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  #21  
Old 05-10-2019, 07:14 PM
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Bobbyoutback
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Quote:
Originally Posted by Wavytone View Post

Alternative along focal length Newtonian(dobsonian),
Fully agree ! a F/7 newt with good optics & correctly collimated will give wonderful performance & cost little

Bobby .
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  #22  
Old 05-10-2019, 07:55 PM
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dannat (Daniel)
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the 6” ed frac in the classifieds here seems a bit of a bargain, grab it
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  #23  
Old 06-10-2019, 12:08 PM
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Rigel003 (Graeme)
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I have a C11 and a premium 175mm refractor. Visually, I would much rather use the refractor on the moon and planets. After a modest cool down period you get perfect images with great sharpness and contrast. Magnifications of 350 - 400x are routine on nights with reasonable seeing. However... it's a very big and heavy tube and a chore to mount, eyepiece position is close to the ground when the planet is near zenith, and it needs a substantial mount for the long tube to be completely stable. It also costs $$$. And despite the perfection of the images, it is only 7" of aperture so there is a limit to resolving power and images dim at very high power (especially noticeable with Saturn).

If you're interested in planetary photography, the SCT is undoubtedly the better choice. The larger mirror gives better resolving power for fine detail, better colour saturation and overall brightness. The increase in light enables you to use shorter exposures and a higher frame rate which will generate more good frames for stacking and a larger image scale. On the other hand, despite my best efforts to cool it well, the SCT will very often be limited by thermal equilibrium issues from giving its best. The number of nights when it really comes up with the goods is small whereas the refractor throws up the best images the atmosphere will allow, night after night.
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  #24  
Old 08-10-2019, 10:13 AM
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Bluknghtv (Daniel)
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I really like my Orion eon 130 triplet, 910mm F7 refractor but i'm now after sct to get closer to the galaxies now it's galaxy season.
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  #25  
Old 11-10-2019, 12:16 AM
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ngcles
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Stick with what you're got

Hi Kevin & All,

Quote:
Originally Posted by cometcatcher View Post
Keep the SCT. No affordable refractor will come close to it.
Succinct and perfect advice.

Your 28cm Schmidt-Cassegrainian is approximately equivalent to a 23cm refractor for visual use that would be prohibitively expensive (for 99.5% of the population). It will require a costly, gigantic mount with a tall pier capable of supporting an approximately 60kg payload and will not be, by any reasonable use of the word, portable. It will probably require a dedicated observatory building to be reasonably usable.

Just to address the second part of your question about magnification. The magnification a telescope provides is calculated by dividing the focal length of the 'scope by the focal length of the eyepiece. For example your C-11 has a fl of 2800mm and when used with a 20mm fl eyepiece, provides x140. A 28cm refractor with a 2800mm fl gives exactly the same magnification with the same eyepiece. A larger telescope or a different design of telescope does not by itself provide more or better magnification. Any telescope can be made to magnify any amount you want simply by putting in a shorter focal length eyepiece.

"Magnification" by itself is no a way to describe the performance of a telescope. There is a practical limit to the amount you can apply on a given night depending on the design of the 'scope, the quality of the optics and most importantly the the atmospheric stability (seeing). For a 28cm Schmidt-Cassegrainian it will be about x500 in the very best conditions imaginable. A 28cm refractor will allow a bit more than that, but again the conditions must be perfect. The major limiting factor on 98% of nights is the atmospheric seeing that will force that high-magnification limit downwards significantly.When viewing a planet, you are better off with a smaller, brighter, sharper and stable image than a big, fat, wobbly blancmange induced by using too much magnification for the prevailing conditions.


Best,

L.

Last edited by ngcles; 11-10-2019 at 12:40 AM.
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  #26  
Old 11-10-2019, 11:05 AM
brian nordstrom (As avatar)
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Great advice all round here , pat yourselves on the back people ! .

I have a really good C9.25 and when collimated and cooled it provides excellent high power views of the moon and planets with ease , because of its 2350mm fl , yes my 127mm 1000mm fl iStar is sharper , easier to set up and cools much faster I have to start using my 3-5mm Radians to acheave the same medium powers the C9.25 gives while idling along using 10-20mm eyepieces .

It only come out on weekends when the nights are good , my smaller refractors get more use because they are easier to set up for shorter week night sessions .

Yes keep the C11 , learn how to collimate and give it at least an hour to cool and settle and if the night allows be ready to be amaized especially on the planets .

One more thing you will have to spend 10x the cost of a C11 on a refractor to get views that better it when the cost of mount and Observatory ? are taken into account , yes a large 7 inch + refractor and mount aint G&G by any means , where a C11 is .

Brian.
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