View Full Version here: : Decisions... Used C9.25 or new CF 8" f4???
09-07-2012, 12:36 PM
Wanting a bit of help in making a decision on what way to go... :help:
I was settled on getting a new GSO cf 8" f4 ota but now I have the opportunity to get hold of a used C9.25 with 0.63 reducer.
The GSO will cost around $950 with CC plus a few $$$ for collimation tools, etc.
The C9.25 I can probably get around $1200 - $1500 with 0.63 reducer, CG5 and some TV 6 to 12mm EPs and a Meade 5000 24mm EP.
I would sell the CG5 straight away as I'll have a iEQ45 to use.
Soooo, knowing the strengths and weaknesses of both, what would be a better buy?
My main use will be an 'allrounder' for now, viewing lunar and planets, also imaging planetary and DSO.
My lil girl is fascinated with the night sky so for viewing, the C9.25 will be good there.
But I want to get into DSO imaging more down the track, so the GSO would probably be better there.
Now with the C9.25, its a few years old (early 2000's I think) the previous owner has been meticulous with it, but he did say that he removed the corrector plate to clean and may not have put it back with the same rotation. If not and there are optical issues, can this be resolved, if so, how?
Also, I don't know if this has the XLT coatings, is this a big issue?
So, the C9.25 sounds like a good buy, especially with the extras it comes with, but considering the above paragraph, and my overall intentions for use, should I go with it, or look at the GSO as first intended?? :shrug:
All helpful information and experiences are gratefully accepted.
09-07-2012, 05:35 PM
I have an older 8 inch f4 and would not recommend this class of scope as an all rounder - good scopes, but they can be difficult to get working properly and you and your child will find it a pain to use on an EQ mount.
suggest an inspection and star test of the C925 to make sure it has no major faults. if OK, should be a good all rounder for visual, very good for planetary imaging and OK for some DSO with the reducer. looks like a very good buy with a mount and good EPs as well.
09-07-2012, 08:22 PM
Thanks for your response.
I was on a similar thought for ease of use.
I've owned mak-cass's and refractors before, so Im used to 'grab-n-go' type scopes, but being a 'techie' guy I would enjoy the challenge of a fast newt. I was looking at a Farpoint coll kit and autocollimator to keep the optics in line if I went this way.
I know the C9.25 would give more wow factor for my lil girl with viewing, but Im thinking more long term, wanting to be more imaging focused, I still feel this little niggling feeling that a newt may give me better results (especially dso).
I guess, those that have owned fast newts for a while (like yourself Shiraz) do you wish you could skip the whole collimation routine every time you use your scope, or are you so impressed with the scopes performance, a little longer in setup isn't such an issue.
And those with SCT's, do you feel you would get better performance (imaging wise) out of a newt to sacrifice a bit of focal length.
If my pockets were deep, Id get both, but a young family and a trigger happy handbrake ;) prevents me... for now :D
Anyway, as suggested, I am trying to arrange getting hold of the C9.25 for a few weeks to really have a good 'test drive'. But if, as I suspect, the corrector lens has been inadvertently rotated, is there a 'how to' on how to resolve this? I will do a star test and correct collimation if necessary if I get my hands on it.
10-07-2012, 10:20 AM
Its not just initial collimation with the f4 - that is just routine after a while.
Some other problems are:
1. the focal depth at f4 is about 40 microns. that means that the mechanical system (focuser etc) has to keep to about that tolerance all over the focal plane. you will need to replace the standard focuser since it will almost certainly sag way out of tolerance with a normal camera and give you odd shaped stars across the field. You will also need a camera with the focal plane exactly aligned with the camera body - my QHY8 was not well enough aligned at f4. Even with an aligned camera, slight misalignment in the locking collar will throw it off enough to slightly tilt the focal plane and distort the stars.
2. you will need a coma corrector and the tolerances on placement and alignment of that element are very tight. the standard focuser is not good enough to keep an MPCC in the right place and you also might be unlucky and get an MPCC with slight misalignment built in - I think mine is off a little bit, but I haven't yet devised a test for this. The MPCC also requires that the collimation be spot on - if the MPCC is not on the centre of the optics, it will distort the stars in very odd ways.
3. the standard GSO mirror mounting systems do not keep good collimation at f4 when you move to a different part of the sky. It is not just collimating once a night, you really need to recollimate if you change to a different target or even if you image for a long period: camera off and warm up, take it out, recollimate, replace camera and hope collar has locked properly, cool camera again and hope no fogging, refocus, possibly redo PHD calibration.
4. the short focal length means that you have significant field curvature - the MPCC does not do so well with coma correction under these conditions and you end up with some coma creeping back in as well as slightly unfocused stars at the extremities of an APS sized chip.
5. low cost eyepieces tend to run out of steam before they get to f4 - you will probably need to buy some that are designed for fast optics (read are expensive) if you want to do visual observing - or buy a good quality Barlow.
Having said all this, f4 is really fast and I still like my scope for imaging wide field DSOs.....mostly. But I would not recommend it as an all-rounder.
10-07-2012, 12:28 PM
You also have the option of going for a Hyperstar on the 9.25 if you want to try some wide field stuff down the track.
Just so you have an idea of the field of view through a c9.25" with an F.63 reducer, here is an image of m42 taken with a sony nex5 at iso 12,800 , 30 seconds unguided exposure with the moon in the sky. No darks , no flats and only one "noisey" exposure.This has been resized to 24kb , it was originally a 14Mp image.
10-07-2012, 02:42 PM
Thanks for the added info Ray.
I didn't realise the rest of the image train would be so finicky on this ota also.
Id be using a self modified 900NC for planetary and my Nikon DSLR for DSO initially. Maybe a dedicated astro imager down the track (have to sneak it in under the handbrakes radar :thumbsup:)
Looks like the C9.25 might be a better (hair saving) scope.
Hopefully I find nothing wrong with it when I finally get to test it.
I was looking at the hyperstar setups, a bit $$y, but definitely something to consider down the track :question:
I actually thought, if I do get the C9.25, I'd sell the CG5 it comes with, and with the proceeds look at a used fast 80mm APO or ACRO for my widefield stuff.
Go the 200mm f/4 if you want to image DSO. C9.25 if you want planetary. I bought my 9.25 to try to get a jack of all trades scope, but I am slightly disappointed with it now that I am imaging even with the reducer.
If you want to see some C9.25 images with an ATIK 320E - I have a few images posted in the beginners astroimaging section.
There's nothing wrong with the c9.25 for deep sky as gem suggests there is. Do have a look at his pictures in the beginners astrophotography forum .i looked at the picture of m20. , detail and colour was great but due to mismatch between camera and telescope the ifield of view is too small. If you look back to my m42 taken with a dSLR size sensor and .63 reducer , shown in previous reply above, you'll see the field of view is very much larger and still flat.
14-07-2012, 10:16 AM
I see your point Philip,
Your image FOV of M42 to me is acceptable, infact I quite like it :thumbsup:. Especially since a dslr chip size is big.
I am leaning more toward the C9.25 as since with a young family, time is a bit more precious, a scope with less tinker time required would be better. But the potential of a fast newt still intrigues me, something for later on perhaps ;).
Anyhoo, still trying to get my hands on the C9.25 to trial, mind you the weather here has been kaa kaa, and looks to stay that way for a while, so no hurry just yet.
Thanks to everyone for your input, been a great read. :thanx: Keep it coming though...
14-07-2012, 11:20 AM
"Nikon DSLR for DSO initially" - would be a good match for the 925 scope.
just for interest, if you want a look at good planetary imaging with a 925, try Troy Tranter's website http://home.exetel.com.au/celestial/html/Home.html
14-07-2012, 06:49 PM
Thanks Ray, will check it out
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