[Shiraz (Ray)]

Hi

Have recently completed a detailed design exercise for a hi res imaging system. One thing that became clear during that process was how interesting the Sony icx694 CCD is. There has been a lot of discussion here and elsewhere on this chip, so this is a summary of where I think it might fit in, based on the results of the modelling. I have also ordered a 694 camera and will be very interested to see how it performs in real life.

The icx694 is a capable 6 megapixel CCD, with an absolute QE of >0.75 peak, very low read and thermal noise and a full 12 bits dynamic range. It is a relatively small chip with small pixels and a combination of characteristics that requires somewhat different design assumptions from those used in the past.

First off, you cannot just bolt one on the back of your existing scope and expect it to be more sensitive than your big pixel Kodak CCD – it won’t be. The small size (4.5microns) of the Sony pixels means that, although the resolution may be better, the signal levels will not – the combination of a given scope and the 694 will be roughly 1/3 as sensitive as the same scope with a typical 9 micron sensor. Expect to see posts along the lines of “the 694 is NOT very sensitive” when people do a simple camera comparison without any consideration of pixel scale….

If you really want to tap into the extra sensitivity of the chip, you need to use a scope with a shorter focal length so that you get an appropriate pixel scale – then the extra sensitivity and low noise will enable you to reduce imaging times significantly (by roughly half at Ha) with the same aperture and resolution. The conventional wisdom that hi res imaging requires a long focal length scope does not apply, since the 694 only needs a medium focal length scope to obtain seeing limited performance. For example, to get under 1 arcsec /pixel sampling with a K11002 requires 2m focal length (eg 10” f8) – the same sampling is available from the 694 at 1m focal length (eg 10” f4). In fact, in Australian seeing, it probably does not make much sense to consider a scope of much greater than about 1.2m fl for any purpose with this chip –with a fast scope at this focal length, you will reach seeing limited resolution in all but exceptional conditions.

So what will you get from the 694? In a nutshell, high resolution and high sensitivity with a moderate field of view from a short to medium fl scope. This chip makes a lot of sense if you currently use a well corrected fast Newtonian or APO for wide field imaging. With a 694 camera, you can switch your wide-field scope over to hi res imaging if the seeing is good. This chip is up to twice as sensitive to Ha as most of the alternatives and has lower noise, so use it with APOs (particularly the fast Petzvals) and fast Newtonian scopes to produce hi res narrow band images with very little noise. The new opportunities for most short scope owners look to be significant and exciting.

The 694 may also help a bit with cost. Although 694 cameras are relatively expensive, you may save on the associated equipment - filters/wheels for example may be less costly and bulky at 1.25”. The chip does not need extensive cooling, a shutter or RBI flooding, so cameras can be lightweight, allowing lighter scopes/focusers to carry them. Since everything can be lightweight, it may be possible to use a less expensive mount. In addition, if you can use one scope for both wide-field and hi res imaging by just changing cameras, the cost savings may be even more significant.

In summary then, if you have already invested in a high res imaging system based on a longer fl scope and a camera with large pixels, don’t bother with the 694 – it is not suitable for your scope. Similarly, the 694 is not a good choice if you want to take panoramic images of wide swathes of sky. However, if you have a well corrected fast Newtonian or a small to medium APO and you want to image smaller nebulae and globulars, galaxies or planetary nebulae, then the 694 offers a new capability to resolve fine detail. It could also be suitable for solar and lunar imaging. The Sony icx285 and Kodak KAF8300 went some way down the short scope path, but the icx694 gives even higher resolution and excellent SNR performance. It is definitely worth a close look for a lot of amateur astronomers.

Thanks for reading. Regards ray