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60mm Refractor FL 432mm - where to place focuser

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#1 m1618

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Posted 14 October 2013 - 12:40 AM

Recently got a classic Tasco 60mm with built-in erector/barlow for terrestrial viewing at a garage sale.
Calculated the FL to be 432mm (only to find it already advertised that way). Currently the tube is ~495mm.

How much would I have to cut to reach focus with adding a diagonal (and removing the existing inline elements)? Current focuser is ~55mm. Is there some rule of thumb to allow play of the draw tube? Thanks for any guidance.

#2 GlennLeDrew

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Posted 14 October 2013 - 02:28 AM

Once you have selected the components, the surest thing is to strap objective cell and focuser to a board and check for the best separation with the range of eyepieces, Barlow, etc., you'll be using. Just use a target a good kilometer or more distant.

#3 PhilHerring

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Posted 15 October 2013 - 02:08 AM

Which model Tasco is it?

#4 m1618

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Posted 15 October 2013 - 08:17 AM

According to the kind folks over at the classic forums, it is a Tasco 5VTE. I should consolidate these posts :)

Found markings on and near what was calculated to be the 432mm focal...suggestive of the previous owner having thought of going down the same path.

#5 m1618

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Posted 16 October 2013 - 03:20 AM

For reference and archive sakes:

http://www.cloudynig...6132650/page...

But from a DIY perspective... it was possible to cut through a 60mm tube with a little handsaw. 1 big error was that I had not measured from the bottom of the focuser thus I was off by several inches at first (I did use the wooden board suggestion though thanks! I just made the wrong marks). Imagine my sore hand from gripping that tube and sawing by hand, twice, in the middle of the night just to get first light! Crazy understands crazy. The way I see it, it was worth it to turn a Tasco from the 60's into an F7...now if I can just attach my DSLR... huh.

#6 PhilHerring

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Posted 16 October 2013 - 05:09 AM

Aah, yes. Here's a picture of mine. It was my first scope, given to me back at the start of the 1970s. I cut it down and added a .965" prism diagonal in the early 1980s. I see from that thread you linked to that I haven't been the only person to do this!

I believe that the FL of the objective in mine is actually about 400mm, measured to the field stop of my favourite eyepiece. I repurposed the original focuser as shown; the tube that houses the varifocal mechanism had a thread at one end that matched the thread for filters in the diagonal, so I cut the end off that, threaded it into the diagonal, and slid the result into the focuser.

Unfortunately, this focuser is way too narrow for this fairly short objective, so only the centre of the field is fully illuminated, making it less than perfect as an RFT. Ideally, you want a fairly short 1 1/4" focuser. A full-size diagram on paper will show you how much drawtube you can live with before it impinges on the edges of the field.

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