Chriske

As a matter of fact I'm working on a new focuser myself. It needs to be installed in a 75mm OD wooden tube. The focuser is a Crayford and will be installed completely in the telescopetube itself. It'll be a challenge ...

This drawing is something like f/6.5. there you will vignetting indeed. I 'm pretty shure a refractor that short will have chromatic errors in case of doublet. And as you were talking about cheap scope I'd think of longer FL. So far less Cr-errors and most important far less vignetting.

Just measured all my 2" eyepieces and the largest fieldlens is 36mm So in my case it would look like this... You could almost park a car in between...😁

The way I see it about that spacing between drawtube and telescope tube. Let's assume the largest fieldlens of your eyepieces is 40mm. Then you have lots of space between the two tubes. Therefore the drawtube needs to be like this.

Is it possible to send me these files so I can open and assemble that focuser in my CAD software.

Not sure I understand the concept you're building here but you do have more space then you think in that OTA. A 2" eyepiece has a 50.8 mm OD. But most of these big eyepieces have a field-lens of only 40 to 45mm diameter. So that give you more space to work with in that optical tube. Yet again I'm not sure what your building, sorry about the ignorance...😟

Indeed, and why not just use threaded rod to do the motion. Of coarse you need to redesign the whole setup.

+1...🙃

One more modification, added a belt tensioner(yellow) About 25mm long Bolt (M3) on the right is pivotpoint of the tensioner Centre bolt is the pusher (brass) Left bolt is the tensioning bolt

😳???

...one tool, two mirrors...

Due to a few reasons we cannot start grinding our 300mmbBrunn telescopes. Another 2 months before we can start grinding. Eager as we are to start building/grinding something we've decided to start another smaller project. We will build a Delmarva Schief each. I don't have to build the scope because I already have a nice wooden version that I got from a friend years ago. Mirrors were not included. I'll be making all the optics and the rest only need to build their scope. The plan is to start grinding 250mm mirrors and cut 3 sets of 108mm mirrors out of the large 250mm mirrors. Because there four of us I need to grind 2 sets of 250mm mirrors. Just finished hogging to the correct depth, so I'll start finer carbo today. This how my Schief will look like. The others will make their own version of coarse. The Delmarva is a anastigmatic version both mirrors have the same ROC. It is a Dave Groski design btw, he knows 'a thing or two' about Schiefs...😁

It's simple, one alu pulley (on the right) is a regular one. the two other (printed) have threads inside.

Vlaiv, Maybe I completely misunderstood, but I think you're referring to this...? Two threaded rods, two (driving)nuts and two LMUU bearings Three T2 pulleys, a closed timing belt and a few bevel gears.

Maybe we'll go this way. Two poullies and a timing belt. Reduction 1:4

Apparently I'm not the first to do this circular movement thing. A guy in the Netherlands used it and still do. And even posted animations about the technique he uses on YT.

AWESOME...!!!😳😳

Well doing some more tests I found that the centre is indeed much deeper then the rest of he mirror. As a matter of fact it is exactly the same phenomenon as a regular long straight stroke during roughing. It also needed some 'repairing' at the end of the deepening session just the same as in the past. To correct this I did not use the classical third stroke as we always do but continued using circular strokes. But this time I used far smaller circular movement. And guess what, the correction using a shorter radius movement did took far less time to end up with a correct sphere. It's all new to me too, but it looks promising...

circular grinding.avi

Hi guys, It's about 42+ years I'm building telescopes and grinding mirrors. Grinding mirrors, especially deepening the mirror with course carbo, I always push the mirror using very long W-stroke most of the time concentrating on the edge of the tool. Thinking about 'concentrating on the edge of the tool', (while I was grinding..😁)I thought why not work a fully 100% on the edge of the tool avoiding the centre of my tool completely. There's only one way to do that and that is using circular strokes with the mirror on top of the tool, not touching the centre of the tool at all.! And so I did... Did it work and most of all is there an advantage using these circular strokes movements...?? The answer is yes/yes/yes Indeed it works, and what's more this way of roughing out is the fasted way of (manually)grinding I ever did...😳 Even the classical very short straight strokes concentrated at fully 100% edge of the tool does not work this fast. The third 'yes' is there because rotating mirror and/or tool is not even necessary. I still do from time to time because I'm used to it all these years, and just to be on the safe side. Why didn't I think of it before...🙄 Maybe someone tested this way of working before, don't know. Never read about it. Did a search on 'circular grinding telescope mirror' but didn't found a thing about it. Except someone on the net(YT) suggested using circular movements parabolizing a telescope mirror. Anyway, the first mirror I did this way, while roughing out, I overshot my target completely. So if you try this, measure very often...!

This exploded view shows how it looks like inside the Crayford. In the white part there's a ball bearing that fits over the brass rod.

Crayford part of the focuser. It has a dual speed knob. The two bolts are for adjusting tension on the barrel.

For maximum performance the eyepiece of this scope needs a tilt of 8.88° So this what it will look like. On the left, Focuser barrel, 8.88° correcting device, 2" to 1"1/4 adapter, and on top the eyepiece. Every eyepiece needs it's own 8.88° adapter, except for the par-focal eyepieces of course.

Next (drawing)step.Secondary cage and also a small shield just below the secondary mirror Between the trusses we will add cross-connection to make it all very stiff.