Talk me into something..

[toc (Tim)]

Originally Posted by g__day

Its a trade off isn't it? A refractors costs way more, has shorter focal lengths, less light reach but gives superb focus and tighter stars (someone please check that last point is true and explain technically why!). An SCT is lower cost, so at a same price point has much longer focal length, far more light reach, but even with optimal focus I presume its stars won't be as tight as the refractors?

Is it just a function primarily of focal length that makes an SCT's stars appear more bloated at optimal focus than a refractors? Put another way if you had say a ten inch apo refractor and a ten inch SCT both shoot the same star field onto the same camera (and you didn't expose over the CCDs well depth - so no blooming) - both perfectly focused and both with say a 2.5 metres focal length - would the refractors stars be the same size as the SCTs - slightly less or significantly small and tighter?

I always thought it was due to the larger CO of SCT's, but I am probably totally wrong. Cant even remember where I heard that from

[Rigel003 (Graeme)]

I think you're totally right as to the principal cause. No central obstruction allows the diffraction patterns of stars to be as pointlike as possible. Other factors also prevent SCTs producing optimal images e.g. thermal issues, the fact that the primary is enclosed and at the bottom of the column of air rather than the top, the fact that the light rays travel 3 times through the tube in SCTs and once only in a refractor. The size of the diffraction pattern from a point source shouldn't have anything to do with focal length or focal ratio.

[xelasnave]

Thanks for all the help everyone. I would love a good refractor and a SCT side by side when I think about it,,,,I should get rid of some stuff when I think about it...
alex

[AlexN]

Thats the way to do it Alex. Although expensive...

I suppose, you could go for the smaller C8, and then use the $1000 difference to buy a good refractor. Something along the lines of a Meade 5000 ED80 Triplet, or a Megrez 90... Both of those are great scopes, "around" the $1k mark give or take.. Can't go wrong with the best of both worlds.. Thats what I always say!

[xelasnave]

I have even been thinking about the money side...I really should get rid of my current gear and some other stuff I have been hanging onto as investment and go for it... I never play my electric guitar for example..some antique furniture you cant even sit on...I am not young ...mmm I need to get organised because I enjoy taking photos more than anything really... It is such a buzz cause it is way past anything I ever aspired to... I am lucky , we are all lucky, to have available to us the excellent gear...

You know what I wanted to set up is four scopes on the one mount so as to have each scope with a dedicated filter color plus HA and cameras for each .... I know I am getting carried away but I think that would be a great system and I would love to try it... four sbigs four astro physics 150 refractors on a paramount mount...
but at the moment just some clear sky would be nice...so bored and cabin crazy

alex

[AlexN]

I hear you on that... I've not had my scopes out properly in quite a while...

I too have thought about the idea of 4 scopes/4 cams... The problem being, How would the computer handle running 4 SBIG cameras all at once? I suppose if you're going 4x150mm AP's on a paramount, the Obs PC is going to be no slouch!!

I do what you're doing in my head all the time... Im constantly wondering what I dont use often enough to justify having, how much I'd get for it and what marvelous astronomy gear I could afford with said money!

I even considered recently my 52" HD LCD TV, for a good triplet APO or a G11 mount What can I say.. Im an Astrotoy Addict!

[Starkler (Geoff)]

Originally Posted by g__day

but gives superb focus and tighter stars (someone please check that last point is true and explain technically why!).

I can think of a few contributing factors.

• Most refractors have a higher optical quality commensurate with their price/aperture ratio
• Most refractors are small aperture scopes and more prone to run out of light before hitting the seeing limits. At low powers they will have a smaller exit pupil than a bigger scope, so again stars will appear sharper unless the observer has zero astigmatism in his eye.
• Less affected by thermal issues

Its not fair to compare a high end refractor to a chinese newtonian or sct that costs many times less per square cm of optical surface.

Now a 6" tak newtonian after proper cooling should give any 5" refractor a good run for its money.

[g__day (Matthew)]

So asked a different way - will a 8" high quality apo give tighter stars than a properly colliminated, and cooled quality 8" RC given both had the same focal length?

I assume the answer should be yes - as the RC will have some thremal currents, it still has a central obstruction. But how much tighter stars will the APO have? Are we talking 5%, 20% or 30% or more?

Curious...

Matt

[AlexN]

I dunno... I rekon the a well manufactured RC (RCOS/TAK/A&M) would compete with an equally size refractor. There would be substantial differences in the images produced, but I dare say that nobody would think those differences made one image better than the other (diffraction spikes etc) The APO might have better contrast with no center obstruction... Its really hard to say... Especially for me, someone who does not own a top quality APO, and does not own a RC scope either.

I do know that my C11 captures tightish stars, often when you take into account the different focal lenght, they seem on par with my Megrez.. Provided its been focused accurately..