Celestron C100ED

[gary1968]

I have been looking for a F10 scope for imaging, with as big an objective as I can afford. So, I was basing my search on a 2nd hand 8"SCT, they are like hens teeth just now. There was a refurbished C8i on fleabay this week but I got beat at the last gasp of the auction.

Anyway, as usual I waffle, so, I am now considering this..... Clearance / Pre-Owned - Celestron C100ED OTA .

So, my questions are, how good is it? is it good for imaging? or will I regret buying that and wish I had waited for a sct? From memory I dont recall many on here using it for either visual or imaging.

Any advice welcomed.......

Gary

[GazOC]

Hi gary,

IMHO its a great scope but a little slow at f9 for imaging.

Gaz

[arad85]

Depends what you want to image. The 8" SCT has a focal length of 2000mm, the 100ED is 900mm. If you're planetary imaging the 2000mm of the 8" will be hugely important... You can't get big scale images of the planets at short focal lengths., DSOs on the other hand are a different beast...

[gary1968]

Cheers Gaz.

I would probably use it with the WO Mk1 reducer flattener so it would be a little under F8 at the end of the day. I also have a Celestron F6.3 reducer so might try it with that too.

I suppose my questions should be, would I get a benefit using the C100ed over my C80ed, or is the extra 20mm aperature not that important. Should I hang of for a sct?

Gary

[gary1968]

Cheers, arad85, DSO's are my main interest. I have been imaging with a ZS66 and 1000d until now, but I am now thinking the FOV is huge and I'd like to get in a bit closer. I have now recieved my sct > 2" adaptor so will be able to fit my WO Mk1 flattener to that, hoping that the FOV will be smaller. To my thinking the FOV of the C100ed will be smaller again.

I am just not sure the extra 20mm of aperature on the C100 over the C80 would be worth the cash layout.

Gary

[arad85]

FOV is directly proportional to focal length. A 1000mm focal length scope will get in twice as close as a 500mm scope. Aperture has nothing to do with the image scale. It is totally dependent on focal length... Aperture just gives you shorter or longer exposure times for a given focal length....

The ZS66 has a focal length of 366mm. The C100ED is an 900mm F9 design which means ~2.5x bigger images and 4x the exposure times as the ZS66 for the same light levels.

[gary1968]

Yes I know aperture has nowt to do with FOV. My question is worded badly....

Maybe this is a bit better, if I were to have say, my C80 and the C100 at a similar focal ratio, by use of reducers, would the C100 be worth the outlay. Or should I hold off for a 'bigger' objective? I have my HEQ5 guiding fairly well, but never tried longer than 10mins, yet!

The C100 would allow me in a bit closer as it is a longer scope, just not sure if I should wait for a bigger objective.

I know what I want answered but I am having difficulty putting it into words..........

Maybe I should just pay more attention to what others are imaging with....

Cheers,

Gary

[ollypenrice]

I can't see that the f9 4 inch would be much of a choice for DSO imaging. Your ED80 will probably out perform it. I don't think much of SCTs for imaging either, TBH! (We have a 10 inch Meade. Working at f10 is a non starter without a world class mount and adaptive optics of some kind. Working at f6 to 7 is better but a good refractor like the SW120 or Meade 127 will give a sharper image that largely makes up for image scale defecit on smaller targets. Then with SCTs there is collimation, dew, mirrior flop and the need for a Crayford to factor in. Life's too short!)

I'd look for a Meade 127 or SW120. (I just sold my 127 for about £850).

But I would also look hard at the GSO photographic Newt available from Telescope Service and elsewhere. It is very good indeed. So also is the SW Mak Newt.

F ratio is what makes a photographic light bucket, not aperture. I wouldn't want to go above f7.5 and less is better. (Our Baby Q at f3.9 is a voracious light-eating little beast, beliieve me.)

Olly

[gary1968]

Thanks for the info Olly....

I fancied getting a 8" scope for its light gathering power, and Blinky has had some very nice results with his SCT.

The refractors you mention are a bit out of my price range, as is the SW Mak / Newt. Unless that is I can pick one up 2nd hand. Do you think my HEQ5 is up to carrying a 190 Mak / Newt and a guide scope?

I was also looking at the new SW 200P DS, its F5 and 8". So it fills my 8" wish and also your F ratio advice. What do you think of that?

Collimation is a concern for imaging, but I am slowly getting my head round that now.

Anyway, thanks again.

Gary

[ollypenrice]

Weight isn't the big issue with mounts. It is accuracy. The EQs are comfortabe with focal lengths up to a metre and patient, crafty and talented souls get them to guide at up to 2 metres of focal length. I wouldn't want to be doing that, though. I would stick at a metre for an easy life. If your longer focal length is also at a slow f ratio then your exposures will have to be longer and so the guiding gets harder still.

I like to enjoy imaging, not spend the night in screaming semi-despair. (It happens though!)

I say again, beware of the term light grasp. In visual astronomy light grasp equates to aperture. In imaging it equates to focal ratio. Entirely different situations.

Olly

[F15Rules]

Gary,

I can only speak as a visual observer..I have an ED100 Celestron I bought new on a CG-5 in January from FLO. It is much better than I expected even though the reviews I did find were all very complimentary..the visual images are superb, no colour at all in focus, sharp, and with good depth, due to the medium F9 FL. It really is a good performer.

I can't speak for imaging, I will bow to the experts' view on that, but if you like to look through your scope (for me the whole point of the hobby!), you can't get a better buy IMHO.

hope this helps.

Dave