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DaveL59

TAL-M refurb

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21 minutes ago, Stu said:

Err hmmm, it’s actually a light thimble, the Telementor is the peashooter 🤣🤣

my bad Stu, the old grey cells don't always play nice, of course I shoulda realised huh, thimble - a kinda teeny tiny bucket... 😄 

Just tried out the barlow combo on a distant (ish) tree canopy that was nice and bright while the sun peeked out. So focuser -> TAL-M x3 barlow -> TAL-1 x3 barlow -> 25mm Plossl. Worked pretty well. OK dim but that's expected with that amount of mag, adjusting focus I was able to see very close into the leaves on the tree as the branch wafted in and out of focus in the breeze. Focus seems soft though, again no surprise.

No idea what mag that actually gave me, the 25mm plossl should give x21 I believe (80/526mm FL) so with x3 barlow that'd make x63. What effect does a barlow in a barlow give I wonder? Would that add another x3 to the first barlow's x63 or be x6 to the EP's x21? So effective result is x189 or x126?

Either way it seems usable as a combo of the 2 barlows and kinda saves me messing about trying to fettle an extension tube. The acrylic tubes would cost around £8 each for the 2 diameters at 100mm length. Not too bad if I do decide to try that one day, easier than trying to work metal with hand tools too 🙂 

 

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Posted (edited)

doing a bit of reading around the web:

Stacking Barlows doesn’t add the factors, it multiples them – thus stacking a 2x with a 3x gets you 6x. Now, on your 1000 mm focal length scope, with a 25 mm eyepiece (40x) paired with that 6x stack, you’re getting a 240x magnification view!

Interesting, that'd mean the combo I have in 32mm fit would give a x9 effect. So I was using the TAL-M at x189 with the 25mm Plossl, wow I'm not surprised the image dimmed and got soft and hard to pin the focus, especially being a tree branch with the sun hitting the leaves. Impressive though that it was useful at all given the 80mm aperture, will be interesting to see how that combo works aimed at Jupiter and Saturn.

Might be worth getting a couple acrylic tubes and making up an extender too for a more in-spec x139 as the stacked barlow could be pushing the scope a bit far, will decide once I've tried it out tho 🙂 

 

PS - source of the above info:
https://orionbearastronomy.com/2019/02/11/pros-and-cons-of-barlow-lenses/

 

Edited by DaveL59

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Decided to have a look at making up an extender today. According to the specs that should allow the scope to reach x139 with the extender, barlow and 15mm eyepiece combo.

The 38mm OD/32mm ID and a 32mm OD acrylic tube had arrived already and the smaller just fits into the larger. Downside is the 32mm OD is a tad over at 32.4mm ish so won't fit into the focuser. The Barlow and eyepiece do slip into the larger tube but a touch loose. Will have to think if how best to address the barrel bore fit later as the older TAL system is push-fit, no lock screws but a spring strip under the lip which I won't be able to replicate. Might tap a 3mm hole so I can pop a thumbscrew in depending on testing results. There's so little play that I won't be able to simply flock the inner to the edge, hmmm.

So on to the first job - cut the tubes. the 38MM to 29.5mm length as per the measurements of an official extender tube. The 32mm I cut to 27mm so that it bottoms out in the focuser tube and overlaps into the outer by 5mm or so. Yeah I got the cuts a tad wonky, always the way huh even with paper rolled onto the tube as a guide. Hand holding against a stop and the saw binding in the acrylic sure didn't help, not that it really matters tho once fitted together, just put the squared end at the eyepiece end so it sits right, the rest is pretty well hidden in use 😄 

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Now to reduce the OD of the 32mm I left the 2 sections push fitted as they are a tight fit and rotated against a file by hand, pressing it against the file with one hand while rotating the part with the other to get a reasonable "roundness". Was going to use penny washers in just the 32mm section and use a drill to speed the process but couldn't locate a suitable bolt, always the way. So by hand and taking regular measurements so I didn't take too much off I eventually got it reduced to just under 32mm OD. Testing against the focuser and is tight so a little more and got a nice smooth fit 🙂 

At this point I separated the sections and popped the 32mm nosepiece into the focuser so I could mark off on the outer how far it'll need to be inserted. Also roughen all the surfaces ready for glue and marker/paint.

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As you can see, I've blackened the nosepiece inner and edges, simple sharpie pen at this stage, I'll do the outer side once glued into place. Probably flock the barrel inner too (or blackboard paint it) as that's a bit shiny right now but we'll see. So mix a little epoxy, lightly coat the inner edge to just before the mark and then slip the 2 sections together up to the mark I made earlier.

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Blackened the nosepiece now, again using the sharpie pen. Don't want to add thickness or it won't fit nor have paint that can flake off and drop into the OTA do we 😉 

Figured I'd smear a very little amount epoxy on the inside of the upper barrel to see if that'll snug up the barlow/eyepiece fit too, may as well use the excess for something 😄 

So does it all fit now?

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The barlow seems more snug in the extender now and all seems to be stable. Will have to wait for the epoxy to fully cure and then I can do the finishing up - paint, flock etc before I can give it a field test. Not a bad result tho I think for an afternoon's play 🙂 

Cost - well approx £16 for the 2 acrylic tubes from the bay, 100x38mm-OD (32ID) and 100x32mm-OD tubes. The epoxy, flock, sharpie, hacksaw etc I already have kicking around as a result of other projects or just general kit. There's enough tube left to make another couple if I accidentally break this one too lol

Oh, please always clean the parts before test fitting so you don't leave acrylic powder inside your lovely optics! For info, I did wash parts and hands each time but just in case someone else wants to try something like this I thought I should mention it 😉 

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Posted (edited)

Figured I'd go the lazy route to finish this up and just flock it. Pah, lazy but not easy... if you've ever tried to flock inside a 28mm bore you'll know it's "fun" lol. Elected to flock the outer larger barrel too to cull any stray light and a thin strip of flock down in the eyepiece barrel too just to help and just enough so the barlow just meets it when fully inserted.

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View down thru the extender

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A bit wonky that flocking job huh 😄 Might redo but let's see how it fairs first as I reckon it'll be well outside the light path anyway. Ignore the glary bit nearest the EP entry as that'll be occupied by the barlow barrel so won't affect the light/image path. The lighter ring where the smaller tube end is - well I am pointing this up at a ceiling with the room lights on, so I guess some more black paint might be needed but not sure it's a real issue given the shape of the barlow and the fact that it's lens element sits lower and into the nosepiece barrel.

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Will decide later if I need to tap for a thumbscrew but I think it'll be ok as-is, the barlow fitting nice and snug with that smidge of epoxy in the barrel inner. Went over it with matt black paint pen just to dull off the surfaces inside the EP barrel area and the tube ends of both sections to control light from brighter objects from finding a way up the sides of the clear tubes.

So there it is, a simple DIY TAL-M focal extender (or Mount as referred in the manuals) 🙂 

Edited by DaveL59

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So tonight the extender got its first test, very frustrating at the start. Jupiter and Saturn low so just above the fence and playing hide and seek behind the branches of the skeleton oak to the SE. So 25mm Plossl and up to 15mm and barlowed, very nice and clear then realised why they looked small, I was viewing via the finder! DOH!!! Switch to the main scope and that's much better, bigger and nice and sharp. I keep doing that with this scope but I guess that's testament to the finder.

So time to try the extender. Manual says... focuser-extender-barlow-eyepiece. All I could get was dark with a bright fuzz and the spider vanes and the obstruction of the secondary. Not possible to focus at all anywhere in the range. Back to the eyepiece/barlow and planet behind branch. After a while trying I was thinking it was just not going to work but then I remembered that for this scope you can use regular 1.25-inch EP's via the barlow so it shifts the focal plane further out, hmmm.

Tried again with the extender but this time focuser-barlow-extender-eyepiece and its a success! I got larger image once the focus was adjusted, on this scope I needed to wind the focuser almost all the way in but I got a nice image and could more easily make out the bands on Jupiter but not much fine detail but it is only 80mm being pushed to x139 after all. Saturn was nice and clear but not able to make out the division in the rings, no surprise there either. 

The refurb has worked well, controls nice and smooth, can't say if flocking has had a huge effect but it can't have hurt either and is a lot darker looking down the tube than the OEM paint that looked light grey. Probably be more noticeable against the moon than the planets. The DIY extender has completed the eyepiece set and allows the scope to reach the max x139 magnification in the spec for relatively little cost or effort so a good overall result I think 🙂 

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