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Old 17-02-2019, 08:07 AM
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The_bluester (Paul)
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Coma in 8" F4 newt

I am in the middle of deciding on a direction to go in for an imaging OTA. The non edge HD C925 has it's limitations as I expected.



Without heading down the road of the standard 80mm frac as a starting point, I am looking to gain a wider FOV. Currently and budget wise well into the future I am using an ASI294 Pro cooled OSC and I am happy with that.


Does anyone have any figures on what the well corrected image circle is for something like the Bintel branded F4 8"? If I can cover a 24mm circle without appreciable coma or other aberrations it would make a good starting point, otherwise a coma correctior needs to be added to the price of admission.

Last edited by The_bluester; 17-02-2019 at 11:23 AM.
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Old 17-02-2019, 10:29 AM
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codemonkey (Lee)
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At F4 you're definitely going to want a coma corrector, and it gets tricky at relatively short focal lengths too. The TeleVue Paracorr Type 2 is a highly regarded (perhaps the most highly regarded) corrector and even it is documented as introducing curvature at focal lengths below 1200mm.

I suspect this was the cause of poor performance off-axis in my 8" F4.5 when combined with the older Paracorr (and also the TS Optics corrector)... I ended up going down to a tiny sensor (IMX 178) to solve my issues.
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Old 17-02-2019, 10:47 AM
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Shiraz (Ray)
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Good advice from Lee.

here is an old image taken through a GSO 8 inch f4 without CC. In this seeing, coma is readily visible at less than 10 arcminutes from the centre.

Also agree that getting a coma corrected fast Newtonian to work properly can be a headache, particularly on a bendy OTA like the GSO. Maybe a CC corrected f5 Newtonian could be a slightly better bet - could be a good match to the pixel size of your 294 and with a good CC it could be effectively free of major aberrations.

Cheers Ray
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Old 17-02-2019, 11:26 AM
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The_bluester (Paul)
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At the moment what I am really doing is assessing options between spending money on my SCT to overcome it's built in issues (Focuser being the biggest) or lashing out on a different OTA altogether.

The current running is spend a fortune on a Moonlite 2.5" SCT focuser as that allows me to fit the SCT reducer/corrector inside the drawtube to keep the spacing where I want it, or throw the SCT overboard as an imaging tube and start again.
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Old 17-02-2019, 12:00 PM
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Shiraz (Ray)
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Difficult decision, but bear in mind that you will need to do a fair bit to a Newtonian as well - including a Moonlite or better focuser in addition to the CC.
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Old 17-02-2019, 12:11 PM
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ChrisV (Chris)
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+1 on the newt. You'd be getting a coma corrector and probably considering upgrading the focuser eventually. That's what I did anyway. And I'm very happy with it. I've got an f5 though

Last edited by ChrisV; 17-02-2019 at 12:50 PM.
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Old 17-02-2019, 01:33 PM
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I really am torn on which direction to go. Probably what I will do is sit tight for the moment and work on getting it all going as sweetly as I can with what I have and go from there.
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Old 19-02-2019, 04:51 PM
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Camelopardalis (Dunk)
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Coming from an SCT, collimating an f/4 newt is a horror story

I took some test shots last night with my ASI290 without the coma corrector, and even on that tiny sensor there was noticeable misshapen stars out towards the edges...besides my collimation issues
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Old 19-02-2019, 08:24 PM
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I am actually in a wait and see mode to see what reports start to come in about the Celestron 8" RASA scopes. On paper at least they are pretty close to something you might call made for my camera. I am curious to see if the focuser on them is a big improvement on the old C925.



Having sold my ST80 as a guider package I would have to buy another guidescope to use, but you never know.


I am toying with refitting a stock focuser to the C925 and getting the Celestron focus motor, at least it would be a known quantity fitment wise, built for the job so to speak.
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