34 replies to this topic

[RichA]

Lets say you could get hold of a lens with a guaranteed 1/8th wave P-V profile, still an achromat.  Would you replace the objective in a Shortube?  Or is it a case of "pearls before swine?"  In other words, does it "violate" the concept of the Shortube 80mm as a very inexpensive, low power sweeper?  Should, if they were available, high-quality achromats be confined to more refined tubes with 2-speed focusers, collimatable cellls, machined components?

[lylver]

I have an old one, known as Photon 127f/9 around year 2000.

It has some blue chromatism but lenses are well made, with a very good star test.

I tuned the air gap (2.5mm now versus 2mm when I got it) and did the lens retro-engineering. Spherical aberration PtV is at the level you said from 535 to 615nm.

As it can magnify a lot now (>x200 on Moon) I made a effort for fine focusing and will do for internal baffling

Edited by lylver, 10 May 2024 - 02:31 AM.

[Jon Isaacs]

Lets say you could get hold of a lens with a guaranteed 1/8th wave P-V profile, still an achromat.  Would you replace the objective in a Shortube?  Or is it a case of "pearls before swine?"  In other words, does it "violate" the concept of the Shortube 80mm as a very inexpensive, low power sweeper?  Should, if they were available, high-quality achromats be confined to more refined tubes with 2-speed focusers, collimatable cellls, machined components?

 

How much would it cost for what focal ratio and aperture?

 

Jon

[bobhen]

Lets say you could get hold of a lens with a guaranteed 1/8th wave P-V profile, still an achromat.  Would you replace the objective in a Shortube?  Or is it a case of "pearls before swine?"  In other words, does it "violate" the concept of the Shortube 80mm as a very inexpensive, low power sweeper?  Should, if they were available, high-quality achromats be confined to more refined tubes with 2-speed focusers, collimatable cellls, machined components?

The concept of a fast (say F5) "standard" achromat from 80mm to 150mm is all about low power, sweeping and deep sky observing. There is no need for anything more than diffraction-limited optics for that purpose. Most all aberrations have minimum impact when the magnification is low. The attractiveness of such scopes is their low price and bang for the buck delivery, if used for that particular purpose. 

 

At one time I owned a University Optics 80mm F6.25 refractor. These are regarded as having a very nice and sharp Japanese lens (using FK5) and rightfully so. That scope was much sharper than the Orion 80mm F5 that I had but not a true ED lens. The tube was all metal.

 

However, under a dark sky, my 102mm F5 achromat (like the Sky Watcher Startravel) was easily the scope of choice for low power deep sky observing, even though the lens in that scope was not as well figured and was faster than the University Optics 80mm. For other uses, the UO 80mm was better. But today, an "excellent" 80mm ED would best the UO 80mm, even though the old UO was an excellent scope for the day.

 

HERE is one link to a thread here on CN about the UO 80mm
 

HERE is another link to a thread on CN about the UO 80mm

 

Below is my old UO 80mm.
 

Bob

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[Echolight]

Are you going to pimp out the 60 ?

[dan_h]

An 80mm f5 achromat has significant limitations that cannot be overcome simply by replacing the objective with a higher "quality" lens that is still an 80mm f5 achromat.   It is pretty easy to throw a lot of money at an inexpensive scope but generally it will not change the character of the scope in the end. 

 

"Should, if they were available, high-quality achromats be confined to more refined tubes with 2-speed focusers, collimatable cellls, machined components?"  

 

Are you implying that there are no high-quality achromats available today?

 

dan

[Echolight]

Sure. I'd put a triplet with two fpl-53 elements in my ST102.

[gnowellsct]

Sure. I'd put a triplet with two fpl-53 elements in my ST102.

Or maybe glue all three lenses in correct order to a yardstick, forget the tube, prop it up on a broomstick and you're good to go.  --GN 

[RichA]

How much would it cost for what focal ratio and aperture?

 

Jon

Not sure.  I'd guess $400.00 or so.

[RichA]

An 80mm f5 achromat has significant limitations that cannot be overcome simply by replacing the objective with a higher "quality" lens that is still an 80mm f5 achromat.   It is pretty easy to throw a lot of money at an inexpensive scope but generally it will not change the character of the scope in the end. 

 

"Should, if they were available, high-quality achromats be confined to more refined tubes with 2-speed focusers, collimatable cellls, machined components?"  

 

Are you implying that there are no high-quality achromats available today?

 

da

I don't know of any high quality achromats with fast focal ratios in amateur telescopes.

[RichA]

An 80mm f5 achromat has significant limitations that cannot be overcome simply by replacing the objective with a higher "quality" lens that is still an 80mm f5 achromat.   It is pretty easy to throw a lot of money at an inexpensive scope but generally it will not change the character of the scope in the end. 

 

 

Poor surface quality, inaccurate curves, will only compound existing problems.

[Jon Isaacs]

Not sure.  I'd guess $400.00 or so.

 

$400 buys an 80mm f/7 ED with very nice mechanicals.  Optically, it would outperform a 1/8 wave 80mm F/5 achromat because of the chromatic aberration.

 

I have a 80mm F/5 with a 2 inch focuser that has a decent lens.  It offers a somewhat wider field than the AT-80LE which is an 80mm F/6 FPL-53 doublet but otherwise, the AT-80LE is a much better scope both optically and mechanically.

 

Jon 

[ericb760]

As a "Proof of Concept" exercise I have mated a William Optics 80mm "Semi-Apo" Megrez II lens cell to a cheap Chinese optical tube (80/500). The pics don't show it but I added a GSO 2-speed Crayford as well. Since the lens cell and body do not mate together natively, and it has turned out to be prohibitively expensive to manufacture a mating collar, I attached the cell to the tube using electrical tape. The resulting Frankenscope performs very well, indeed. I need to add more baffling, but so far I am impressed with the performance.

 

 

[topomountain]

i have two japan f5 achromats from vixen

 

80 f5 tasco 10vr

60 f5 tasco 9vr

 

these are nothing like the st80

 

they both have very sharp lenses, of course they are f5 but it is easy to tell the difference.... about once a year i see a vixen 80 f5 achro come up on CN and they sell fast

 

i would gladly pay for a 100 or 120 f5 for that kind of lense

[Echolight]

I actually don't think I'd buy another 60 to 80mm conventional telescope.

 

Maybe a spotting scope with ED glass.

[RichA]

$400 buys an 80mm f/7 ED with very nice mechanicals.  Optically, it would outperform a 1/8 wave 80mm F/5 achromat because of the chromatic aberration.

 

I have a 80mm F/5 with a 2 inch focuser that has a decent lens.  It offers a somewhat wider field than the AT-80LE which is an 80mm F/6 FPL-53 doublet but otherwise, the AT-80LE is a much better scope both optically and mechanically.

 

Jon 

Except the ED will likely violate two positives of the Shortube:  Lightweight and size.  The idea isn't to be optically perfect, just better  while preserving the attributes of the Shortube.

[ABQJeff]

Except the ED will likely violate two positives of the Shortube:  Lightweight and size.  The idea isn't to be optically perfect, just better  while preserving the attributes of the Shortube.

 

Thing is, the F/5 achro lenses could be P-V 1/10 wave and it still not be any better.  The issue with an F/5 achromat is not the lens figure, the issue is it is an F/5 achromat.  The Chromatic aberration swamps things like spherical aberration (SA) that are corrected with a well figured lens.  Then in an 80mm F/5 you also have field curvature which would be more noticeable than the delta of 1/8 P-V performance over diffraction limited P-V 1/4 wave performance.    Some people can cope with the field curvature or are only doing high mag, but there is still the CA issue.

 

So take care of the CA of an F/5 with SD glass (or go to F/10), then you can start addressing the SA and if you have P-V 1/8 wave performance.

[Terra Nova]

What would be the point. An achromat is still an achromat and a short achromat is a short achromat with all that that implies. I built (CNC) a completely custom ST80 and put a defraction-limited Vixen lens in it. It still had the achromatic color blur that comes with such as the magnification was increased. 

[Echolight]

So the triplet with 2 fpl-53 elements it is!

[RichA]

 

As a "Proof of Concept" exercise I have mated a William Optics 80mm "Semi-Apo" Megrez II lens cell to a cheap Chinese optical tube (80/500). The pics don't show it but I added a GSO 2-speed Crayford as well. Since the lens cell and body do not mate together natively, and it has turned out to be prohibitively expensive to manufacture a mating collar, I attached the cell to the tube using electrical tape. The resulting Frankenscope performs very well, indeed. I need to add more baffling, but so far I am impressed with the performance.

 

 

 

 

I had a semi-apo WO scope that was far far sharper than a Shortube and that wasn't down to just colour.  It was a 70mm f6. 

[25585]

Lets say you could get hold of a lens with a guaranteed 1/8th wave P-V profile, still an achromat.  Would you replace the objective in a Shortube?  Or is it a case of "pearls before swine?"  In other words, does it "violate" the concept of the Shortube 80mm as a very inexpensive, low power sweeper?  Should, if they were available, high-quality achromats be confined to more refined tubes with 2-speed focusers, collimatable cellls, machined components?

Yes, for a good optical finder. say a quality spotting scope lens which at 80mm are better than short achro refractors.

[betacygni]

They actually used to sell such a scope, the Vixen 80SS, an 80mm f5 achromat made in Japan. Not sure what wave optic they have, but better than Chinese short tubes for sure. Great Vixen build quality with a 2” rack and pinion focuser. Rare to find used, but are a steal at the price they go for.

Edited by betacygni, 12 May 2024 - 03:56 PM.

[Jon Isaacs]

Except the ED will likely violate two positives of the Shortube:  Lightweight and size.  The idea isn't to be optically perfect, just better  while preserving the attributes of the Shortube.

 

Upgrading an ST-80 means a 2 inch focuser which adds to the weight.  The lack of a sliding dew shield adds to the size.  A clamshell or tube rings are necessary for proper mounting.  With the better optics, one needs something better than the plastic cell that comes with most ST-80s.  

 

This is my current ST-80 though it's one long term loan.  It is unlike the vast majority of ST-80s in that it has a metal lens cells that threads on to the OTA and a metal dew shield that threads onto the lens cell.  The optics are sufficient to easily split the double-double.  

 

 

Just what are you thinking the better optics will achieve?  As others have said, it's still a 80mm F/5 achromat with plenty of CA.  It will never be a planetary-double star scope.  As it comes, it's an inexpensive scope that is rugged with decent optics but rather poor mechanicals.  The obvious upgrades are the rings to mount it and a better focuser.  That turns it into a wide field monster... 

 

 

Jon

 

 

[Jon Isaacs]

So the triplet with 2 fpl-53 elements it is!

 

 

The 80mm F/5 Skywatcher Esprit is the obvious choice.  It's an FPL-53 triplet and only $2230... 

 

Personally, upgrading the optics in an ST-80 doesn't make a lot of sense.  If one is going to spend the money, an F/6 or F/7 ED/apo is the way to go.  

 

Jon

Edited by Jon Isaacs, 13 May 2024 - 05:49 AM.

[COMtnSkies]

Interesting to hear the Vixen A80SS refractor mentioned. I bought a used one years ago and use it primarily for bird watching but it is very sharp for what it is. The 2-in focuser is nice. I use a 38 mm a lot for birds.