John

That is probably the best solution, I agree.

Here is a link showing the disassembly of one of the older type C8 focusers: NGC1514.COM I have a few bits of the assembly which might help - the part they call the brass focusing mechanism, the "lead screw" and the plastic cover / bushing that fits over the brass section. The first 2 items in this pic, plus the "lead screw" that sticks out of the hole in the back plate of the scope, but not the 3 screws, the small circlip or the plate with 3 holes in it (I did have one but gave it to someone else who had lost theirs a few weeks ago): I can let you have some measurements if any of what I have might help.

I believe that would have been the late Paul Dellechiaie, who sadly passed away in June 2022. Paul worked with Tele Vue from 1985 until the time of his passing.

I've highly rated Tele Vue products for many, many years (and bored many on this forum with that no doubt 🙄). I have not yet had the pleasure of meeting any of the TV team but it is good to see that they are such a motivated group of people maintaining high standards and setting the standards that others try to emulate. Thanks for posting FLO's impressions after visiting the company and meeting the Tele Vue staff. Very encouraging 🙂

You know they are no good when your nephew comes back and says "Uncle Chaz, these binoculars you gave me are rubbish !" 😁

The secondary star of Kappa Leonis is magnitude 9.7 I think. With the bright moon nearby I think that would be quite a challenge with a 4 inch aperture. I got Iota Leonis (mags 4.6 and 6.7, separation 2.3 arc seconds) with 4 inch refractors this evening but it was not dead easy by any means.

The Nagler zooms are a 5 element design. They do seem to work in a similar way though. I have been using the Svbony 3-8 and Nagler 2-4 this evening. Both very good performers although one costs nearly 4x as much as the other 🙂 I wonder how the APM Super zoom works ?

Dominant moon again. The crater Pythagoras is looking very fine close to the lunar limb. It's central peak has a very distinctive appearance under this illumination. This image captured in 2006 by Michael A Phillips, USA:

4 inch fun and games 🙂 After posting this: It occurred to me that I have not actually compared these scopes "back to back" for quite a while. So here they are, side by side, trained on Iota Leonis. Vixen ED102SS vs Takahashi FC100-DL:

Not always the case 😉

My Vixen ED102SS F/6.5 doublet needs 30 minutes or so to cool down. Prior to that, high power images are not quite as sharp as you might expect and it's really not worth pushing the magnification over 130x or so. Once cool the scope shows excellent performance and that is maintained to 220x-250x, seeing conditions allowing. My Tak FC100-DL F/9 doublet seems to perform excellently, even at very high magnifications, almost straight out of the house. I've put the scope straight out and split 1.5 arc second doubles at 257x, for example. The Tak performance at very high magnifications is slightly better than the Vixens. It's pretty comfortable even at 300x on suitable targets and the diffraction pattern around brighter stars is more tightly controlled - a very thin 1st diffraction ring, thinner than the Vixen's, even when the Vixen is fully cooled. The cool down difference between these scopes I put down to a) thicker lens elements and b) the fatter Vixen tube (112mm vs 95mm). The performance edge at high powers I put down the Tak having a higher precision objective plus, possibly, the use of a fluorite element rather than the Ohara FPL-53 that the Vixen uses. Both these scopes are extremely enjoyable to use though and present very nice images of the astro targets that I point them at. Their characteristics vary, which is why I'm happy to own both of them 🙂 Nothing to do with eyepiece cool down though, apologies for that 🙄

I wonder if those older ones are slightly better than the more recent ones ? I've owned 4 of the 150 F/8's, 2 Skywatcher ones, 1 Konus (yellow !) and 1 Helios. I recall the older ones, the Helios and the Konus, were the best optically.

Yours sounds a good 120. Many (most ?) of the chinese achromats have varying degrees of spherical aberration which scrubs the edge of their higher resolution performance. The Chromacor correctors were about correcting that SA as much as they were about correcting CA in such scopes. The Chromacors were aimed at the 120 F/8.3 and 150 F/8 chinese doublets produced by Synta.

I find good O-III and UHC filters very useful under my bortle 5-ish skies. I say good ones because I've tried a few lower cost ones that were not very effective but staying with brands such as Astronomik and Tele Vue (which are now made by Astronomik) has paid noticeable dividends. With small to medium apertures the O-III filter especially can make the difference between seeing practically nothing and having quite a nice view of targets such as the Veil and Owl nebulae. The UHC's are a bit more subtle but subtle over a slightly wider range of targets, if that makes any sense !. If possible it's worth having both in your tool kit though. I'm talking about visual use of course. Imaging filters are different although still very valuable I believe. Maybe worth a separate thread on filters ?

I do a bit of that when the moon is bright as well. Adds some variety 🙂 I've packed in now (wimp !) - keep on trucking @Mr Spock 👍

Irresponsible behaviour: Nagler 2-4mm zoom on 2mm + 2.2x barlow (the Parks GS 2x is 2.2x really) in a 900mm FL scope = 989x 😬 Apollo astronaut viewpoint gliding over the lunar surface. I'll pretend that the floaters are a dirty command module window 😉 Highly un-scientific, but fun 😄

Good seeing tonight. I've been able to get really sharp and crisp views of Gassendi, it's rilles, central peak and fractured floor using the Svbony 3-8mm zoom (4mm setting) AND a Parks GS barlow which gives a combined magnification of a rather ludicrous 494x. Stepping up to 566x and a touch of the sharpness is lost although the views are still quite impressive. The scope is my old ED120. Schiller looks like a huge divot close to the terminator. The Parks GS barlow is the same optically as the old Celestron Ultima. Nice glass 🙂

The moon dominates the sky here tonight. Lots to see but following the Hesiodus "ray" phenomenon a couple of nights back, I could not resist a look at the smaller neighbour, Hesiodus A with it's double ring structure. It's small but quite well illuminated just now: Image Credit: ACT-REACT Quickmap / LRO / NASA

My old ED120 eyes the moon tonight at 257x

I believe that making an optically flat window, properly flat that is, is quite difficult. I have seen it offered as an option in the past but it was quite an expensive option. I used to have a 6 inch F/6 maksutov-newtonian but the meniscus on that had a figure of course, to correct the spherical primary. Nearest thing to refractor images that I've seen from a reflecting design though. Tiny secondary - just 18% of the aperture of the primary.

I think the ED80, 100 and 120 will still use FPL-53 plus a schott element, despite the above 🙂

Thats my experience with refractors as well. I don't have an F/5 newt to try it out in now.

My understanding is that the ED80, ED100 and ED120 still use an Ohara FPL-53 ED element mated with a Schott glass element. The ED72 and ED150 use undisclosed glass types. Changing the glass type used in the long established aperture models would, IMHO, be complex and risky for the brand. The decision by Skywatcher not to disclose the ED glass type used in the 72mm and 150mm models initially caused some angst from potential customers judging by the comments made on forums but it seems to be accepted now and those models seem to be judged on their merits.

Plus, there might be a martian dust storm to obscure the features, as there was in 2018 🙄

I have only briefly looked through a newish ED120. Mine is one of the very early ones (gold tube / cream trim) and is an excellent scope. The newer one seemed good too but it was only a short look that I had. No chance of a proper comparison. It's quite possible that the coatings have changed during the life of the model (around 18 years and counting I think) but I don't think the glass types used in the objective will have changed.