refractor design poll - what would you use it for?

[Guest Altair]

Guys, please vote on this poll and help influence our next large refractor design.

We are thinking Fluorite or equivalent ED Triplet APO, SFT focuser 3-3.5", probably oil spaced lens assembly, able to handle a good degree of thermal shock. Optional optical test, hand made case, large format flattener.

We are trying to get a feel for your perception of scopes of this calibre, and what you feel you'd like to do with them.

Edited July 2, 2012 by Altair

[Guest Altair]

come on folks we need more info for meaningful data

[Guest Eddy]

I'd vote but it would be out of my budget so it wouldn't really be any help.

[Sheila]

Sorry don't know enough yet to make a comment :-(

[cosmic dave]

I love refractors, but they are very expensive. Give me a shout if have any free samples lol

[Guest Altair]

To answer some questions, it's all about contrast if you're a planetary observer, and focal ratio if you're an Imager. Resolution which is a result of aperture is also important.

Here's a comparison of some designs I'm considering for the future and their theoretical performance comparison:

Telescope type: 160mm Super APO approx. F6.8 FPL53 ED Triplet

Cost: Significantly more than the Super Planetary Cassegrain

Resolution: 0.725 arc sec. (Dawes formula)

Contrast: Excellent contrast. Best possible uses 100% of aperture

Photographic: Excellent for deep sky medium to wide field imaging with flattener. Respectable webcam imaging, but not enough resolution for very small details.

Lunar/Planetary visual observation comments: Very good. Image not as bright at high power as the Cassegrain due to smaller aperture.

Deep sky observation comments: Good under dark skies where relatively brighter objects and high contrast optics compensate for smaller aperture. Able to use OIII and UHC filters for wide field objects.

Summary: Does it all visually, and works well for medium field imaging of DSOs.

Telescope type: 10†Super Planetary Cassegrain 25% central obstruction, F16-20 DK or Classical Cassegrain

Cost: Medium

Resolution: Excellent. 0.5 arc sec. (actually theoretically better, but this is Dawes "Ceiling")

Contrast: Very good, almost as good as a refractor of equivalent aperture, and significantly better than an SCT.

Photographic: Don’t bother with deep sky imaging. Webcam imaging of planets is however very good.

Lunar/Planetary visual observation comments: The winner under good conditions for Lunar/Planetary viewing. Can use long FL eyepieces for more comfortable viewing. Image very bright, compensating for lower contrast.

Deep sky observation comments: Good light grasp for smaller objects like planetary nebulae and globular clusters.

Great for double stars. Able to use OIII and UHC filters for small objects. Not good for large, extended objects as field of view too small.

Summary: Specialist scope for those who want a large aperture planetary telescope for visual and webcam use. (Also very good for double stars and small concentrated DSOs like Globular clusters).

[Guest Tweedledum]

Hi Nick,

For me it is about compromise, I don't think I would consider an apo of that size. My next big purchase (when house gone) is a 12" sct. And an achromat (you know the one!!!!)...

Having had, large newt, small frac, big frac, medium frac,medium mak (fries) and a 10" sct, then the sct on forks wins hands down every time for me anyway. Its quick to move in alt az, stuff a wide angle ep in and you don't need goto except for the faint fuzzies.

I guess this apo would be aimed at the dark art of astrophotography, it would be interesting to put one in the hands of one of the practiced folk here and see what results they get.

However its really great to see the forum potentially steering some design and stocking of new scopes, and its great that you thought to do also, and to bring it to market.

Best Regards

Damian

Edited August 3, 2012 by Tweedledum

[philjay]

Hi Nick

I am a refractor nutter, (just ask anyone here) and I have several so unfortunately filling in a poll on what refractor I would buy would be a little bit of a problem because I have a refractor for most disciplines from F15 achros for lunar and planetary to 98mm Apos for widefield and imaging so I cant be of much help there; except to say that my most used refractor is the Meade 127 EDT.

This scope is about as close to a general purpose refractor as I can get, At f7.5 and with good colour correction its a stonking planetary and lunar scope but it takes exceptional images as well. I use it frequently with or without a reducer with a canon 1000d or Atik 314 ccd cameras. The apeture and focal length make it a good compromise between light gathering power and portability, its big but not too big.

So from my experience, if I were to have to own one refractor it would be a 127mm F7 - 7.5 triplet apo, they are about as general purpose as you can get so if your after a refractor design that will be useful for a wider market then I would gop for that.

But unfortunately these days a lot of folk seem to be hung up on short focal length apos even for visual but from experience these dont necessarily make the ideal general purpose scope.

Hope thats of some use

Phil

[Guest Altair]

thanks for the feedback guys, Phil I know what you mean, I love refractors - always have! Damian, my thinking is that we've pretty much covered the apertures up to 130mm, so I'd like to explore the larger apertures.

This scope would bridge the gap between imaging and visual use - a kind of "crossover" design around F6.5 to F6.8 or thereabouts. On a large APO, aside from colour correction and a larger usable field, the real benefit of a scope this size is going to be aperture and therefore resolution while still retaining the qualities of an APO refractor. That's a performance improvement both visually and photographically. If we use FPL53 we can get down to F6.5 or faster without any visible colour aberration even at this large size, though the lenses would have to be aspheric which is an intensive process, hence a high cost per unit and at least a 3 month wait time. Another reason I don't want to go too long in focal ratio, is that or a scope of this aperture, you're adding 160mm per F stop in tube length, and that's important from a practicality point of view. The view through a 130mm scope compared to a 100mm scope really is night and day. (Our 115 kind of straddles that area with very good blue colour correction photographically), but a 160mm would be in a league of it's own compared to a 130mm and would noticeably outperform a 150mm scope too, both visually and photographically.

In terms of a planetary Cassegrain, I think that would be a specialist scope for the planetary enthusiast only, curved spider vanes to get rid of diffraction spikes, a very high Strehl mirror, probably Supremax, and a small central obstruction of around 25%. That would theoretically come very close to a large refractor but with a lot more aperture and therefore resolution to overcome the theoretically slightly lesser contrast. Purely designed for planetary visual and webcam use only really - although double stars, globular clusters, and small nebulae would be great. F18 seems to be the sweet spot according to calculations, yielding a focal length of 4500mm. That extra aperture, and long focal length would mean you could use eyepieces like the longer Televue Plossls and still retain good eye relief with very high sharpness. The field needs to be coma free at around 25mm, fully illuminating a 1.25" eyepiece. An open truss-tube design would be nice for managing the thermal side of things, and reducing tube or mirror convection currents. That's pretty much an optimum design for planets. I have compared my Mewlon 250 (then owned by someone else) to a 175mm Astro physics long FL refractor in Devon a long time ago, in some of the best early morning seeing I have ever experienced in the UK, and it beat the refractor hands down on ring and "surface" detail on Saturn. Thing is the scope has to be cool and the air steady - and it took most of the evening to cool it down. An open truss-tube design with fans would enable very fast cool-down by comparison.

So I guess you'd have a large 160mm APO which has plenty of aperture, which does very well on the visual and photographic side of things, and a 250mm Cassegrain scope which is optimised for planetary observing. The APO does photo-deepsky/visual/planets, and would cost significantly more than the planetary Cassegrain, but if you're a planet enthusiast, then you wouldn't want all those extras the APO provides. Therefore the two products would address two separate needs in the market. Anyway that's just an idea, next some CAD drawings starting with the Cassegrain, then the dreaded cost estimates - might as well check it out for feasibility.

(edited my spelling!)

Edited August 4, 2012 by Altair

[Guest Tweedledum]

Hi Nick,

Agreed, thats why I want one of those 152mm achro's you are going to import, and a nice shiny new ( To me) 12" SCT!!.

Cheers

[Caldwell 14]

I only own one scope, an ED100, I have no intension of up-grading, I seem to be immune to aperture fever. 3 out of my 4 scopes have been 4 inch refractors, the odd one out was a Newtonian reflector which I only kept for 3 weeks, I sold it and bought another refractor as fast as I could

0n edit...BTW I voted, come on its only a telescope

Edited August 4, 2012 by Caldwell 14

[cosmic dave]

I'm very happy with my 120mm refractor, I will go to a 150mm one day but am in no rush to upgrade. I to tried a reflector bit got rid, think its just what u get used to.

[Guest Altair]

Here's an interesting comparison between a Cassegrain and and APO under good conditions and very similar to my experience in Devon.