What do you think is the best scope in the world under 20 inches at the moment?

[gregbradley]

A contentious question. I mean for deep sky imaging as visual would have a whole lot of different values.

Perhaps it should be by aperture. 4, 5,6,8, 10, 12, 14, 16, 17, 20 inch aperture of any type.

Greg.

[glend (Glen)]

There can be many different answers based on circumstances. What mount, fixed or transport to dark site, weight to lift, type of imaging, budget of buyer, room for storage or observatory, etc etc....

Personally I believe the 'sweet spot' is around 10" , still light enough to handle and transport, doesn't require a monster mount and cost effective in most optical designs except refractors. Big enough for DSOs.
If there were no barriers to entry, i'd opt for a Planewave CDK - the smaller one.

[alocky (Andrew lockwood)]

Mirror, mirror, on the wall, who has the highest Strehl of all?

I would have to argue that it would be a 19.999" something with good optics. Assuming it is appropriately mounted, the sensor samples the image adequately for the focal length, and the system is well designed to eliminate 'problems' ie flex, stray light. I've guessed the CDK style, which is why I'm buying one. Others are opting for RC, and the best will always come from one of the custom optics houses. The fact that there are multiple options still commercially viable means there is no correct answer!
I heard the r-h was ok too, but I'd hate to think what a mangin mirror near 20" would cost.
Cheers,
Andrew.

[Eratosthenes (Peter)]

Andrew,

I think a 19.999745" is the way to go - overall it performs better under various conditions.

I think it's worth the extra cost and the additional weight compared to your 19.999" should be okay if you have a friend to help you with loading onto the truck. (I would also consider the 19.99999614" which is heavier and out of my price range. Of course there are some 19.9" scopes coming out of China on ebay for about $75 - stay clear of them, the plastic mirrors are light but are near impossible to allign)

[LewisM]

I dunno.

I had once a Takahashi FCT-100 - considered the BEST 4" telescope of all time, heck, even considered the best optically of ANY refractor before or since. Yes, it had a reputed Strehl of 0.998 (call it parity really) - absolutely diffraction limited in every sense of the word. Also, VERY rare telescope (I paid a ransom for it). A telescope Roland happily tips his hat to. Only downside to it was it was pre-MC days, so the contrast wasn't as good as recent offerings (but still better than many!). Tak still makes a limited custom offering in the guise of the FCT-250 and FET-300 - both telescopes that I recall costs in excess of $100,000 (and come with their own custom mount, installed by Takahashi personnel).

It is both a visual (mind blowing) and imaging scope, as it laid the foundations for the FSQ legend.

So, as 4" goes, FCT-100 hands down. It is legendary, though many have not even ever heard of it.

For modern renditions for imaging, of course the FSQ-106N cannot be beaten. The FSQ-106ED may have slightly better correction, but I, and many others, feel the contrast and colour rendition of the N better to the ED, plus many of the N's can be found with better optics (higher Strehls) than the ED's.

I think for the larger refractor apertures, the Tak TOA 130 and 150 are right up there, but Roland and Yuri sure give them a VERY good run for the money and surpass them in many ways. The Tak FSQ130 is an unproven beast right now, so the jury is out on that one.

Mirrors? They hang on a wall... I know NOTHING about them, as my ugly mug tends to rack them

[alocky (Andrew lockwood)]

Originally Posted by Eratosthenes

Andrew,

I think a 19.999745" is the way to go - overall it performs better under various conditions.

I think it's worth the extra cost and the additional weight compared to your 19.999" should be okay if you have a friend to help you with loading onto the truck. (I would also consider the 19.99999614" which is heavier and out of my price range. Of course there are some 19.9" scopes coming out of China on ebay for about $75 - stay clear of them, the plastic mirrors are light but are near impossible to allign)

True - but that aperture can exceed the 20" criteria on a warm day, although i believe you can get the optics made from a single flawless piece of diamond from Zeiss.
After Greg's recent purchase, this question and the coincidence of my children watching Disney's Snow White I had visions of Greg in his observatory preparing a poisoned apple...
Cheers,
Andrew.

[RugbyRene (Rene)]

Originally Posted by alocky

True - but that aperture can exceed the 20" criteria on a warm day, although i believe you can get the optics made from a single flawless piece of diamond from Zeiss.
After Greg's recent purchase, this question and the coincidence of my children watching Disney's Snow White I had visions of Greg in his observatory preparing a poisoned apple...
Cheers,
Andrew.

Seriously you guys? No wonder these forums seem so impsoing to new posters. Someone asks what is a very valid question and you guys turn it into a joke. Instead of making fun of the question maybe, just maybe you could contribute something worthwhile.

[alocky (Andrew lockwood)]

Originally Posted by RugbyRene

Seriously you guys? No wonder these forums seem so impsoing to new posters. Someone asks what is a very valid question and you guys turn it into a joke. Instead of making fun of the question maybe, just maybe you could contribute something worthwhile.

OK, fair enough. Since I've never contributed anything worthwhile before I shall attempt to now. The 'best' optical system I assume to be defined by spot size across the imaging sensor area, since Greg asked about imaging (which, incidentally, he is not exactly a novice at). This will be the smallest for the largest aperture of any given optical design. End of story - very basic physics. Assuming the designer and optician have done their job there are a few options commercially available and I would argue none are 'the best'.
Now as to why anybody actually considering undertaking the purchase of an imaging instrument of this size would be relying on the opinion of an open Internet forum -? Then I sincerely hope they do some more detailed research. You are not going to buy it from Australian Geographic, and it is probably not for the easily intimidated or humourless.
Better?

[marc4darkskies (Marcus)]

Originally Posted by alocky

... Assuming the designer and optician have done their job there are a few options commercially available and I would argue none are 'the best'.

+1. Although, I think the most often suggested answer to questions like Greg's is ... "mine". (Think about it)

Originally Posted by alocky

Now as to why anybody actually considering undertaking the purchase of an imaging instrument of this size .... You are not going to buy it from Australian Geographic, and it is probably not for the easily intimidated or humourless.

+2

[LewisM]

Originally Posted by marc4darkskies

+1. Although, I think the most often suggested answer to questions like Greg's is ... "mine". (Think about it)



+2

Make that +2 and +3

[sn1987a (Barry)]

Originally Posted by RugbyRene

Seriously you guys? No wonder these forums seem so impsoing to new posters. Someone asks what is a very valid question and you guys turn it into a joke. Instead of making fun of the question maybe, just maybe you could contribute something worthwhile.

http://www.motifake.com/image/demoti...1235455903.jpg

[LewisM]

Originally Posted by alocky

True - but that aperture can exceed the 20" criteria on a warm day, although i believe you can get the optics made from a single flawless piece of diamond from Zeiss.
After Greg's recent purchase, this question and the coincidence of my children watching Disney's Snow White I had visions of Greg in his observatory preparing a poisoned apple...
Cheers,
Andrew.

Ahhhh, thank the Gods they weren't watching Frozen.

[alocky (Andrew lockwood)]

Originally Posted by LewisM

Ahhhh, thank the Gods they weren't watching Frozen.

Noooo 😱!
Last time they put that on I had to listen to Led Zeppelin I-IV three times each to clear out the ear-worms....

[Eratosthenes (Peter)]

Originally Posted by alocky

True - but that aperture can exceed the 20" criteria on a warm day, .....

...good point. You should always buy OTAs in the winter time. A mate of mine managed to buy a classic 19.9999997621" research grade scope for the price of a 19.9999954" (he used the money he saved on a couple of Barlow lenses and one of those electric tooth brush gadget thingies)

[sn1987a (Barry)]

How about one of these with each sub 20"?.

http://www.diyphotography.net/wordpr..._Telescope.jpg

[alocky (Andrew lockwood)]

Originally Posted by sn1987a

How about one of these with each sub 20"?.

http://www.diyphotography.net/wordpr..._Telescope.jpg

That's definitely the way to optimise the weather window for narrowband! Either that or the owner is a trilobite...

[AstralTraveller (David)]

Originally Posted by alocky

That's definitely the way to optimise the weather window for narrowband! Either that or the owner is a trilobite...

Well he won't be needing it then ... being extinct and all.

[Paul Haese]

How long is a piece of string? You don't really expect a rational answer here Greg do you? Isn't that a budget related question?

My take on it.

If you had the money you could buy the technical best, but you would want to put it in the best conditions otherwise it is just a trinket really. It would never perform at its best in sub standard conditions and a lesser scope in better conditions might just perform better.

There are plenty of scopes I would love to own, but non would be considered the best for one reason or another. The same as there is not one screw driver that will drive all screws; there is not one scope that can do all the imaging that can be done.

So the best scope does not exist in my opinion. It is a technical myth. Something to dream about.

BTW a 4" scope would not qualify as the best scope under my criteria and many others I'll warrant. It might be a good work horse if it got worked.

[gregbradley]

Some funny responses. Poisoned apple is in fact already made.

4 inch scopes for imaging usually boil down to Tak scopes. TSA102, FSQ106N and 106ED. I am not sure how good the AP versions were, I've heard variable opinions. The 106N gave great colour and was very sharp but it vignetted (that strange black bar through bright stars along the perimeter), had very little backfocus but a rock solid focuser. FSQ106ED focuser is not rock solid, the focus lock is defective, the colour correction may be good but the scope has a slight colour bias and colour rendition is not as good as 106N. The FSQ130 may take all that a step forward.

125-130mm you've got AP 130 GT and TOA130 and TSA120. APM make some in this area.

140-180 you've mainly got TEC and APM and 2nd hand AP (quite rare).
I always thought one of the best imaging scopes of all time was the AP155. The AP140 is a scaled version of the AP155 (same lens, same body just a different lens cell). TOA150 is superb as well. Although I do see people selling them after a while. Not sure why, maybe they are portable imagers and don't like the weight. The TEC140 is the most popular scope in the 140 band. Yuri kicked a goal with that one. Yuri though does not make reducers/correctors (he's a visual guy).

RC's of course are fabulous. The latest 20 inch Dall Kirkhams etc don't seem to be making the same incredible images the RCOS 20 inch scopes made famous by Russell Croman, Jay Gabany, Ken Crawford and others.
Pauls 12 inch RC images show the GSO scopes can hold their own with some work and bang for buck may well be the best.

So possibly the older RCOS 20 inch is the best all time imaging instrument. Tiny stars, super sharp, they work well with the STL11 camera.

The only images I have seen rival the older RCOS 20 inch images (you don't see many these days) have been Martin's CDK17 images from Sierra Nevada. Otherwise they are largely unrivalled except for Adam Blocks RCOS 32 inch images!

Greg.

[Peter Ward]

Chasing an eclipse?.... then a 20" CDK is probably the wrong choice.

Like to visually observe planets?.....then a Honders Astrograph is also a bad choice.

I've found one instrument simply can't do it all....so it comes down to "horses for courses"...and sure, while we'd all like price not to be an object, that often doesn't apply.

A 99.9% Strehl, zero-CTE mirrored, carbon OTA, half-metre diameter optic is as probable as a lotto-win for many.

However, some excellent, purpose built optics can be acquired at very affordable levels.
Rolf's truss reflector is a superb example of an imaging machine that shames many commercially made telescopes.

That said, T-minus 80 days (or so) until my all-singing-all dancing 16" Alluna arrives...so I might have a different perspective then