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A Tale of Two Focusers

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#26 Redbetter

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Posted 07 December 2019 - 12:30 PM

I think Steve was referring to the aftermarket GSO focuser, which has 50mm of travel. 

No, he said the stock Orion focuser had 5-1/4" of travel, which is what I find confusing.  A draw tube long enough to provide that much travel would stick through an inch or so past/through the baffle when racked all the way in..   

 

The GSO's (two Lunt 2" dual speed that I have) have ~73mm of travel.  The Orion 1.25" is about 1" longer racked all the way in (less back focus), because of that threaded visual back piece on the draw tube.  This is in comparison to the 2" GSO without the 1.25" adapter inserted. 



#27 Shorty Barlow

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Posted 07 December 2019 - 01:35 PM

No, he said the stock Orion focuser had 5-1/4" of travel, which is what I find confusing.  A draw tube long enough to provide that much travel would stick through an inch or so past/through the baffle when racked all the way in..   

 

 

This, or something similar, is what I was told about the 2" GSO afermarket focuser. Which apparently cuts into the light cone in effect diminishing the size of the objective by a few millimetres. This is getting way too complicated lol. At the end of the day, the GSO's worked fine in my ST80's if you didn't rotate the focuser. With a 2" diagonal I don't recall needing an extension tube, but with a 1.25" diagonal I often had to use an extension tube.



#28 Redbetter

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Posted 07 December 2019 - 02:57 PM

This, or something similar, is what I was told about the 2" GSO afermarket focuser. Which apparently cuts into the light cone in effect diminishing the size of the objective by a few millimetres. This is getting way too complicated lol. At the end of the day, the GSO's worked fine in my ST80's if you didn't rotate the focuser. With a 2" diagonal I don't recall needing an extension tube, but with a 1.25" diagonal I often had to use an extension tube.

That bit about the 2" GSO focuser in the ST80 is almost certainly wrong for several reasons.  The most obvious is that a chief complaint about the ST80 is that the baffle itself restricts the fully illuminated field to either a few mm, or possibly even reduces the aperture in 1.25" with the stock focuser.  But if you move the baffle back to open things up the 1.25" draw tube is still narrower than the 2" drawtube.  So it becomes a matter of how far racked in the draw tube is to achieve focus.  And for 1.25" operation the draw tube is racked way out, so the front edge of the draw tube is less of an issue.    At f/5 in 2" mode the draw tube is limiting to the fully illuminated field even with the baffle moved all the way back, but that is for a much wider field.

 

However your focuser is not the same GSO focuser typically used in the 80 f/5's.  That 50mm drawtube range is the tip off. 



#29 Shorty Barlow

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Posted 07 December 2019 - 03:44 PM

That bit about the 2" GSO focuser in the ST80 is almost certainly wrong for several reasons.  The most obvious is that a chief complaint about the ST80 is that the baffle itself restricts the fully illuminated field to either a few mm, or possibly even reduces the aperture in 1.25" with the stock focuser.  But if you move the baffle back to open things up the 1.25" draw tube is still narrower than the 2" drawtube.  So it becomes a matter of how far racked in the draw tube is to achieve focus.  And for 1.25" operation the draw tube is racked way out, so the front edge of the draw tube is less of an issue.    At f/5 in 2" mode the draw tube is limiting to the fully illuminated field even with the baffle moved all the way back, but that is for a much wider field.

 

However your focuser is not the same GSO focuser typically used in the 80 f/5's.  That 50mm drawtube range is the tip off. 

I honestly don't know. The GSO focusers I used were the heavy duty ones I think. I have two ST80's, the first was a Sky-Watcher.

 

med_gallery_249298_10284_206061.jpg

 

I spent a lot of time trying to get a tripod/mount combination that worked with the ST80. The AZ3 had serious altitude drift. I eventually ended up with a Porta II/Hal-130.

 

gallery_249298_5348_197006.jpg

 

I also bought new eyepieces etc. The SW had a stuck dew shield and other problems. So it was a 'no brainer' to just replace the OTA. I got an Orion as I could buy it without a mount. 

 

gallery_249298_5348_81285.jpg

 

When I decided to upgrade the focuser I used the Orion first. I bought the heavier duty focuser. At the time it was the only type I could easily purchase. But it worked pretty well and I kept buying the same sort as I upgraded.


Edited by Shorty Barlow, 07 December 2019 - 03:53 PM.



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