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Post by jamesling1000 on Oct 2, 2012 23:58:31 GMT -5
Hi All....
Lately in CN, a lot of discussion on the subject of what aperture size of refractor is equivalent to a mirror type, such as SCT.....
The answer ends up we need bigger aperture size of refractor to match the mirror or SCT, as the light through put is much lower....even the mirror type minus away the central obstruction....
I just wonder for ISTAR scopes, how much will be the light through put ?
80% higher or lower?
And how much more will be affected between a doublet and triplet....
And in this case, my 8" ISTAR could be equivalent to any 7" SCT aperture.......or even lower.......
I am sure many members here are experts in providing both their views and the calculated figures, especially Mike and Ales should be able to give us more accurate one....
But from my experience of using both 8" SCT and the 8" ISTAR, there is no way the SCT can come close to match the refactor, in terms of resolution and details.....
You will need bigger aperture like my 10" Mewlon to take the challenge.....
And not forgetting my Celestron is the Ultima C8 goto which is guarantee diffraction limited during the late 90.....
Regards
James Ling
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Post by criverside on Oct 3, 2012 1:14:23 GMT -5
Hi James,
They have it reversed, assuming 2% loss or less per-surface on well coated lens, 8% or less for a doublet. Compared to the best telescope mirror coatings claiming 95% transmission per surface for a 10% loss with secondary mirror. Now factor in the center obstruction of at least 30%. Lastly, if you factor in full field illumination, most reflectors have a much narrower field to get full illumination.
Craig
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Post by jamesling1000 on Oct 3, 2012 1:48:13 GMT -5
Hi James, They have it reversed, assuming 2% loss or less per-surface on well coated lens, 8% or less for a doublet. Compared to the best telescope mirror coatings claiming 95% transmission per surface for a 10% loss with secondary mirror. Now factor in the center obstruction of at least 30%. Lastly, if you factor in full field illumination, most reflectors have a much narrower field to get full illumination. Craig Hi Craig.... That is also exactly why I couldn't understand from the detail debate in CN...... I have been frequently using the 8" ISTAR, for the past 2 years, and I do not see why the light through put is less than my Ultima C8, although is starbright coating..... I do not own any XLT C8, as such I thought the XLT is 15% to 20% better light transmission than the older starbright model.... And the debate is refractor against XLT or ..... I even confirmed with my own eyes at the recent prior Venus transit night gazing at NUS university, the 12" meade with superior coating is not that bright , as compared to my 8" ISTAR..... In fact, during that night when we are pointing at saturn, the ISTAR is using a 4mm TMB ep, at 450X, is showing a better image than the 12' meade........ Perhaps is the cooling down of the ISTAR scope that really give the advantage...... Shy to say, beside my C14 and the 10" Mewlon, my ultima C8 can never power up to 400X.....as it stays good around 300X at most.... Regard James Ling |
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Post by criverside on Oct 3, 2012 13:56:44 GMT -5
Hi James,
Another point I didn't mention, on a SCT you will have an additional reflection loss of 4% in the First element corrector lens.
I agree with your assessments through observations, but even with proper cool-down I feel the refractor has many advantages over the same size reflector, thats part of the reason I joined the ISTAR Club.
Light Buckets, SCT's and Mak's ( and I am a Mak lover! ) have advantages and a place in observing, but there are unique features of a refractor that make it irresistible for me, and I suspect the majority of ISTAR Club members.
Craig
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Post by jamesling1000 on Oct 3, 2012 23:35:43 GMT -5
Hi Crag.....
For me , although I started off with a celestron 80mm refractor, and then I have the C8 and C14.....,
Only after 5 yrs of using , then I really wanted to explore the refractor
again....
But the C6R F8 really does not live to its quality of std I want....
It did not perform better than my C8......
But to pursue the refractor like performance scope, I went for the 10" Mewlon, instead of a 6" triplet.....
If I did not get the copy of astronomy magazine, that advertised the ISTAR 8" F9, I would have not come back to refractor again....
Really is amazing that I now is using this scope more than my other precious collection, hahaha......
Regards
James Ling
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Post by criverside on Oct 4, 2012 0:29:12 GMT -5
Hi James,
I guess large refractors just "grow on you". Something about the larger fully illuminated field, and high contrast images.
Craig
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Post by jamesling1000 on Oct 4, 2012 4:32:03 GMT -5
Hi Craig....
Coz of the ISTAR, now the number of refractors I owned is more then my 3 mirror types.....Hahaha....
1. 8" ISTAR F9
2. 6" C6R F8
3. WO88ED, which I use it at my oversea working place....
4. 70mm Vixen refractor....
Regards
James Ling
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Post by Watcher3 on Oct 4, 2012 23:18:27 GMT -5
I had a c-8 for many years. I replaced it with an Omni 120 achro after a prolonged layoff from observing led to the c-8 falling into disrepair. Aside from more color saturation on Jupiter and Saturn, the c-8 didn't do anything that the refractor couldn't do!
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Post by jamesling1000 on Oct 4, 2012 23:53:32 GMT -5
Hi Mike.....
I know the celestron Omni 120 achro is a very good scope.....
So you don't need a 6" acrho to beat the C8.....
So now just wtg for you to show us how good the 150 R30 performs .......
One of our regular side walk team member uses this for most of his side walk , in a park.......
Regards
James Ling
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