Seasons greetings!!
I have the chance to purchase a Celestron 9.25in SCT tube assembly for a good price with some extras thrown in that I don't want, which of course I will on-sell.
It is about 4 years old and in "mint" condition with a 6.3 focal reducer. I was wondering if this would make a decent imaging scope and what do I look for in this type of scope to tell whether it will be ok to purchase.
Thanks.
Indeed, the c9.25 is probably one of the better SCTs to image with due to its lower magnification secondary. This produces a flatter fov. If you're only starting off in imaging, I would suggest a scope with a wider fov. The reducer will help achieve this. If the price is good - go for it.
I already have all the gear, including EQ6, remote control, guide scopes and cameras etc, its just a chance to upgrade my OTA. Might have to get an electric focuser but!!
Well, i wasnt going to say anything but i guess since you are thinking of an SCT then you may as well check the mirror flop. In any case even if this was quite bad a lot of people upgrade with a crayford focusser. Still it may be worth checking.
The feathertouch 10:1 microfocusers help reduce mirror shift (vs mirror flop) by a factor of about 3 times. Large dovetail bars - like Losmandy D series are a good idea too.
I did some research on mirror flop and I'm not impressed. There seems to be a lot of work and expense in working around it if it exists and seems to be inherent in this type of scope, from minimal to severe. I do like reflectors for their light gathering ability, I might just get a Skywatcher SW252 and upgrade the focuser and motorize it. Then if I need to I can use EQMOSAIC but I think the SW252 is f4.7 anyway. Whaddaya reckon??
I did some research on mirror flop and I'm not impressed. There seems to be a lot of work and expense in working around it if it exists and seems to be inherent in this type of scope, from minimal to severe. I do like reflectors for their light gathering ability, I might just get a Skywatcher SW252 and upgrade the focuser and motorize it. Then if I need to I can use EQMOSAIC but I think the SW252 is f4.7 anyway. Whaddaya reckon??
Ive never found mirror shift to be a problem with the C8 - from what I have seen its only slight on mine, but I wouldnt consider mirror shift to be a deal breaker.
Ive never found mirror shift to be a problem with the C8 - from what I have seen its only slight on mine, but I wouldnt consider mirror shift to be a deal breaker.
Hi, I just mentioned it as something you should check. All SCT's that focus by moving the main mirror have it to some extent but this doesnt mean it's unuseable. Plenty of people image using Meade and celestron SCT's. Also there are strategies to minimise the shift and Crayford style focussers are not that expensive.
My C11 suffers from mirror shift a bit, it usually gets progressively worse as you move up in mirror size... however adding a crayford fixed the problem completely.. rough focus with the primary mirror, compose subject, perform critical focus via crayford, and fire away...
The williams optics SCT crayford is around $250 if i recall correctly, and the GSO model is closer to $150.... they are a good place to start.
I have a carbon fibre C9.25 to which I added a Meade motor focuser (-> JMI USB convertor -> a PC. I then added a added a Lumicon OAG (removed the focal reducer) added a 35mm barrel extension -> Hutech light pollution filter -> Canon 400D, guided with a Meade DSI II Mono Pro using PHD on 10 second frames between pulses if needed.
Stacking 6 * 10 minute shots and 3 * 15 minute shots of NGC 2070 (so far) gave me stars that were beautifully round and sharp (for an SCT) and not a pixel's drift after almost 2 hours of capturing so far (attached with basic - not very well done colour adjustment - Photoshop CS2 processing). This isn't one of my better shots (and the image processing was quite rushed and lost alot of detail shrinking a 60MB tiff to a 150Kb jpg. But accept that and ignoring the poor star colour - look to see if you see worrying mirror shift in this image.
A well tuned OAG basically eliminates your need to worry about mirror movement.
Matthew, Im astonished by the stars in that image... I assume with no focal reducer, you were running the system at F/10..
I've got a Lumicon OAG on the way for my C/11.. I was hoping that it will give me good results for 10 min subs at F/6.3... If you're able to get 15min at F/10 then I think I'll be very happy with my purchase!!
I run a Vixen Atlux (the second generation kind with the SkySensor2000-PC - not the starbook toy edition ) on a large home made steel peir. My rig uses Losmandy sidesaddles and O rings to hold a C9.25, side saddle 5" MAK and a Megrez 80 on top of teh C9.25. These scopes are counterbalanced with about 25 Kgs of weight. The mount seems to deal with all this weight very well.
For me the OAG and the more sensitive DSI II mono pro working correctly in light polluted skies were a revelation last night. I really botched up the Photoshop curves processing of the shots above (the real star images stacked where alot tighter - maybe half or 1/3 the size of the stars in this shot - each star got a halo) - but yes at F/10 the stars were beautiful up to 15 minutes at 2.3 metres focal length - that is some pretty good guiding. For the first time ever - ever - the stars after 2-3 hours were in exactly the same position (to the pixel) on my last shot as the first. Even guided - I normal see some drift over the hours - meaning stars bloat. On these shots there was absolutely none - meaning tracking and guiding were as close to perfect as I have ever achieved!
Fantastic! You must be pleased! I can only hope to achieve similar results with my EQ6+C11 with losmandy saddles counterbalanced with 15kg of weights... Having no drift from frame to frame would be good indeed.. I usually get 2~3pix over 2hrs or so... not enough to be a problem, but enough to have you wondering why/how etc... Quite the annoyance.
I'm thinking now that a really sensitive auto guider - very, very, very well focused - and a really tuned (to your rig) guiding program coupled to really rigidly mounted gear (to eliminate differential flex) is essential to getting much, much better results.
Personally I am growing to like the C9.25 (especially the carbon fibre one) more and more each day! I've said elewhere - I only have to re-focus mine about once or twice a year! And with a Bhatinov mask cut with a scapel from art paper (cost $2 and 30 minutes effort) I can achieve perfect focus in about 8 minutes!
I'm in heaven
PS
A 900 second shot with slightly better PS colour processing to combat the blue shift. I must stress there are no flats, darks or bias shots applied to this shot.
Look mainly at how the stars are - I'm still learning PS techniques and have a long way to go - but the stars tightness reveals a bit more about what this scope can do.
PPS
Back end of my imaging / viewing gear if anyone was interested.
PPPS
Note the small thin wire stuck into the back of the Meade DSI pro - trick I learnt. The USB cable has to be really snug - if it isn't with the amount of current needed to power Meade gear a loose cable making bad contact is enough short and immediately re-boot your PC! Took me six months of mysterious PC re-boots - once or twice a night to catch this error when imaging remotely!
Thanks for all your replies. Ive been doing some thick research into what I think I really want and I was looking at this one from Andrew's.
Any good?? Great for wide field and going narrow band?? Chromatic aberration/colour fringing?? http://www.andrewscom.com.au/images/...egrez110ed.jpg Megrez 110 ED APO f/5.9 Doublet OTA package
$1999.00 AUD
2.5" 10:1 micro-focuser
Must be the best value high quality 110mm ED OTA, ever!
Includes 114mm mounting rings and aluminium case, too
If you can stretch more money wise, the FLT110 is a much better scope... Otherwise, the Megrez 90FD will likely have better color correction being a slightly slower scope.. An F/5.9 doublet is approaching the faster end of the refractor scale, and therefore, more likely to suffer CA problems than the slower Megrez 90 @ F/6.9.
The Megrez 90 also uses FPL-53 glass in its objective, where the Megrez 110 uses FPL-51. Another reason why color correction may not be as good using the 110... For imaging purposes, the Megrez 90 will be an all round easier (and likely better) scope.. Its lighter, has better glass, slightly slower, (Can be reduced to F/5.6 using the 0.8 reducer) and is $200 lighter on the hip pocket!
The extra 20mm of aperture of the 110 will not make much if any difference...
The Megrez 90 also uses FPL-53 glass in its objective, where the Megrez 110 uses FPL-51. Another reason why color correction may not be as good using the 110...
Whilst I have no specific knowledge of these 2 scopes and which is better than the other the fact of FPL51 does not necessarily mean lesser colour correction.
Roland Christen has stated several times in the Yahoo refractors group that it is the mating element that determines colour correction not the material of the main lens.
So knowing what one element is made of is something scope manufacturers play on and they do not say what the mating elements are made of.
For example the Tak FS series has a mating element which is cheaper than the superior mating element of the Tak FC series which they changed apparently for cost saving reasons. But both are fluorite doublets.
The TEC 160mm ED which gets fabulous reviews uses FPL51 as the main element in its triplet.
Its a bit like the concentration on megapixels in marketing of cameras - it isn't necessarily the factor that will make the best image between 2 competing cameras of different megapixels.
It would be safer to assume (and again a generality here is only a practical thing not a rule) that a well made triplet will outperform a well made doublet.
An alternative (perhaps more expensive though) is the WO FLT110 with a TMB triplet lens. If the Megrez 90 is a doublet than I would check out the FLT110 as a triplet generally speaking will outperform a doublet for colour correction. TMB also have an excellent reputation.
Greg.
Last edited by gregbradley; 24-12-2008 at 11:52 PM.
There seems to be a lot of work and expense in working around it if it exists and seems to be inherent in this type of scope, from minimal to severe.
I do like reflectors for their light gathering ability, I might just get a Skywatcher SW252 and upgrade the focuser and motorize it. Then if I need to I can use EQMOSAIC but I think the SW252 is f4.7 anyway. Whaddaya reckon??
