PLANETARY VIEWING ?? -aperature rules?or telescope type?

[wavelandscott (Scott)]

Originally Posted by robz

Hmmmmm?.....that's interesting Mark, and I have heard this from time to time.

Does anyone know what the BINTEL mirrors for newtonians are like?
I know we're back to mass produced optics here again, but the specs are quite impressive (if true) : surface accuracy at least 1/16 wave RMS (typically better)BK7 substrate.

It all sounds better than your average plate glass slab of Chinese junk does it not?

They are (were) sourced from GSO and rebadged for Bintel. They do check them all before they go out.

I've been very please with my 8 inch Bintel reflecting telescope.

[wavelandscott (Scott)]

Originally Posted by robz

:
It is clear that aperature is superior and that the B.S. spread on the central obstruction is not the'' image/contrast killer'' it's made out to be

Apples and Oranges using astrophoto as a proxy for visual evaluation...

Astrophotography benefits over visual in that you can stack, add/delete images to build a better one over time. Your eyeball can;t do that.

I'm not saying that an SCT is a bad type of scopes, they have some advantages in size relative to portability but from my own experiences I'd not list contrast as an advantage of the SCT design.

Anyway, enjoy your choice!

[Satchmo]

Originally Posted by wavelandscott

They are (were) sourced from GSO and rebadged for Bintel. They do check them all before they go out.

I've been very please with my 8 inch Bintel reflecting telescope.

For the sake of technical accuracy Bintel telescopes are collimated and mechanically checked -but not optically tested.

I don't know of any other shop in Australia besides Bintel that will perform this excellent service on a Chinese import.

[Satchmo]

Originally Posted by robz

It is clear that aperature is superior and that the B.S. spread on the central obstruction is not the'' image/contrast killer'' it's made out to be

Yes- central obstruction is indeed a contrast killer. Damien Peach has access to exceptional seeing and does a load of image processing/stracking. You will never see views like that _through the eyepiece_ , but a high quality simple optical system with clean optics and a minimised central obstruction can give glimpses of it.

[Satchmo]

Originally Posted by robz

but the specs are quite impressive (if true) : surface accuracy at least 1/16 wave RMS (typically better)BK7 substrate.

It all sounds better than your average plate glass slab of Chinese junk does it not?

Only if you don't understand about the tendency in the industry for `wave inflation' via terminology. '1/16 wave RMS on the surface' translates to about 0.5 P-V on the wavefront. I recently tested a couple of 16" Chinese mirrors for a well known Australian telescope builder. They came in at around 0.5 and 0.66 wavefront . The smaller mirrors may test out better than that I don't know.

Given the issues of a relatively thick non low expansion glass it is a moot point whether such a system would benefit from a higher accuracy mirror. If you are a casual observer and not out for high planetary performance they are great value there is no argument there. As always in life , you get what you pay for and usually no more.

[casstony]

Rob, I like Schmidt Cassegrains but they do have a serious disadvantage with regard to high magnification in temperate climates - it's difficult to get the tube cooled. Typically the scope will lag falling temperatures until sometime around midnight and you won't get the best view until an hour or so after temperatures stop falling. Some owners cut holes in the tube to add cooling fans or other devices to get the main mirror cooled.

With that in mind the new Celestron Edge with it's cooling vents is worth considering.

[wavelandscott (Scott)]

Originally Posted by Satchmo

For the sake of technical accuracy Bintel telescopes are collimated and mechanically checked -but not optically tested.

I don't know of any other shop in Australia besides Bintel that will perform this excellent service on a Chinese import.

Good clarification! Thanks!

[robz (Robert)]

Thanks to all again for your comments and assistance.

Yes, I have said that I have settled on the SCT at the present point in time until funds permit ..........daughter's wedding coming up in January.......DOH!!......having a hard time juggling finances!
BUT...................until that purchase is made, there is a lot to think about........especially ''REFRACTORS'' ?????????

I can't seem to get them out of my head knowing and seeing how good even the small aperature achromats can be

What does concern me over this tremendous discussion is that optical quality of sufficient level in newtonians really needs to be in the hand made catagory over here in Australia which blows the price way past my budget in anything around the 16 inch mark.Plus I would have trouble keeping such a large tube steady on an EQ6PRO

Then there's the cooling problem - SCT, MAK, R.C. NEWT. and so on. I have seen the differences in my 8 inch newtonian astrograph, F4.4 and know the amazing transformation that occurs

This all brings back the PITA factor which I do not want........I'm getting too old, have two stuffed knees and little time to wait for optics to cool down all night with the fans blowing their guts out !

Someone once said to me : '' if I were to go back before I bought and collected all my various scopes and spent the money on the ultimate scope, it would have been a TAK 150 and I would be done.......... and in heaven ever since''
Yeah well I can't afford a ''TAK'' and would be looking at ED's and APO's that are affordable and still perform well.

We can support or slam the Refractor as much as we want, but deep down inside, we all know how extraordinary they can be...............

What''s the use of aperature when high power won't work due to temperature equalisation problems, sub standard optics and mis-collimation when ever a passing bat farts?

[mbaddah (Mo)]

Originally Posted by robz

Someone once said to me : '' if I were to go back before I bought and collected all my various scopes and spent the money on the ultimate scope, it would have been a TAK 150 and I would be done.......... and in heaven ever since''
Yeah well I can't afford a ''TAK'' and would be looking at ED's and APO's that are affordable and still perform well.

We can support or slam the Refractor as much as we want, but deep down inside, we all know how extraordinary they can be...............

What''s the use of aperature when high power won't work due to temperature equalisation problems, sub standard optics and mis-collimation when ever a passing bat farts?

The Tak150 is a superb refractor that will offer excellent views more consistently (generally speaking) than a Dob with large aperture on low contrast objects such as planets, or splitting doubles. But for the same price you can get a premium 15" dob which will blow the Tak out of the water on dso (every time) and on planets when seeing permits, there's no ifs or buts about it. Had this not been the case, there would be no point in people purchasing 20"+ dobs. It's not all about high power! Collimation is no where near as bad as some people make it out to be (trust me I've been there).

Unfortunately your going to have to compromise on something with each scope, that's the reality of this hobby. I've bought and sold so many scopes over the years that I've realised you can't have one that does it all (yes even an SCT is full of compromises). It takes time to work out what your observing habits are like and from there, you can settle on a scope which would best suit your observing needs.

I personally think the best combination is a 100-120mm refractor with a 10-12" dob. Those two scopes will cost less than an 11" SCT!

Best of luck.

[robz (Robert)]

Originally Posted by mbaddah

The Tak150 is a superb refractor that will offer excellent views more consistently (generally speaking) than a Dob with large aperture on low contrast objects such as planets, or splitting doubles. But for the same price you can get a premium 15" dob which will blow the Tak out of the water on dso (every time) and on planets when seeing permits, there's no ifs or buts about it. Had this not been the case, there would be no point in people purchasing 20"+ dobs. It's not all about high power! Collimation is no where near as bad as some people make it out to be (trust me I've been there).

Unfortunately your going to have to compromise on something with each scope, that's the reality of this hobby. I've bought and sold so many scopes over the years that I've realised you can't have one that does it all (yes even an SCT is full of compromises). It takes time to work out what your observing habits are like and from there, you can settle on a scope which would best suit your observing needs.

I personally think the best combination is a 100-120mm refractor with a 10-12" dob. Those two scopes will cost less than an 11" SCT!

Best of luck.

O.k................without the budget for a ''premium dob'', what brand that is readily available on the Australian market has good optics and reputation in a 16 inch????

We have the Meade Lightbridge, the Bintels, The SkyWatchers, the GSO's..............

[rmcconachy]

Originally Posted by robz

O.k................without the budget for a ''premium dob'', what brand that is readily available on the Australian market has good optics and reputation in a 16 inch????

We have the Meade Lightbridge, the Bintels, The SkyWatchers, the GSO's..............

In the 16" size all of the scopes you mention use the same optics supplied GSO.

[andrew2008]

Must say I agree with mbaddah. A 12" dob and a 100-120mm ED refractor would be my ideal setup. The 8" sct I owned never came close to giving me the views from my old 10" skywatcher dob on planets, and just about everything else. Tube currents drove me nuts as it never seemed to reach equilibrium. On one occasion an ED80 gave a better view of mars in side by side comparison.

I've gone back to a 12" dob and now looking for a 100-110 mm ED refractor.

[Sylvain (Jon)]

What''s the use of aperature when high power won't work due to temperature equalisation problems, sub standard optics and mis-collimation when ever a passing bat farts?

Cooling down can be an issue it's true, but large refractors will suffer from the same problem though to a lesser extend. A 150mm refractor is a thick piece of glass. Takes time to cool down.
Easy solution to solve cooling down issues: take the scope out 2 hours or so before observing. Easy.

What's the use of aperture? Greater brightness at a same magnification will help you see tiny details of low contrast. (see the illustrative picture posted previously).
It is however true that instruments require proper collimation. This is also true for refractors although they typically do not require the user to perform collimation. That being said, my Mak does not require collimation either. Holds it extremely well. Regarding optical quality, maks are usually good performers due to the relative ease with which spherical optical surfaces can be produced.

For good optical quality (never tested it myself though), have a look at the orion optics website in the UK where they certify the optics up to lambda/10 for newtons. Some models are relatively affordable.

[ngcles]

Hi All,

Well lots of opinions here but for what it's worth, I agree with Satchmo -- spot on as usual.

Yes, a 4-5" APO will be very good, but the smallest detail it will resolve on a planet is around the 1 arc-second diameter mark. Resolution is tied intrinsically to aperture. It doesn't matter how good the optics are or whether you've got 99.8% Strehl ratio optics etc, it won't (indeed can't) do better than this.

A clean, well-cooled and collimated, say 10" f/6 or f/7 Newtonian with quality optics around 1/25th wave RMS mark and a secondary obstruction <20% in near perfect conditions will do twice as well as the 5" APO in the same conditions -- ie in the smallest detail to be potentially visible. It will probably cost little more than half as much. If the seeing is mediocre or poor there will be little difference in detail visible except in those occasional moments when clarity prevails for a moment or two -- and the Newtonian wins again. The larger 'scope will produce a significantly brighter image that will take much more magnification before it becomes unacceptably dim and uncomfortable to view.

Don't get me wrong, refractors are beautiful telescopes inch for inch, but they are practically limited in aperture. (well they are aperture-limited by the depth of your pockets I guess). Aperture of the primary mirror/lens is the prime determiner of how much detail is potentially visible in a planetary image.

In poor, mediocre or average conditions a SCT of similar size to our Newtonian will perform about as well on planetary detail as the Newtonian. In very good conditions or excellent (rare) the Newtonian will produce a somewhat crisper image due to the much smaller secondary mirror used. It is a simple matter of physics due to the size of the secondary obstruction and the wave-nature of light. Increase the secondary mirror size and you push more light out of the Airy-disc and into the surrounding diffraction pattern. As Foghorn Leghorn said to the young chicken-hawk "Son yer can argue with me, but yer can't argue with figures" -- and that's a fact, not an opinion!

Also, it is a simple fact of life that in a typical commercial SCT used with a diagonal, you need to get 5 optical surfaces right for it to work well. In a Newtonain there are only two.

If you are looking for a quality, visual-use, portable "APO killer" for an Eq6 mount, get yourself an 8" f/7.5 Newtonian with a 25mm secondary. Longer focal length Newtonians are easier to collimate and much more forgiving of slight errors. Additionally, they are much easier to fabricate! Your eyes (and bank-balance) will thank you for a long time -- it will flog the pants off any 4" APO on any subject in the night-sky save perhaps ultra-wide field viewing. The image will be 4x brighter at a given magnification and will show twice as much detail in the right conditions.


Best,

Les D

[robz (Robert)]

Getting back to the article by Damian Peach further back on this thread, it clearly seems to me that most have not paid much attention to it or am I wrong?

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.

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

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.

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?

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!

[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%.

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; collimation is not a problem; cooling can be a big problem, which is why I would point you to the Celestron Edge(has vents) in preference to the Meade.

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.

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.

[robz (Robert)]

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?

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

I have had a 16 inch Newtonian(pipe fitting mount), have at present,an 8 inch F4.4 Orion on a Gazer E.Q.(undriven) 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.

The newtonians have and are a proper nuisance, in having to check and collimate the optical path, rotate that bloody tube to get to the eyepeice and are heavy and long, making set up just that bit harder.

Have never owned a SCT, but have allways wanted a big one............wish me luck coz I'm gonna need it

[casstony]

Rob, 11.1% probably refers to the area of the secondary compared to the primary - equates to deceptive advertising. 4"/12" = 33% sceondary obstruction by diameter, which is about the same as my C11.

You'll still need to tweak the collimation regularly on the SCT to get the best high power views - and you'll certainly need to add some kind of active cooling if you get the 12" Meade and high power is the priority.

The Meade 12" also weighs about 38 lbs while the Celestron 11" weighs about 28 lbs.

[Sylvain (Jon)]

The C11 is already a pretty massive beast and it's not an easy task to move it around.
I suggest you really consider the weight issue. It's always something we think is going to be fine prior to purchase, but it ends up being one of the most (or the most?) annoying thing for daily use afterwards.

[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