[ngcles]

Hi All,

Where do I start ?

Originally Posted by robz

I realise that the images and comprisons are examples of perfect conditions(and are stacked and manipulated captures) with all necessary criterias met etc., but it is NOT the central obstruction that causes loss of fine details.

I'm apologise Rob but I believe you are incorrect here. See this paper here:

http://articles.adsabs.harvard.edu//...00455.000.html

and a slightly more lay-approach here:

http://www.astrosurf.com/legault/obstruction.html

The physics of this has not changed in the last 50 years (trust me!). Leaving aside the effects of seeing and imperfections in the optical surfaces, aperture determines the size of the smallest details that are resolved. However the size of the central obstruction expressed as a percentage of the diameter of the primary mirror has a substantial effect on contrast (produces contrast depletion) and the human eye's ability to discern a contrast difference between this "spot" and an adjacent "spot" on the disc of a planet.

This is not particularly noticeable on a target like the Moon which has high-contrast detail -- mainly in black & white, or splitting a close binary, but on say Mars, Jupiter and Saturn where most of the detail (particularly the fine detail) is low contrast it is significant. Below a 20% obstruction, the effects of this contrast depletion are not noticeable visually. For practical purposes it can be disregarded. Once you pass about 25% obstruction by diameter, they become increasingly significant. Most commercial Schmidt-Cassegrainians are between 35 and 40% obstructed by diameter.

You might legitimately ask at this point about why so many very high quality images are produced by significantly obstructed telescopes -- and it would be a fair question. The answer is simple. Pure, raw resolution is governed by aperture diameter. Commercially available software used in the processing of raw images manipulated and draws out the fine contrast that is there but hidden by the effects of a large secondary. Your eyes see in real-time. There is no image-processing software in your brain, or between the retina and brain apart from what God gave you. You can't un-muddy contrast depletion visually.

Originally Posted by robz

The main conclusion is what some have mentioned here : APERTURE .RULES...................end of story.

True, subject to what I've said above.

Originally Posted by robz

After some more Googling, let's now go back to the 12 inch Meade SCT I have been considering along the way :it has all that is required for planetary viewing : large aperture , large focal ratio so I don't need to barlow the crap out of it(talking about extra optical surfaces!)and high quality aberation reduced optics all in a compact tube and not an F7 or F8 long ,heavy, wind catching cannon.!

Most of this is perfectly fair comment. Large aperture is desirable as it is the sole determiner as to theoretical resolution. However, even for a 12" telescope that produces an airy-disc around (I haven't done the maths -- this is from memory) the 0.35 arc-second size, the number of times you will get seeing that will permit the 'scope to resolve to that level is very small. Perhaps only a night or two a year (depending on where you live). If you are expecting to see fine detail on a windy night, dream on. But I agree completely that solid-tube Newtonians are more wind sensitive than a smaller package.

Long focal length is also desirable for a planetary visual-use 'scope because it means you won't have to use short focal length eyepieces to achieve moderately high and high magnification. Very short fl eyepieces usually have teensy-tiny eye lenses you have to screw your eye-up or squint to see through. This increases eye-strain and makes using the telescope uncomfortable. If the user is uncomfortable, you will find it hard to see ultra small low contrast detail.

High quality, aberration reduced optics? Just because you buy a commercial Schmidt-Cassegrainian, don't necessarily expect as a matter of course, higher grade optical surfaces than a home-made optic, or a Chinese made Newtonian. I've seen many optically good and occasionally very good S/C telescopes ... and I've seen several I'd describe as sub-standard and also a few out-and-out lemons.

A friend of mine spent two years fighting with a manufacturer (no names, no pack-drill) over his S/C that was obviously, clearly spherically aberrated (by about 1/2 a wave) -- a dead-set lemon. He sent it back overseas to the manufacturer once at his own expense to have it checked. They returned it (at his expense) and said it was fine. He had to send it back again several months later (at his own expense) with several testimonials as to the lack of optical quality. They re-checked it, apologised profusely, replaced the corrector and it is now a good telescope. It took 2 years of fighting to resolve (pun not intended ).

Originally Posted by robz

Central obstruction????...............rated at 11.1%..........shocking!....ain't it???.............I'm sure that a newtonian with an average obstruction of usually more than 20% is muuuuuuuuuch better?!

Hmmm ... that 11.1% figure is perfectly true -- as expressed as the area of the objective mirror aperture lost by the central obstruction. They express it that way on purpose, can you work out why??). However, it is misleading because the important factor here is the percentage of the diameter of the aperture. Expressed that way I'm certain the Meade 12 S/C it will be in the realms of 35-40%. This will cause contrast depletion, no ifs, no buts. That contrast depletion will be detectable in very good seeing and significant in excellent seeing.

Many commercial Newtonians (like the various Chinese brands and the Meade light-bridges) are typically 20-25% ocstructed. Good, could be better. If you want to optimise a Newtonian, unless the f/ratio is very, very short the obstruction is very commonly less than 20%. Examples: My old 25cm f/6 (254mm paerture, 54mm secondary) was 21.25% -- not optimal but close. My 31cm f/5.3 (307mm aperture, 54mm secondary) is 17.5% -- optimal. My 46cm f/4.9 (456mm aperture, 78mm secondary) is 17.1% -- optimal. It takes very little work or expense to optimise a commercial Newtonian. You can't optimise a commercial Schmidt-Cassegrainian.

Originally Posted by robz

I appreciate the help of all and am grateful..........but c'mon everyone...........let's keep the ''myths'' at bay next time as I have been confused to buggery!

No myths in what I've said, sorry.

Originally Posted by casstony

Rob, central obstruction is conventional stated as a percentage of the diameter of the objective. The Meade 12" SCT will have an obstruction of around 35%.

Agree, correct.

Originally Posted by casstony

The 12" SCT will deliver greater resolution than a perfect 6" refractor, but only under perfect conditions (stable atmosphere, good collimation, temperature equalised). Your local conditions may or may not limit the magnification you can achieve ...

Agree mostly here. If the seeing is the same as or worse than the Dawes limit for a 6" aperture, the resolved detail will be pretty much the same in both 'scopes. If the seeing permits smaller airy discs than the 6" can physically form, the 12" will achieve better resolution. Up to twice as good (depending on how much smaller) in fact because it is twice the diameter.

Originally Posted by casstony

I like SCT's for their convenience (compact tube, comfortable observing position, can store the tube inside my house) but my local conditions rarely let me use high magnification so I'm not too bothered by tube currents - it's a trade-off I'm prepared to accept for convenience. An 11" or 12" aperture gives decent deep sky views at lower magnifications and I'm prepared to wait until midnight for occasional higher magnifications that come with those lazy high pressure systems.

Good comments, agree!

Originally Posted by casstony

Refractors, SCT's and Newts are all good scopes - you'll probably only know which one suits you best after you've owned each type.

Agree 100%. The best telescope for you is the one you will use the most often. If you feel its too heavy, too inconvenient, too fiddly etc etc you won't use it as often and thereby becomes an inferior choice for you. This is a perfectly good reason to go down that track -- no argument from me at all. In the end it is all about your priorities

Originally Posted by robz

Hi casstony, thanks for your input.
According to the specifications on the Meade 12'' SCT, they state a 4 inch(physical?)/11.1% (optical ratio?) secondary obstruction figure................so what gives?

As I said above and Tony has also said, that 11.1% figure is an "area" obstructed, not a percentage of the diameter and it is the latter figure (about 35-40%) that bears on the question of visual contrast depletion.

Originally Posted by robz

Anyhow, with all the compromises that exist with telescope types, I will most likely go the SCT route.

All telescopes are a compromise. You have to trade $ per inch of aperture -v- ease of use -v- portability -v- quality -v- image perfection. I've looked through a lot of S/C telescopes over the years and there are a large number of perfectly legitimate reasons to own one. They pack significant aperture into a small package (compared to a Newtonian or refractor of identical aperture), they are inch-for-inch much cheaper than a refractor all-up and because the tube is short and mass lowish for that aperture, don't need such a heavily engineered mount for their aperture and are generally low-maintenance telescopes. These are all perfectly desirable things and if they are near the top of you list of priorities when you shop will probably tip you to the Schmidt-Cassegrainian.

Originally Posted by robz

... and an EQ6 PRO mount with standard and extra large dovetail heads ready for whatever new ''PLANET KILLER'' I decide upon and a TASCO 60mm refractor.

Sorry, no commercially manufactured Schmidt-Cassegrainian compared inch-for-inch against a similar quality Newtonian or Refractor could be described as a "PLANET KILLER". Due to the compromises made in that optical design, it will never compete because of contrast depletion induced by the relatively large central obstructions. There are a lot of perfectly valid reasons to choose a Schmidt-Cassegrainian. They may well be the telescope of choice for you having regard to all those things that must be considered as I've outlined. But, high contrast visual planetary observing is not one of them.

... Pt 2 follows

Best,

Les D