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Old 16-02-2013, 04:02 PM
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Galaxy imaging system design - which camera/scope to use

Hi

Have just completed the final stage of designing a high res system for imaging galaxies and am posting this summary in case anyone else is trying the same thing – would be interesting to compare notes, particularly on cameras.

An earlier study determined that there is vanishingly small resolution gain from a bigger scope once you get above about 10 inches aperture in my expected best seeing of 2 arc seconds (there will be more photons with a big aperture though, so bigger is still better). The study also showed that around 1 arc second per pixel was the optimum scale for best resolution plus maximum sensitivity in the chosen seeing. On that basis, I investigated three system options based on 10 inch scopes. Mike Sidonio recently posted details of his fast 12” scope and Sony 694 camera combo and that is also included for interest, since it was designed for the same purpose.

Available cameras divide into two groups – 9 micron and about 5 micron pixels. These match well with 10”/f8 and 10”/f4 scopes respectively, with the following combinations all having a little less than 1 arcsec/ pixel scale:
  • 10 inch f8 with Kodak 11002
  • 10 inch f4 with RCC1 and Sony 694
  • 10 inch f4 with Paracorr and Kodak 8300
  • Mike Sidonio’s 12” f3.8 with Sony 694 (included for interest)
The analysis included a fairly complete model of the responsivities of these systems, along with shot, read and thermal noise. The operating temperature was chosen to be -10C for the Sony and -20C for the Kodak chips.

The sensitivity results are presented in terms of the single exposure time required to obtain a broadband SNR = 5 with NGC247 as the extended target (nominal surface brightness of roughly mag 23 per square arcsec ). Other source and SNR levels could be chosen, but the results would be generally similar - except for much dimmer sources. Sky brightness has been ignored, as has noise from dark/flat processing, since this will depend on technique. Camera data came from the Starlight Express website and I estimated the broadband QEs (450-700nm) from their graphs.

The results are summarised in the attached spreadsheet capture and are as follows:
  • The most sensitive system is Mike’s 12” f3.8 with the 694 chip, which takes 3.2 minutes to get to an SNR of 5 on NGC247 – it also has the smallest field of view.
  • This is followed closely by the 10 inch f4 with the 694 chip – it can get to an SNR of 5 in 3.5 minutes, but also has a relatively small field of view.
  • The 10 inch f4 8300 system needs 5.6 minutes to the same SNR, but the field of view is significantly larger.
  • The 10 inch f8 11002 system has the largest field of view, but it needs 11.7 minutes to get to an SNR of 5, due mainly to the relatively high read noise and low QE of the CCD.
  • The dynamic ranges of the systems are similar and all have significant overload margins.
  • The 8300 system is easily the lowest in cost, the 694 system is $1200 more expensive and the 11002 system is about double the cost of the other two.

Any one of these systems could do the job of imaging galaxies – they all have about the same angular resolution and dynamic range, although Mike’s will provide better resolution in exceptionally good seeing.
Comparing the 10 inch systems that I am interested in, the 694 f4 system wins out over the 8300 f4 on the basis that a smallish field of view is OK to capture most galaxies and the extra cost of $1200 is a small enough price to pay for the extra productivity due to the very much shorter imaging times. The extra sensitivity will also bring fainter stuff within reach before the noise floor takes over. The 11002 f8 system is much less sensitive than the alternatives in this high resolution application, but it could still be considered where a large field of view was desired or if the alignment/correction/focus advantages of working at f8 were considered to be as important as imaging efficiency.

After going through this exercise, I think that a system modelling approach provides a sound basis for decision making, since it is possible to see what parameters matter for a given application and to investigate "what if" combinations of components. I have had enough of buying gear on an educated hunch, only to find after trying it out that it is not really ideal for what I wanted to do. I will be ordering most of the bits of an f4 system next week, so will find out just how good the system model is in capturing my requirements – provided I can get the real hardware to perform at design levels (in particular am hopeful that an RCC1 does not introduce anywhere near as much SA as an MPCC).

Thanks for reading.
Regards ray
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Last edited by Shiraz; 16-02-2013 at 04:56 PM.
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Old 16-02-2013, 05:09 PM
binofied
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Others may have varying opinions but I feel your worrying to much about a few of the less important aspects of the systems you describe. Yes it is important to match the sensor to get a sensible pixel scale. However you say nothing about optical design, image circle size, correctors, criticality of collimation, sky brightness (country vs suburb), quality of the mount needed for the different tube lengths and weights. All these things make a massive difference, more than being simply slightly over or undersampled.

For example you could be talking about a 10" F8 RC vs a 10" F4 newtonian. Totally different in almost every respect, different focal length, different corrections needed etc....

Your right to do at least some research on the pixel scale as this should be within sensible bounderies.

Dave
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Old 16-02-2013, 05:11 PM
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The spreadsheet doesn't come up when you click on it so I can't read it.

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

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.

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).

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. 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. 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.

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).

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).

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.

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.

Greg.
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Old 16-02-2013, 08:05 PM
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Quote:
Originally Posted by binofied View Post
Others may have varying opinions but I feel your worrying to much about a few of the less important aspects of the systems you describe. Yes it is important to match the sensor to get a sensible pixel scale. However you say nothing about optical design, image circle size, correctors, criticality of collimation, sky brightness (country vs suburb), quality of the mount needed for the different tube lengths and weights. All these things make a massive difference, more than being simply slightly over or undersampled.

For example you could be talking about a 10" F8 RC vs a 10" F4 newtonian. Totally different in almost every respect, different focal length, different corrections needed etc....

Your right to do at least some research on the pixel scale as this should be within sensible bounderies.

Dave
Thanks for the input Dave.

Now that I have decided on the most efficient imaging solution, the implementation details are practically self evident. I will be getting:
  • 250 f4 CF Newt
  • Moonlite digital focuser
  • RCC1 (or Paracorr if RCC1 does not work out)
  • OAG/filter wheel/694 camera combo
  • EQ8 (feeling brave)

regards Ray
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Old 16-02-2013, 08:17 PM
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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.

Last edited by Shiraz; 17-02-2013 at 02:08 PM. Reason: get Dave Rowe's name right
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Old 16-02-2013, 08:18 PM
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Hi Ray,

just want to say that an f4 system is not ideal for doing galaxy imaging. It is just too short in focal length. Personally I would forget that and look more at a system around f8. Probably an RC or a CDK is going to serve your purposes best. Use a camera with large wells sensors like the 11000 will give you great stars and allow nice deep imaging.

Anyway just my opinion and good luck with your selection.
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Old 16-02-2013, 08:25 PM
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10" @ f4 is going to make for pretty small galaxies surrounded by a lot of black.

At 9.25" @ f10 I find I sometimes wish for more mag and always wish for more aperture.
Something like the upcoming 16" RC might be a better choice.
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Old 16-02-2013, 10:09 PM
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Thanks for your reply Ray.

With the 694 which is in regular camera terms a micro four thirds chip so if it were in a DSLR it would be a 2X crop factor compared to a 35mm chip like STL11.

So 1 metre focal length with that chip gives you the equivalent of a 2 metre focal length in terms of field of view or crop factor. 2 metres is a reasonable focal length for most people's seeing and to get a reasinable image scale on the larger galaxies (the smaller ones will still be small).

The well depth issue is a bit vague and shows up in some images and not in others. It seems to vary with the setup, the sub length and the focal length.

What seems to happen is a bright star saturates those shallow wells easily. Then as the star has a spread function the next pixels out overflow then the next and then the next. I realise the overflow is taken away. I don't know how perfectly these anti blooming channels work or if the microlenses have light scatter as well.

So the effect is on a bright star the number of pixels out from the centre that overflowed is much higher on these small well chips than the massive wells of the 16803 type chips. Hence the bloated looking overexposed stars. A better way of putting it would be these chips overexpose bright stars more easily resulting in nasty looking halo effects like say a Horsehead image with Alnitak in the frame. I assume the dynamic range calculation is simply read noise divided into well depth and the assumption there is it will display a wide dynamic range because the read noise is so low. That's a different concept to overexposure. DSLRs dynamic range varies with ISO. Its usually best at lowest ISO and falls off as you boost the ISO.

Couple that with a fast F4 and you will probably need to do short exposures to prevent blowing out the highlights ie. the bright stars.

The Bunyip scope - 12 inch (12.5 inch?) Newt has shown tremendous images and tends to support your argument except it is 12.5 inches or so and I would contend 12 inches is more of a sweet spot for most people's imaging situations.

694 versus KAF8300 is a different argument. 694 has smaller pixels, higher QE, slightly lower noise, but even lower full well capacity (the main worry for me - it may pan out to be not a concern though). 694 is smaller than 8300 chip in size (I think).

On paper 694 may be a winner if the small wells turn out to be no worry.

Another point as well. I bet the QE of these chips varies with the angle of the light hitting them. If too sharp an angle performance drops off on most chips heavily. Hence the microlenses on DSLR chips.

I look forward to your results as it will be an interesting cutting edge system. If you go for 10 inch I would go for super high quality mirrors.
Mark Suchting? Are you making the scope?

I notice the latest incarnation of CDK has 6 or 7 fans. 3 or 4 at the back and 3 at the side (I think just in front of the primary, no doubt to get rid of the thermal layer on the mirror more efficiently. That would be worth looking at.

Greg.
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Old 17-02-2013, 09:49 AM
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Hi Ray,

just want to say that an f4 system is not ideal for doing galaxy imaging. It is just too short in focal length. Personally I would forget that and look more at a system around f8. Probably an RC or a CDK is going to serve your purposes best. Use a camera with large wells sensors like the 11000 will give you great stars and allow nice deep imaging.

Anyway just my opinion and good luck with your selection.
thanks very much for the input Paul. I am convinced that the small pixels compensate exactly for the short fl, so will continue down this path - maybe give you the opportunity to say @I told you so@ at a later date -

Quote:
Originally Posted by Poita View Post
10" @ f4 is going to make for pretty small galaxies surrounded by a lot of black.

At 9.25" @ f10 I find I sometimes wish for more mag and always wish for more aperture.


Something like the upcoming 16" RC might be a better choice.
thanks for the comments Peter. I put your system into the spreadsheet and, assuming you use 9 micron pixels, what I am proposing will have almost exactly the same image scale and resolution as you currently use - plus a bit more aperture. Since you feel a need for better resolution, I guess that my assumption of 2 arc sec seeing may be a bit pessimistic and I will investigate the effects of a slightly longer fl.

Quote:
Originally Posted by gregbradley View Post
Thanks for your reply Ray.

Hi Greg - thanks or the continuing discussion

With the 694 which is in regular camera terms a micro four thirds chip so if it were in a DSLR it would be a 2X crop factor compared to a 35mm chip like STL11.

So 1 metre focal length with that chip gives you the equivalent of a 2 metre focal length in terms of field of view or crop factor. 2 metres is a reasonable focal length for most people's seeing and to get a reasinable image scale on the larger galaxies (the smaller ones will still be small).

Yes, this is exactly the basis of the design - a 1m fl f4 system with 4.5 micron pixels works out to be geometrically equivalent to a 2m fl f8 system with 9 micron pixels. Werner posted an interesting image on IIS from a 10 inch f4 scope using one of the configurations I looked at - although the camera was an 8300 and not a 694, it gives some idea of how much resolution and image scale is available with small pixels on an f4 scope http://www.iceinspace.com.au/forum/s...d.php?t=103350

The well depth issue is a bit vague and shows up in some images and not in others. It seems to vary with the setup, the sub length and the focal length.

What seems to happen is a bright star saturates those shallow wells easily. Then as the star has a spread function the next pixels out overflow then the next and then the next. I realise the overflow is taken away. I don't know how perfectly these anti blooming channels work or if the microlenses have light scatter as well.

The other thing that may be at work here is crosstalk between pixels - unfortunately all of these issues are pretty much hidden from anyone buying a camera.

So the effect is on a bright star the number of pixels out from the centre that overflowed is much higher on these small well chips than the massive wells of the 16803 type chips. Hence the bloated looking overexposed stars. A better way of putting it would be these chips overexpose bright stars more easily resulting in nasty looking halo effects like say a Horsehead image with Alnitak in the frame. I assume the dynamic range calculation is simply read noise divided into well depth and the assumption there is it will display a wide dynamic range because the read noise is so low. That's a different concept to overexposure. DSLRs dynamic range varies with ISO. Its usually best at lowest ISO and falls off as you boost the ISO.

My understanding is that overexposure results when you run out of headroom. If you have a lot of noise you need to integrate longer to get a given SNR and you will run out of headroom on bright objects. If you only have a little bit of noise, you can get the given SNR without so much integration time, so are less likely to overexpose stars. the close tie up between noise and well depth is captured in the dynamic range - well depth by itself does not tell the whole story.

Couple that with a fast F4 and you will probably need to do short exposures to prevent blowing out the highlights ie. the bright stars.

I looked more critically at the SX website images - they are all taken with small scopes, so I think that the blooming effects are most likely due to the inherent PSF of a small aperture.

The Bunyip scope - 12 inch (12.5 inch?) Newt has shown tremendous images and tends to support your argument except it is 12.5 inches or so and I would contend 12 inches is more of a sweet spot for most people's imaging situations.



694 versus KAF8300 is a different argument. 694 has smaller pixels, higher QE, slightly lower noise, but even lower full well capacity (the main worry for me - it may pan out to be not a concern though). 694 is smaller than 8300 chip in size (I think).

On paper 694 may be a winner if the small wells turn out to be no worry.

Another point as well. I bet the QE of these chips varies with the angle of the light hitting them. If too sharp an angle performance drops off on most chips heavily. Hence the microlenses on DSLR chips.

could possibly be, but from what I have read, I don't think the light cone angle is much of a problem until you get to about f2

I look forward to your results as it will be an interesting cutting edge system. If you go for 10 inch I would go for super high quality mirrors.
Mark Suchting? Are you making the scope?

I will be modifying a commercial scope - I don't have a machine shop and modifying existing bits is a much more efficient option than trying to build something from scratch. And at my age, I may not have time to wait for bespoke optics - I think twice about green bananas .

I notice the latest incarnation of CDK has 6 or 7 fans. 3 or 4 at the back and 3 at the side (I think just in front of the primary, no doubt to get rid of the thermal layer on the mirror more efficiently. That would be worth looking at.

yes, I noticed that too. I have been toying with the idea of front face cooling on my current 12 inch Newt and it may be an option for the new scope. I recall reading a very interesting article from an old BAA journal on front surface forced air cooling that shows how effective it can be in getting rid of the boundary layer.

Greg.
comments in the above text


Regards ray

Last edited by Shiraz; 17-02-2013 at 10:09 AM.
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Old 17-02-2013, 11:04 AM
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thanks very much for the input Paul. I am convinced that the small pixels compensate exactly for the short fl, so will continue down this path - maybe give you the opportunity to say @I told you so@ at a later date -


I am not sure the sampling rate will be what you expect with short focal length and small pixels. Think of it like this; that with a longer focal length you will get more data for an object over more pixels. I recommend that using larger pixels with a longer focal length. Just like planetary imaging if you don't use a barlow or powermate you can only capture so much detail and can only resample to a certain extent. You will never get the same sort of detail that you would with a longer focal length scope. Just check out any galaxy which is reasonably distant (not the close ones) and go looking for images of that object via google. Then check out the scopes used. It will be uncommon to see short focal length scope capturing detailed images of those galaxies.
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Old 17-02-2013, 11:37 AM
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I am not sure the sampling rate will be what you expect with short focal length and small pixels. Think of it like this; that with a longer focal length you will get more data for an object over more pixels. I recommend that using larger pixels with a longer focal length. Just like planetary imaging if you don't use a barlow or powermate you can only capture so much detail and can only resample to a certain extent. You will never get the same sort of detail that you would with a longer focal length scope. Just check out any galaxy which is reasonably distant (not the close ones) and go looking for images of that object via google. Then check out the scopes used. It will be uncommon to see short focal length scope capturing detailed images of those galaxies.
The exception I can think of this Paul is David's 12.5 inch "Bunyip" Newt.
His NGC6744 and other images rival my CDK17 which is surprising but they do.

Greg.
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Old 17-02-2013, 11:45 AM
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The exception I can think of this Paul is David's 12.5 inch "Bunyip" Newt.
His NGC6744 and other images rival my CDK17 which is surprising but they do.

Greg.

I have been thinking about this recently, I too will be looking at a galaxy rig later in the year and I wonder if the reason these newtonians do so well is they can get away with a small (relatively) obstruction. Punching some numbers into newt I can make a 8 inch f8 newt with a 9% obstruction (by area) with a 100% zone large enough for my kaf8300 to be 200mm outside the tube, plenty of room for an AO unit. Not sure if I would need a corrector at f8 though. I always wonder if this would be better than a rc8.

If I dont bother with the extra room for an AO unit, and put in 55mm for camera instead of the 200mm I can get away with a 50mm secondary which is a 6% by area obstruction.
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Old 17-02-2013, 12:20 PM
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strongmanmike (Michael)
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Originally Posted by Paul Haese View Post
Just check out any galaxy which is reasonably distant (not the close ones) and go looking for images of that object via google. Then check out the scopes used. It will be uncommon to see short focal length scope capturing detailed images of those galaxies.
Yeh..? Not sure I completely agree. While I certainly wouldn't give one back should I be handed one these long FL scopes can't fit in the bigger objects so a time starved imager on a realistic budget is often looking for the holy grail compromise, so 1200mm FL with as much apperture as practicable and in average levels of seeing experienced in Oz, can be pretty much the sweet spot for most IMO .

150mm FL 1100mm FL

12" FL 1120mm

12" FL 1120mm

152mm FL1300mm

152mm FL1300mm
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Old 17-02-2013, 12:27 PM
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I tend to agree with you Mike. 1200mm focal length doing double duty with a small chip high QE for galaxy work is very practical.
I would add to that if possible a small wind profile to make it useable on more nights if windy. AP RHA 305 may well be the ideal telescope for the average imager.

Greg.


Quote:
Originally Posted by strongmanmike View Post
Yeh..? Not sure I completely agree. While I certainly wouldn't give one back should I be handed one these long FL scopes can't fit in the bigger objects so a time starved imager on a realistic budget is often looking for the holy grail compromise, so 1200mm FL with as much apperture as practicable and in average levels of seeing experienced in Oz, can be pretty much the sweet spot for most IMO .

150mm FL 1100mm FL

12" FL 1120mm

12" FL 1120mm

152mm FL1300mm

152mm FL1300mm
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  #15  
Old 17-02-2013, 12:34 PM
clive milne
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Hi Ray,
I'm in the process of doing pretty much the same thing.
The (equipment) selection process I have employed goes basically like this:

The equatorial mount I have at present has excellent drives but doesn't
have the payload capacity that I would like. A C14 is the largest aperture
telescope I could reasonably use. The short moment arm of schmidt cassegrain
OTA's means that they transfer less mechanical energy to the mount during
wind gusts and from the effects of stresses induced by cables. I subsequently
managed to pick up a C14 off
astromart for a pretty good price.

I have since ordered a 0.77x reducer corrector made by Phillip Keller which
has a plate scale of 14 microns per arc second on the C14.
A moonlight focuser with stepper motor was purchased second hand through
the IIS classifieds, and as it turned out required a 20 minute drive to pick up.
Just 2 inches to the left of my keyboard as I type this is the sharpsky digital
focusser controller kit I am assembling for the project.

The CCD camera I will be using is an ST10. The choice was pretty simple
in as much as I already own it, and even to this day the Kodak KAF-3200 is
still arguably the best front illuminated CCD out there. It has a similar QE to
the Sony 694 albeit more weighted towards the red end of the spectrum,
but 4 times the well depth. It also has very low noise. The one aspect of
the KAF-3200 that has seen it fall out of favour is that it simply doesn't have
the area coverage of the scientifically inferior 16803 (etc) chips. If you are
only interested in galaxies, field coverage isn't nearly as important as the
signal to noise ratio and overall quantum efficiency.

Putting it all together, I realised that AO would be desirable at 3m of focal
length but impracticable with the back focal length available (97mm) using SBIG equipment.
ST10+CFW8+A08 needs more back focus than 97mm and will rarely be usable
with colour filters.
ST10 + Orion Nautilus CFW + Orion steady star AO with built in off axis guider
and field rotator adds up to pretty much bang on the money.

~c
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Old 17-02-2013, 12:45 PM
strongmanmike's Avatar
strongmanmike (Michael)
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Quote:
Originally Posted by gregbradley View Post
I tend to agree with you Mike. 1200mm focal length doing double duty with a small chip high QE for galaxy work is very practical.
I would add to that if possible a small wind profile to make it useable on more nights if windy. AP RHA 305 may well be the ideal telescope for the average imager.

Greg.
Yes a wind break is necessary for sure .

Yes the AP RHA is certainly an excellent choice in thsi regard... but it is rather expensive compared to an equivalent fast Newt with the same specs (which when built and tweaked properly will likely out perform the RHA) plus it is notoriously hard to get your hands on one.

Mike
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Old 17-02-2013, 01:06 PM
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Quote:
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I am not sure the sampling rate will be what you expect with short focal length and small pixels. Think of it like this; that with a longer focal length you will get more data for an object over more pixels. I recommend that using larger pixels with a longer focal length. Just like planetary imaging if you don't use a barlow or powermate you can only capture so much detail and can only resample to a certain extent. You will never get the same sort of detail that you would with a longer focal length scope. Just check out any galaxy which is reasonably distant (not the close ones) and go looking for images of that object via google. Then check out the scopes used. It will be uncommon to see short focal length scope capturing detailed images of those galaxies.
thanks Paul. I know that long fl = hi res is the reigning philosophy, but I think that the new small pixel sensors with high QE open up an alternative way of getting the same results. The same sort of thing starting to happen in the planetary imaging field where pixels are getting smaller. We only use f25 because we have 5.6micron pixels. the next generation of sensors with around about 3 micron pixels will only require f15 or so to get the same pixel scale and if we get to 2 microns, you will not need a Barlow at all. It is the same in DSO imaging. That is not to say that the new approach is any better in principle, just that it opens up an alternative way of doing things. At least that is the way I understand it. Interesting times. Regards Ray

Quote:
Originally Posted by Peter.M View Post
I have been thinking about this recently, I too will be looking at a galaxy rig later in the year and I wonder if the reason these newtonians do so well is they can get away with a small (relatively) obstruction. Punching some numbers into newt I can make a 8 inch f8 newt with a 9% obstruction (by area) with a 100% zone large enough for my kaf8300 to be 200mm outside the tube, plenty of room for an AO unit. Not sure if I would need a corrector at f8 though. I always wonder if this would be better than a rc8.

If I dont bother with the extra room for an AO unit, and put in 55mm for camera instead of the 200mm I can get away with a 50mm secondary which is a 6% by area obstruction.
Hi Peter. FWIW, my understanding is that the resolution of a 200mm scope is so much better than the average atmosphere (maybe 2 arc sec or worse), that things like obstruction have little effect on DSO imaging results - the atmosphere dominates to the extent that even systems like the Riccardi Honders work OK with extreme obstruction. I haven't done enough work to be sure of this yet, but I think that the only thing that can make a mess of things is bad SA - but even that can be acceptable as is shown by the number of successful systems using the MPCC, which is reported to probably introduce about 0.7 wave of SA.

Suggest that a 200 f8 system with the 8300 might be slight overkill, giving about 0.7 arcsec per pixel. f6 might be better, giving >0.9 arc sec per pix and nearly doubling the sensitivity in the process. would be a nice system at either focal ratio.

regard Ray
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Old 17-02-2013, 01:29 PM
clive milne
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Ray,
The RCC1 was designed by Dave Rowe (not Jim Rowe)
He did indeed design the optical configuration for a number of CDK's including
those made by Planewave, Hubble optics, The Alt-Az initiative
and also one or two custom jobs, all them ex-gratis to my understanding.

As for the RCC1, it does not introduce spherical aberration to the wavefront
when used in conjunction with a paraboloid. It has something in the order of
100mm back focal length so it has significant advantages with respect to the
equipment you can fit in the imaging train.

As for the bad write up on IIS... all I can say is that the RCC1 design is
quite a bit better than the MPCC1, so if results were obtained that didn't
reflect this, then the problem is most likely the result of incorrect mechanical
assembly either due to spacing of the focal plane, or customs have
inspected it by pulling it apart and putting it back together ass-about.

Also... it is not necessary to preserve diffraction limited performance for
an imaging telescope unless you are talking about long focal ratios designed to
operate at the diffraction limit, such as the HST F/D=24.
It is only at that level of over-sampling at the focal plane that you will
notice 1/2 wave of SA or the effects of a 50% central obstruction.

There is a reason why planetary imagers use powermates.

At F5 or so, you can get away with a level of aberration that you simply wouldn't tolerate for a visual instrument.

best
~c

Quote:
Originally Posted by Shiraz View Post
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 Jim 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
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Old 17-02-2013, 01:39 PM
clive milne
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Ahh.. also I forgot to add wrt the RCC1 corrector.

If you would like more detailed information with respect to its performance,
vignetting, ghosting, ray trace results, etc) There is no better source than
from the horses mouth.

Here's Dave's email address:
(redacted for privacy PM me if you need it)
He's a pretty approachable guy so I'm sure he would be glad to help.

Last edited by clive milne; 17-02-2013 at 01:51 PM.
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Old 17-02-2013, 01:43 PM
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Shiraz (Ray)
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Quote:
Originally Posted by strongmanmike View Post
Yeh..? Not sure I completely agree. While I certainly wouldn't give one back should I be handed one these long FL scopes can't fit in the bigger objects so a time starved imager on a realistic budget is often looking for the holy grail compromise, so 1200mm FL with as much apperture as practicable and in average levels of seeing experienced in Oz, can be pretty much the sweet spot for most IMO .

150mm FL 1100mm FL

12" FL 1120mm

12" FL 1120mm

152mm FL1300mm

152mm FL1300mm
Wow Mike, that is an impressive set of images. Thanks for posting them. regards ray

Quote:
Originally Posted by clive milne View Post
Hi Ray,
I'm in the process of doing pretty much the same thing.
The (equipment) selection process I have employed goes basically like this:

The equatorial mount I have at present has excellent drives but doesn't
have the payload capacity that I would like. A C14 is the largest aperture
telescope I could reasonably use. The short moment arm of schmidt cassegrain
OTA's means that they transfer less mechanical energy to the mount during
wind gusts and from the effects of stresses induced by cables. I subsequently
managed to pick up a C14 off
astromart for a pretty good price.

I have since ordered a 0.77x reducer corrector made by Phillip Keller which
has a plate scale of 14 microns per arc second on the C14.
A moonlight focuser with stepper motor was purchased second hand through
the IIS classifieds, and as it turned out required a 20 minute drive to pick up.
Just 2 inches to the left of my keyboard as I type this is the sharpsky digital
focusser controller kit I am assembling for the project.

The CCD camera I will be using is an ST10. The choice was pretty simple
in as much as I already own it, and even to this day the Kodak KAF-3200 is
still arguably the best front illuminated CCD out there. It has a similar QE to
the Sony 694 albeit more weighted towards the red end of the spectrum,
but 4 times the well depth. It also has very low noise. The one aspect of
the KAF-3200 that has seen it fall out of favour is that it simply doesn't have
the area coverage of the scientifically inferior 16803 (etc) chips. If you are
only interested in galaxies, field coverage isn't nearly as important as the
signal to noise ratio and overall quantum efficiency.

Putting it all together, I realised that AO would be desirable at 3m of focal
length but impracticable with the back focal length available (97mm) using SBIG equipment.
ST10+CFW8+A08 needs more back focus than 97mm and will rarely be usable
with colour filters.
ST10 + Orion Nautilus CFW + Orion steady star AO with built in off axis guider
and field rotator adds up to pretty much bang on the money.

~c
Thanks for the info Clive - you are using a similar set of priorities in the design, with an emphasis on high QE and optimised sensitivity/scale. What sort of seeing do you expect? It's the main design input parameter and all I have been able to do is guess what it might be here - maybe I will get some better idea after I get the system going. I regularly watch planets jumping around and distorting by many arc sec when doing planetary imaging, so it's not likely to be very good much of the time.

the 3200 was way ahead of its time - you are lucky to have one.
I also have one of Dave's focusers, what a nice bit of work it is - it just worked as advertised from day 1 - I haven't calibrated the temp compensation yet, but even that seems to be perfectly functional.

Thought about AO, but it was a pretty short thought with an f4 system The RCC1 would make it a bit easier, but still just beyond the realm of the doable.

Really looking forward to seeing how your system performs.

and thanks very much for Dave's email. I now have a copy, so you can take it off the post if you wish.

Regards ray
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