are plenty of good 10 inch GSO RC images around but they aren't as good as large aperture scope galaxy images.
Small well cameras are more susceptible to bloat on fast scope systems. So that would mean shorter subexposures to compensate which in turn is not ideal for faint galaxy imaging.

Not really. The wells on the 694 are 20,000 electrons, on the 8300 they are 25,000 electrons. The thing that matters is that the dynamic ranges of all of the systems considered are practically identical.

The seeing calculation refers more to focal length than aperture. Long focal length scopes need good seeing and larger pixels. Aperture is still king and having good seeing is still a very important factor (Martin Pughs Sierra mounted CDK17 images are about as good as you see anywhere).

My understanding is that seeing is an angular measure. Long focal lengths matched to large pixels and short focal lengths matched to small pixels will produce images with identical resolution when they are seeing limited. Martin’s wonderful images from the Sierra are clearly from exceptional seeing conditions and I would not expect a small scope to resolve as well as big one under those conditions.

The best 10 inch scope shots I have seen (not many were galaxy shots though) were from the discontinued RCOS 10 inch astrograph by Roth Ritter (google his name and you should get his site).

Thanks – will look this up.

I had a lot of fun with my Takahashi BRC250 and Microline 8300 though and it did work well for galaxies but as I say my Proline 16803 on CDK17 is a far better combo.

A light sensitive narrow field of view camera makes a lot of sense with a large aperture fast Newt design that tends to flex easily. I think Mike's setup will be hard to beat without seriously large and very expensive gear.

Agree that mechanical problems are the Achilles heel of Newtonians. However, I have been able to tame a couple of fast Newts over the past few years and I am fairly confident that I now know enough to get good results from them. Mike’s system will still work better than mine – the model shows that quite clearly.

10 inch is probably a tad small for galaxy work in my experience and you would be stretching it to get enough light no matter what F ratio configuration (F ratio myth, CCDs are linear in performance unlike film). At the end of the day its all about aperture and QE of the chips and matching pixel size to focal length and shooting with a setup that will perform in your local seeing and light pollution.

Yes, completely agree and that is what I have tried to do.