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Helical focuser for apochromat

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4 replies to this topic

#1 astrosatch

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Posted 22 June 2021 - 12:38 PM

Hi!
Does anyone have any experience using helical focusers for imaging with apochromats?
Any pros/cons using it?

Andrej

#2 chvlat

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Posted 22 June 2021 - 02:54 PM

Haven't had any, but did do some due diligence when looking on for my current telescope. In short I'd say, even though I'm not an imager:

 

(+) Easier to do fine adjustments vs. R&P/Crayford due to large circumference

(+) Lighter, esp. for larger diameter setups

(-) [usually] much more limited focuser travel

(-) Hard to make and maintain, more prone to breakdown and/or to backlash in the long run

(-) Expensive

 

That being said, they're usually quite rare too - so far I remember only 4 makers using them so:

  • Pentax, on their "SDP" and "EDUF/SDUF" scopes
  • Takahashi on the old (but gold) FCT-76 pre-1987
  • Borg, as an option on most of their refractors
  • Vixen VSD-100 (essentially an involved Pentax SDUF)

Of the above, only the last one is available new. I've not read the best comments about the second option (lots of backlash, even Tak went R&P eventually and secondhand ones do usually come modified to counter backlash -- see some old koji postings on AM)

 

On the Borg side I understand most who go for the helical option are usually after extreme portability (mostly when used as a telephoto lens). My current setup was previously used as an imaging-only rig and the guy who sold it to me had gone for the M57 FTF option (instead of the 7835 helical), even though the latter would have more focus travel (19mm vs. 16mm) but I got from him he trusted the FeatherTouch more

 

I've not seen any of the old Pentax helicals (or their current Vixen incarnation) but I've generally heard good things about their precision, smoothness and stability. I understand though they are quite hard to make (supposedly there's only a single machine in Japan which caters towards the VSD focuser production) and as a result uber-expensive even as old scopes on the secondary market



#3 H-Alfa

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Posted 22 June 2021 - 03:05 PM

I use the helical focuser that came with my Borg and it didn't worked in the beginning. The play in the internal thread introduced tilt, so I had to modify it by adding some kind of (=diy) ball bearings. After that modification it worked quite well, even shooting at f/4.

Enviado desde mi Mi 9 Lite mediante Tapatalk

#4 astrosatch

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Posted 23 June 2021 - 01:18 PM

Thanks for your replies.
What about flexure under heavy load? How do helical focusers compare against crayford or other models?

#5 Nippon

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Posted 27 June 2021 - 01:35 PM

Haven't had any, but did do some due diligence when looking on for my current telescope. In short I'd say, even though I'm not an imager:

 

(+) Easier to do fine adjustments vs. R&P/Crayford due to large circumference

(+) Lighter, esp. for larger diameter setups

(-) [usually] much more limited focuser travel

(-) Hard to make and maintain, more prone to breakdown and/or to backlash in the long run

(-) Expensive

 

That being said, they're usually quite rare too - so far I remember only 4 makers using them so:

  • Pentax, on their "SDP" and "EDUF/SDUF" scopes
  • Takahashi on the old (but gold) FCT-76 pre-1987
  • Borg, as an option on most of their refractors
  • Vixen VSD-100 (essentially an involved Pentax SDUF)

Of the above, only the last one is available new. I've not read the best comments about the second option (lots of backlash, even Tak went R&P eventually and secondhand ones do usually come modified to counter backlash -- see some old koji postings on AM)

 

On the Borg side I understand most who go for the helical option are usually after extreme portability (mostly when used as a telephoto lens). My current setup was previously used as an imaging-only rig and the guy who sold it to me had gone for the M57 FTF option (instead of the 7835 helical), even though the latter would have more focus travel (19mm vs. 16mm) but I got from him he trusted the FeatherTouch more

 

I've not seen any of the old Pentax helicals (or their current Vixen incarnation) but I've generally heard good things about their precision, smoothness and stability. I understand though they are quite hard to make (supposedly there's only a single machine in Japan which caters towards the VSD focuser production) and as a result uber-expensive even as old scopes on the secondary market

Didn't the Zeiss APQ scopes use a helical focuser?




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