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80-100mm with lots of spare backfocus?

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#1 peter k

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Posted 27 November 2022 - 08:09 PM

I've got a 2" filter drawer that works well on my C8 for comparing views with OIII, UHC and H-beta filters. I'd like to use it on a shorter focal length refractor to view larger nebulae, but it eats up about 50mm of optical path, so I can't reach focus with my AT102—even using a 1.25" Celestron prism diagonal. Does anyone know of an 80-100mm refractor, preferably around f/6-7, preferably ED that would give me sufficient backfocus? I guess it'd be possible with some Borg modular kit, but I want to keep it under $1k.



#2 wrvond

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Posted 27 November 2022 - 08:12 PM

I just got an AT80EDL - it's f/7 FL560mm and the focuser has 95mm of out focus. 



#3 peter k

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Posted 27 November 2022 - 08:28 PM

Thanks, Warren. My AT102ED also has over 90mm of focus travel. It's not how far out the focuser can travel—extension tubes can solve that if needed. I need to move the focuser further in than it goes on my scope. The terminology is confusing and maybe I'm not using it correctly.


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#4 Lentini

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Posted 27 November 2022 - 09:41 PM

Yes, my Borg 100ED with a shorter 80mm tube… Loads of in/out focus. But you’re right that all pieces and parts may go over $1k. The next size down is 77mm I think.



#5 gnowellsct

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Posted 27 November 2022 - 11:35 PM

My 92 mm has 175 mm back focus. But it's not under $1k not even close.

One of the reasons I had to get the 92 was that my lomo stellarview triplet Apo did not have enough back focus for me to do solar h alpha. So I sold it and got one that did.

One of the things that budget refractors teach you is their limitations. Back focus is one of them. When you find three or four things that your current scope can't do and that a higher tier telescope might be able to do then you can think about how to budget it.

Of course even the most expensive scopes have their limitations so there is no such thing as the unlimitedly capable telescope.

#6 betacygni

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Posted 28 November 2022 - 02:16 AM

Safest bet is to look for “binoviewer friendly” scopes. These should have plenty of back focus for your needs. Trouble is most are only available used. Stellarvue made quite a few models that were binoviewer friendly. Some models of the SV80ED and SV102 had a removable tube section. They don’t come up used often, but are good performers and would be 1k or less.

#7 peter k

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Posted 28 November 2022 - 07:04 PM

Thanks, all. APM carries a TS Photoline 80 that's supposed to have 140mm backfocus, which ought to be sufficient for my needs, but I've have to get it shipped from Europe, which would involve some expensive shipping—both ways if it turned out not to have the claimed BF.

 

Maybe just get out the old Hackzall...



#8 Nippon

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Posted 28 November 2022 - 07:28 PM

By back focus do you mean the focuser won't rack in far enough? Wish it were called in travel and out travel.



#9 peter k

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Posted 28 November 2022 - 08:17 PM

By back focus do you mean the focuser won't rack in far enough? Wish it were called in travel and out travel.

Yeah, that's what I'm talking about. The term refers to how far inside the objective's focal plane you can rack the focuser. Makes more sense from a photography perspective where you're trying to move the focal plane toward the back (where the sensor is) of the camera. At least that's how I think it goes—I'm strictly visual.


Edited by peter k, 28 November 2022 - 08:17 PM.

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#10 godelescher

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Posted 28 November 2022 - 08:35 PM

TS-Optics sells a bino-friendly 102ED, but it's f/11. It has a removable section of the tube.

 

Is you're focuser threaded on? If not, it's easy enough to cut down the tube and reattach the focuser yourself.

 

If it is threaded, most machine shops should be able to cut it and rethread it farily easily. I imagine it would cost a few hundred bucks to have it done + the cost of an extension for when you don't want to use the filter drawer, but still, a lot less than $1000 for a new scope


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