Maximising field-of-view with an RC/large sensor combo - Forums

Topic starter

Jul 15, 2021

I have just purchased a full frame mono sensor ZWO ASI 6200MM Pro. I had intended to use it mainly with my Esprit 100 which has an imaging circle that covers the full frame.

However, I would also like to use it with my GSO RC8. Unfortunately the (uncorrected) image circle of the GSO RC8 is much smaller than the full frame of the 6200MM, due to the field curvature inherent in the RC design.

Normally I take images with my RC8 using a 0.67x focal reducer (the CCD47) giving me a focal length of 1080mm, but I discover that the image circle for round stars is 15mm or less - wasting a large fraction of the 35 x 24mm sensor.

What are my options to use more of the sensor. Ideally I would like to stay at a focal length of around 800-1200mm, or a image scale which gives me no more than 3-4pixels FWHM my stellar profile under the 3-4arcsec seeing I enjoy at best.

1) Get a better focal reducer. I have seen claims that the Astrophysics CCDT67 0.67x focal reducer gives "tack sharp" images out to a 24mm imaging circle. But I have also seem claims that the CCD47 (the reducer I currently have) is an identical design to the CCDT67.

2) Go for a corrector without reduction. This would put me back at 1600mm focal length, and I would have to Bin x 2 to better match my seeing. Although this reduces by a factor of 4 the field covered by sensor, this would be OK if the image quality across the full field was better. There is a TS 2inch RC corrector which claims a corrected field up to 44mm.

3) Go for a corrector/reducer combo. I have seen 2.5inch and greater RC reducer/flatener combos and while this would be ideal, it would mean investing in a new focuser. At which point I would also start to worry about the weight hanging off the back of my RC.

4) Give up on the RC and get a CDK. Perhaps I am simply pushing the field size of the RC too much, and I would be better off just to accept what I currently have. If I really want a wide field in this type of telescope design (and I think the RC has much to recommend it), I need to invest much more in a more sophisticated telescope design tailored for widefield.

Options three and four are expensive (but so was the ASI6200!) and I would probably look to options 1 or 2 first - but only if they were effective.

Grateful for any thoughts or experiences that my AB friends might have had with this, or similar, kit.

With the increasing availability of large sensors, I am sure that there are other people out there trying to push the image circle of their scopes to match.

Many thanks in advance and clear skies to all

Brian

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andrea tasselli

Frequent contributor

Jul 15, 2021

In terms of absolute performance the best results is for the field corrector alone, no reduction and bin the sensor as needed. This is most effective solution of your troubles (for a host of optical reasons). The only better option is when the design include both reducer and corrector such as some like the modified Dall-Kirkham or optimised RC or even Rumak/Simak Maksutov-Cassegrain but obviously this will require a new scope and I doubt that they exist in anything smaller than 10". I have used a RC (larger than yours) with the TS field flattener and full frame sensor and I couldn't see but a hint of astigmatic field curvature at the corners.

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Brian Boyle

Topic starter

Jul 15, 2021

Andrea, That is a really helpful and encouraging response. Great to hear of your success with the Field Flattener. Brian

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Alan Brunelle

Jul 16, 2021

I have little experience with the RC design, but have been looking to go in that direction. As such, from what I gather with most of the commonly available RCs is that the sensor you want to use may be a bit large for the designed image circle of the 8 inch (corrected). No experience there, just what I have seen posted on usable image circle for the different RCs. So you may find that there is no corrector or combo that will work all the way to the edges of your field of view.

Having said that, if just the corrector with no reducer works for you, I cannot imagine that binning would be the worst thing. Other than a better match for your seeing, and likely spot size for you 8 inch, the reduction in the size of the sub files generated has to be a benefit as far as workup and processing time… I have heard others on this site state that the full res subs from these cameras can put major strain on their computers and storage. Since you already use the camera on another setup, I will assume you have that aspect all under control. I use the C-sized version of the same chip and the file size is ok for my computing rig. But it still is an effort!

Brian Boyle

Topic starter

Jul 16, 2021

Thanks, Alan. I suspect you may be correct in suggesting that no flattener will correct over a full frame, but Andrea's experience is encouraging. Even if I don't get the full frame corrected - it sounds like correcting and not reducing is this way to go. That is, I gain more field-of-view correcting, than I lose field-of-view bt not reducing. Fortunately my CPU and disks handle the data going off the ASI6200MM OK. Yes it takes a little longer, but I make heavy use of the Batch Pre-processing routine in PI, which I can set running and come back to later.

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John Hayes

Frequent contributor

Jul 16, 2021

I completely agree with Andrea that going for a field flattener without a reducer is the way to go. The trick is to find the right field corrector that matches your scope. You are running into the main reason that I gave up on RCs and went with a CDK. RCs are great for a lot of applications but without a field flattener, wide field isn't one of them.

John

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Brian Boyle

Topic starter

Jul 16, 2021

John, thanks for the confirmation about the correct route. I will go ahead with the corrector - and bring back the results to this forum.

You make a great point about the RC/CDK comparison. If I were to make a "next step" in this game, it would be to CDK. If I had the money, I would likely be a Planeware CDK12.5 with 0.66x focal reducer. Would be interested to hear your thoughts on that direction.

Of course, it would also mean rebuilding my entire rig from group up; mount, focuser - and possibly even observatory so it could fit. But still - its nice to dream.

Brian

Ross Salinger

Jul 22, 2021

It's almost impossible to use the .66 PW reducer with the 12.5" CDK. There's just not enough back focus if you are using a mono camera, filter wheel, and off axis guider. Take a look carefully at the PW website for more about this. I'm considering junking my mono camera so that I can get rid of the wheel and get within the back focus requirement to use the reducer on my scope.

Rgrds-Ross

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wesm

Apr 25, 2023

Brian Boyle:
However, I would also like to use it with my GSO RC8. Unfortunately the (uncorrected) image circle of the GSO RC8 is much smaller than the full frame of the 6200MM, due to the field curvature inherent in the RC design.

What is the diameter of the uncorrected image circle? I have the same scope, just branded by TPO, and have difficulty covering all of a micro 4/3 sensor (ASI294MM).
Thanks

YingtianZZZ

Aug 17, 2024

I read and do with combining 6200MM with a Hotech flatenner for reducer, and it works well on my 8'' RC. Now I'm starting to wonder will CCDT67 will happen to also work with this flatenner, but I don't know where to add it now.

Wei-Hao Wang

Aug 17, 2024

I have a CFF 12" RC with CFF's dedicated flattener. I used it with a 44mm*33mm sensor. Based on the result, I would say this combo should work perfectly on FF.

No matter flattener or reducer, the key is for these rear refractive elements to cancel out the field curvature of the RC. If the manufacture of the RC also produces a reducer or flattener that's matched to the scope's field curvature (like the case for CFF), then this is usually not an issue. The problem is the difficulty to find a matched reducer or flattener. There are several generic reducers or flatteners in the market, and their specifications are all very vague about the match of field curvature. It's very hard to know which one works on which RC.

One thing that's important is that the level of correction of a reducer or flattener can be changed by adjusting the back focal distance. For example, you don't feel your CCD47 give you round stars outside 15mm circle. This may be simply because the back focus isn't optimal. If you can find ways of adjusting the back focus and test, you may be able to get nice stars. Or perhaps there is a limit of how far you can push. Also, even if changing the back focus can improve the stars, if the reducer is too small, you will still be limited by vignetting.

If I were you, I probably would look for a reducer with sufficiently large clear aperture, and whose manufacture claims that it works well on FF and RC. Then prepare series of spacers to try different back focus (starting from the back focus recommended by the manufacture). This should give you the best odd of finding a working reducer while minimizing the cost.

Of course, switching to a CDK would be even better, but it costs much more than a reducer.

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