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Refractor project — good idea or bad?

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8 replies to this topic

#1 RefractoryTWO

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Posted 11 February 2019 - 11:10 AM

Recently saw a listing for an upgraded 102mm F/7.7 OTA (dovetail and better focuser added) for about $100. For that price, I assume it’s an ordinary achromat.

 

My first thought was, What if I buy this and swap out the objective with an ED doublet? Could be a fun project.

 

I searched online, and specifically in CN, but didn’t find mention of any companies that sell ready-to-install lenses. Is this even possible? Would it be as simple as ordering a 102mm F/7.7 doublet and installing? Would it be cheaper than buying a new 102? Thanks.



#2 Stellar1

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Posted 11 February 2019 - 11:17 AM

i'm no scope builder but I'm assuming if the new objective has the exact same focal length it may work, i'm sure others will weigh in.


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#3 J A VOLK

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Posted 11 February 2019 - 11:21 AM

not worth the trouble IMHO, look for a good deal (used?) for the scope you want

Edited by J A VOLK, 11 February 2019 - 11:22 AM.

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#4 SeattleScott

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Posted 11 February 2019 - 11:24 AM

You could call Mr StarGuy to see about one of their 103mm lenses. They are F7.7. That being said you would probably have to machine an adapter for the lens cell. APM sells their 6” ED doublet objective separately but it is designed to fit a C6R tube. They provide the adapter.

Unless you have a metal machine shop and a lot of spare time, I wouldn’t bother with it. Or just use it as an achro.

Scott
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#5 John Huntley

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Posted 11 February 2019 - 11:24 AM

I think you would find that an ED doublet objective lens (assuming you could find one) would cost close to as much as a whole ED scope. Your overall costs (given that you have already invested $100) could be higher than just buying an ED doublet scope off the shelf.


Edited by John Huntley, 11 February 2019 - 11:25 AM.

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#6 ngc7319_20

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Posted 11 February 2019 - 11:36 AM

Not worth it.  Save your pennies and just buy a ED scope.  Maybe get one second hand?  Much better re-sale later than some home brew.  Also, the entire scope needs to be well made to get full benefit of ED lens.  You cant just drop the elements of a ED doublet in an achro cell and expect proper collimation, spacing, etc.  Lens cell needs to be mounted accurately square to the tube, etc.


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#7 Astro-Master

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Posted 11 February 2019 - 12:06 PM

If you are only using the scope for visual observing $100 is not a bad deal.  Just have some fun with it.


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#8 db2005

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Posted 11 February 2019 - 12:07 PM

Recently saw a listing for an upgraded 102mm F/7.7 OTA (dovetail and better focuser added) for about $100. For that price, I assume it’s an ordinary achromat.

 

My first thought was, What if I buy this and swap out the objective with an ED doublet? Could be a fun project.

 

I searched online, and specifically in CN, but didn’t find mention of any companies that sell ready-to-install lenses. Is this even possible? Would it be as simple as ordering a 102mm F/7.7 doublet and installing? Would it be cheaper than buying a new 102? Thanks.

TS in Germany do sell various APO objective assemblies:

 

https://www.teleskop...l--Linsen-.html

 

When you see the listed prices it's easy to understand why such an undertaking is not likely to be a worthwhile project if your goal is to reduce cost.

 

Rather, if anything, I'd probably go the other way: buy a new or lightly used run-of-the-mill ED scope and consider upgrading the focuser with the focuser harvested from the old OTA. If it's a really good focuser that alone might be worth considerably more than $100.


Edited by db2005, 11 February 2019 - 01:52 PM.

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#9 RefractoryTWO

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Posted 11 February 2019 - 01:34 PM

Not what the tinkerer in me was hoping to hear, but good to know. Thanks, everyone!




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