38 replies to this topic

[betacygni]

They are nice looking scopes, in my last post I was just questioning how good the optics were at F6..

Significantly better than any achromat, that much is for certain, even a reasonably longer focal ratio one.

Edited by betacygni, 06 April 2025 - 02:17 PM.

[RichA]

This is a great little 70 with a metal focuser! And it’s readily available:

 

https://astronomics....-ota-fpl-53-f-6

Didn't the poster say "achromats?"  It quickly veered off into ED / APO scopes of course..  The reason there are no metal or decent focusers in 70mm achromats is there is no way to build it into the price because the market for 70mm achros caters to Christmas gift buyers who want it cheap because the recipient is a child.

[Jon Isaacs]

They are nice looking scopes, in my last post I was just questioning how good the optics were at F6..

 

The scope in question is an AT-70ED with an F/6 doublet based on FK-61, an ED glass very similar to FPL-51. Mine has quality optics. I was able to separate eta Orionis. StelleDoppie lists it at 1.8" , magnitudes 3.6-4.9. Given that the Dawes limit for a 70 mm is 1.65", I'd say qualifies the optics as decent.

 

Jon

[maniack]

Didn't the poster say "achromats?"  It quickly veered off into ED / APO scopes of course..  The reason there are no metal or decent focusers in 70mm achromats is there is no way to build it into the price because the market for 70mm achros caters to Christmas gift buyers who want it cheap because the recipient is a child.

Well yes, that's all true. Once you add a good focuser you're competing with a nice 90mm achromat (Svbony SV48P) or 70mm EDs.

Synta can probably build a version of their cheap metal R&P focuser for a 70mm. But the price might even be higher than the ST80, since such a scope would have a smaller market than their 80mm options (the ST80 and 80mm f/11 share the same focuser, with the f/11 having a longer drawtube).

[Messyone]

A while back I did this for my 70x900 SW achro with great results.

 

https://www.cloudyni...ts-a-2-focuser/

[VA3DSO]

Didn't the poster say "achromats?" It quickly veered off into ED / APO scopes of course.. The reason there are no metal or decent focusers in 70mm achromats is there is no way to build it into the price because the market for 70mm achros caters to Christmas gift buyers who want it cheap because the recipient is a child.

I’ve owned three different 70mm achromats, yet I’m 56 years young - hardly a child (although I’m sure my wife would disagree!!).

[hfjacinto]

If you REALLY want a 70 MM ACHRO with a metal focuser. 

 

Here is the ACHRO:

 

https://agenaastro.c...e0LpCACUVqfqB0s

 

You'll then need a diagonal

 

https://agenaastro.c...ssion-ring.html

 

So cost comes out to $237

 

Or you can get this :

 

https://astronomics....jgLOTEXqb2K6B48

 

Which is much nicer for $100 more. There is no reason to get a 70MM Acro.

[rob1986]

The above is not a long achro

[RichA]

Well yes, that's all true. Once you add a good focuser you're competing with a nice 90mm achromat (Svbony SV48P) or 70mm EDs.

Synta can probably build a version of their cheap metal R&P focuser for a 70mm. But the price might even be higher than the ST80, since such a scope would have a smaller market than their 80mm options (the ST80 and 80mm f/11 share the same focuser, with the f/11 having a longer drawtube).

80mm's like Orion's now dead Shortube would seem to be the lower cost limit as far as aperture in achromats go WITH mostly metal parts.  But, you could make a 70mm with say an f7 instead of f5 focal ratio and sell it for around the same price which when the Orion ceased production was around $110 for the OTA.  But, Joe Public can't buy one stripped to the OTA (because he has no other accessories) so the "astro" model with accessories to actually use it pushed the price to $189.00.  The cheap 70mm achromats out there now even come with rudimentary tripods.  All-in-one.  Compared to the 80mm Shortube, a 70mm costs just as much, or would, if it existed, if you added up the retail cost of the components (which you can buy now) it would be more expensive.  $27 for the lens, $35.00 the focuser, unknown for the lens cell, unknown for the tube, unknown for the ring to adapt the focuser to the tube, $10 for the short Vixen rail.  Given that, I think it's pretty clear that there is ZERO savings going from an 80mm you can buy now, down to a 70mm scope built the same way.  So what would you be left with?  A small reduction in physical size over the 80mm and a weight savings.   The best solution is to find the slimmest decent focuser and adapt it to a 70mm's tube with an adapter ring which would have to be made.   You can buy a metal focuser which can be made to function very well for $35.00.  You can buy a 70mm telescope for $35.00.  You'll need an adapter ring 3D printed or machined to accommodate the focuser.  So it's likely, even if you do  the printing yourself or get a friend to do it, it's going to cost around $80 to "create" a 70mm with a decent focuser.  For a 2 speed focuser, any economy goes out  the window and you are looking at $200 for a GSO focuser and the cheap scope PLUS an adapter.  Not worth it since you can often buy a 70mm ED scope for not much more used.

 

70mm telescope:

 

https://www.temu.com...mRoCMTwQAvD_BwE

 

metal focuser:

https://www.ebay.ca/...kp:BFBM9IOW0sFl

[maniack]

Well said RichA, but I'll make a plug for a 2-speed 3D printed modification that can be adapted for almost any scope (I've done an ST80, an 80mm f/11, a C5, a C6, and a C8 so far).

[maniack]

There's an AT72EDII for a very good price in the classifieds, sold by a well-respected CN member and refractor aficionado. If I didn't have one already I'd spring for it myself.

[Oldfracguy]

There's an AT72EDII for a very good price in the classifieds, sold by a well-respected CN member and refractor aficionado. If I didn't have one already I'd spring for it myself.

Unfortunately, that one sold late last night:

 

[Chris1485]

All good answers, thank you. It's nice to have other view points and opinions.

I'm not sure about the remark that entry level scopes are aimed just at children.

Anyone can have and enjoy an entry level scope, I did at the age of 45, it cost me £60 (about $70) used with AZ-3 mount and 2x SW EPs.

I think it's a valid point that the cost to produce a 70mm achro with a metal focuser would be close to the ST80, and it is down to supply and demand, there may not be the demand for a 70mm metal focuser achro that there is for an ST80, so the production runs would be smaller and hence more expensive to produce.

I think I'll look at the options for fitting a metal focuser to a 70mm scope, or see if I can get a friend to do some 3D printing for me..

I enjoy tinkering with scopes so it would be a nice project.

 

Many thanks again,

 

Chris (UK)

[RichA]

All good answers, thank you. It's nice to have other view points and opinions.

I'm not sure about the remark that entry level scopes are aimed just at children.

Anyone can have and enjoy an entry level scope, I did at the age of 45, it cost me £60 (about $70) used with AZ-3 mount and 2x SW EPs.

I think it's a valid point that the cost to produce a 70mm achro with a metal focuser would be close to the ST80, and it is down to supply and demand, there may not be the demand for a 70mm metal focuser achro that there is for an ST80, so the production runs would be smaller and hence more expensive to produce.

I think I'll look at the options for fitting a metal focuser to a 70mm scope, or see if I can get a friend to do some 3D printing for me..

I enjoy tinkering with scopes so it would be a nice project.

 

Many thanks again,

 

Chris (UK)

It may come down to affordability.  People can afford an 80mm achro.  No need to go down to 70mm?