comparison study 2

[PSALM19.1 (Shaun)]

Ok, so had the opportunity on Monday to observe with my 8" Dob and my mate's C11 SCT and noted a few things: although larger, the image in the SCT was not more detailed as I thought it would be, in fact, when looking at Jupiter, my Dob was slightly better than the C11 and so was the moon in a 32mm EP. My mate was quite surprised and so was I to be honest! Then we looked at the Orion Nebula and the difference was very minute, in fact, the C11 won out in that the larger aperture gave a bigger image but again, the images were comparable for sharpness. I guess it showed me just how good for viewing a Dob can be: of course, the SCT is obviously made for photography and that is why my mate bought it (about 15 years ago for around $10, 000! I'll settle for observing with a Dob for $500!

[raymo]

Hi Shaun, You're comparing apples with oranges. It's not surprising
that your 8" Dob gave views as sharp as the C11 given that the C11's
optics are 15 yrs old, and may be not quite as sparkling as they once were. Also, newer SCTs have better coatings. Secondly, SCTs have a larger central obstruction, which affects the image quality slightly.
If you had viewed some fainter objects you would have seen the
benefit of the extra aperture immediately, given that the 11" has
almost double the light grasp of the 8". Finally, if you had looked
carefully at some craters on the moon you would have discovered that you could resolve smaller ones with the 11". However, whether the 11"
SCT is worth 20x the 8" Dob comes down to personal preference
or requirements.
raymo

[barx1963 (Malcolm)]

The thing to remember is that these are quite different designs which have been put together with different aims.
An SCT main strength is it has a compact OTA and its long focal length ( I assume it is an f10 so about 2800mm) makes it a high power scope. High power will always come at a cost in the image as you also magnify any aberrations or atmospheric issues.
To give a fair comparison you really should ensure that the magnification is the same.
Also a couple of things with your post. Firstly more aperture will not make an image bigger, that is a function of the focal length either of the OTA and/or the eyepiece. A larger aperture can make a scope more amenable to higher power. My old 12" was struggling to push much past 120x on most nights while I can often use 250x in my 20".
Secondly, SCT's have never been designed as an imaging scope, rather as a reasonable compromise to be used for either imaging or visual. Personally I would always prefer to use a newt for visual before an SCT ( based on personal experience, admittedly limited as I have on looked through an SCT on maybe half a dozen occasions) and they are not the commonest imaging scope. However as a scope that can do both reasonably well they are not a bad compromise.

Malcolm

[PSALM19.1 (Shaun)]

Thanks guys. Yes Malcolm, I realised after posting that aperture doesn't increase the image size! Duh! Was fun to look through both...we had great viewing until Mars and Saturn rose and then clouds came and ruined further viewing....

[Camelopardalis (Dunk)]

Sounds like seeing to me. And I've said this before and I'm not ashamed to say it again...bigger does not mean better! Aim for crisp rather than blurry and you're more likely to see more.

[Camelopardalis (Dunk)]

Oh and also bear in mind the overall range and quality of scopes on the market 15 years ago was very different from now...you'd certainly not get 8" of decent Dobsonian for that back then, and imagers got sucked into all sorts of expensive contraptions to defeat field rotation...

[brian nordstrom (As avatar)]

Interesting results , and I have to ask , what was the night ( seeing , as you say ) like ? if it was good I will say that the C11's collumnation was out , because I have looked thru many C11's over the years and they all have thrown up excellent images , especially lunar/planetary .

I will happily put my C9.25 against your 8 inch f6 dob any night of the week , me using my 16mm Nagler giving 146x and your Newtonion using my 9mm Nagler ( both T2's and the best high power eyepieces made ) giving 133x on any object .

Not to put yours down Shaun , as Simmo's 10 inch GSO Dob here in Perth has a very special mirror , perhaps you lucked out and ended up with a sweet one as well , sounds like it and when these mirrors are good ,, they are very , very GOOD ! no Q's asked .

Yes my C9.25 is a very good one as well . .

Its only when we hit 300x plus that a SCT on its driven mount really shines , as Simmo's easily took 300x on mars , but it took a deft hand to keep it in the FOV , done but it got old fast . Your 8 inch will be exactly the same , sorry to say and at 2 am after hours of driving ,,,, well ???

Great report tho , please report back after a few more sessions , we do the same thing here in Perth and one night Peters 16 inch would not even split A Crux at 150x WTF ? but 20 miniutes later .even a sweet 110 triplet APO failed at 150x as well ,,, yes the seeing went to 'Custard ' , I will place my money on that being the difference you guys seen .

ps. Dunk is right on the quality of these chinese optics of today , they are way better than 10-15 years ago , great value today .

Brian.

[PSALM19.1 (Shaun)]

Thanks Brian: yes, seeing was very good initially; jupiter and moon were excellent. As the night moved on, Jupiter started to turn, as you said, to custard! I asked my mate about collimation, he said it had been a while since he had, so that may indeed be a factor for sure! I certainly hope I did get a very blessed mirror! I do get some smashing views at times....

[brian nordstrom (As avatar)]

No worries Shaun , the quality of Asian optics have improved dramatically as said here , and these days it has to be 80/19 % good optics vrs average optics with the last 1% being lemons , but if you buy from a shop here you will get a replacement easily so its not like the 'Bad old Dark ages " of 10 -15 years ago when it was 50/30 at best , with 20% lemons and no internet .
Yes 'Custard ' thickened up some what ! .
Keep looking up Brother .
Brian.