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Old 17-02-2019, 07:03 PM
RyanJones
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Guide scope options

Hi all,

I've just aquired a C9.25 SCT with the help of another IIS member Carlton.

I plan to start my guiding adventures on this scope because at a focal length of 2350mm, I don't think it's worth not doing. And hey, I've got to start guiding sometime.

I have a Nexguide auto guider ( I'm well aware there are better options but this is what I have ) .

The question is;

I have a 102mm refractor in my collection already but would need to find some reasonably priced guide rings big enough for it ( it appears that size isn't all that common ) and there's the additional weight of a full sized scope. It does have the benefit of a reasonable focal length of 580mm.

OR

Do I buy a small guide scope ? I can get an 80 / 400 for $99 pp and a pair of guide rings for $45. Smaller, lighter, and easier to get BUT quite a bit shorter focal length and less apature.

Your thoughts ?????


Cheers

Ryan
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Old 18-02-2019, 06:11 AM
kens (Ken)
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For 2350mm focal length SCT you should use an Off Axis Guider. Otherwise you'll be battling differential flexure issues.
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Old 18-02-2019, 07:15 AM
tvandoore (Tim)
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Theres a TS optics thin OAG for sale in the trade section, could be worth a look.
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Old 18-02-2019, 08:42 AM
RyanJones
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Thank you for your input guys. I have read about OAGs on these OTAs on other forums and they are saying that it's a real pain to get it working and most have given up and gone to a guide scope. That's why I had discounted it as an option. I personally thought that this would be the best because obviously it's seeing what the camera is seeing.
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Old 18-02-2019, 10:44 AM
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traveller (Bo)
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Just a word of warning, guiding at that FL is very demanding on your mount.
You need excellent polar alignment plus a finely tuned mount of 1" or under.
I'd be advising you to try at short FL first ~500mm or so to get the hang of guiding first before plunging into 2000mm territory.
Cheers
Bo
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Old 18-02-2019, 11:34 AM
RyanJones
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Quote:
Originally Posted by traveller View Post
Just a word of warning, guiding at that FL is very demanding on your mount.
You need excellent polar alignment plus a finely tuned mount of 1" or under.
I'd be advising you to try at short FL first ~500mm or so to get the hang of guiding first before plunging into 2000mm territory.
Cheers
Bo
Thank you for your input Bo. I generally spend a fair bit of time getting my polar alignment as close as I can. When you say a finely tuned mount of under 1". What do you mean by that ? Are we talking a polar alignment accuracy of under 1" ? I have a couple of OTAs which much shorter FL that I can learn with first. Then I have the C5 that's 1250 I can step up to so that's good advice that I'll take. Thank you
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Old 18-02-2019, 12:00 PM
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traveller (Bo)
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Hi Ryan,
There is a formula to follow to obtain good match with your telescope and camera and therefore the demand for good guiding.
Your SCT and 350D combo will result in 0.56"/pixel, a very demanding resolution on your mount.
See this link for more info https://astronomy.tools/calculators/ccd_suitability
Cheers
Bo
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Old 18-02-2019, 04:13 PM
RyanJones
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Quote:
Originally Posted by traveller View Post
Hi Ryan,
There is a formula to follow to obtain good match with your telescope and camera and therefore the demand for good guiding.
Your SCT and 350D combo will result in 0.56"/pixel, a very demanding resolution on your mount.
See this link for more info https://astronomy.tools/calculators/ccd_suitability
Cheers
Bo
Thanks for the link Bo. Interesting reading.

Given that my setup with this particular OTA isn't ideal, I will practice on my newt first. With the C9.25 , I have to accept it will have its challenges. As you mentioned before, I will try to learn guiding before expecting it to work with the 9.25. This returns us back to the original question then is what guide setup I should use. Will my 102mm refractor be good ? Should I get a smaller 80 / 400 setup ? Or as Ken suggested should I scrap those ideas and go for an OAG ?
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Old 18-02-2019, 07:32 PM
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The_bluester (Paul)
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I am using an Orion Thin OAG on my C925 with a 0.63 reducer on an Orion Atlas Pro (AZEQ6 painted black) The challenge is getting the spacing just so with the reducer so that you can get decent guide stars and the imaging and guiding cams in focus together.



You also need a good guide camera, I am using an ASI290mm mini. I tried with an old Orion SSAG guide camera I had but it just was not sensitive enough so I sold that to another IIS'er with an Orion ST80 scope as a guider package. Hopefully it is doing the trick for him.



Even with my reducer and current spacing which gives about 1500mm focal length getting the guiding happening nicely is a challenge. I am still working on it after a couple of weeks, just chipping away and improving it bit by bit. But the mirror shift inherent in the SCT is going to make a guidescope a frustrating thing.
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Old 18-02-2019, 10:05 PM
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traveller (Bo)
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Quote:
Originally Posted by RyanJones View Post
Thanks for the link Bo. Interesting reading.

Given that my setup with this particular OTA isn't ideal, I will practice on my newt first. With the C9.25 , I have to accept it will have its challenges. As you mentioned before, I will try to learn guiding before expecting it to work with the 9.25. This returns us back to the original question then is what guide setup I should use. Will my 102mm refractor be good ? Should I get a smaller 80 / 400 setup ? Or as Ken suggested should I scrap those ideas and go for an OAG ?
Rule of thumb is you can do a guide scope for under 1000mm FL. I have a 50 mm guide scope (modified from a straight through finder) with my 770mm refractor and it works pretty well.
Once you go over 1000mm, then an OAG will be better because if differential flexiture etc.
Your 120mm f5 will work well with a guide scope given its 600mm FL. Being an achromat, you’d be expecting significant fringing, esp at f5. So it will be a test scope to fine tune your guiding and you won’t be able to image with it.
An OAG can be quite fiddly to set up as Paul said, plus you will need a sensitive guide cam due to dimness of stars caused by the prism and restricted number of stars to guide with.
My suggestion (and a suggestion only) is to get a 70-80mm doublet (or triplet if budget allows) and start with a guide scope to get the hang of guiding. A lot of DSOs are surprisingly large so you can get plenty of shots at around 500mm FL, plus it will be a good match to your 350D.
Hope that helps.
Bo
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Old 18-02-2019, 10:47 PM
RyanJones
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It does help thank you Bo. Thank you for all your information. It's funny because I've been discussing which way I'm going to with my missus. I said " what's the bet I'll be told that I need both a guide scope for my Newt and an OAG for my SCTs " more gear......

I really appreciate yours and everyone else's input. Thank you all
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