badhex

Thanks @wookie1965- sorry I missed this before. I'm work my way through @Ags excellent Discovering Double Stars so I'll message you when I run out 🙂

Looks great @johninderby! Looking forward to hearing about first light. What are your impressions on the build quality so far?

Argh! No picture!

Oh - I almost forgot - for clarity I made these measurements using my TS102 F7 ED, as I don't have a SW 72ED. Thus the in-out focus numbers are obviously irrelevant to your scope, but the light path difference in the diagonal should hopefully be quite similar, if not slightly better at F7.5. Someone with a better understanding of f-ratio and focus distances may be able to offer more info if I am mistaken!

Ah yes, very similar to the one that comes with the GSO.

Okay here's the data. I ended up measuring most of my main EPs to get more accurate results, and measured the two Morpheus EPs 2" mode as well. I tried to be as accurate with the markings as possible, but only went for nearest whole mm, and bear in mind that this is not at infinity but a chimney in the distance. It was also a bit tricky to focus at higher powers due to some shimmer/heat haze. The figure in each 'diagonal' column is the marking on the focuser, so the 'difference' column shows how much shorter the diagonal light path is for the GSO vs Synta. I would have expected the difference to be the same for each EP, so I out the variance down to measurement and/or focus errors. One thing that is obvious immediately is that the difference when using the 1.25" is huge! This must be what I noticed previously when observing, and unfortunately not that helpful for your use case. This is due to the design of the adaptor, which is much lower profile in the GSO (right). It also has an inset which some EPs fit into, some don't. In summary, we can say that you will gain ~2-3mm in 2" mode, and a whopping ~10mm in 1.25" mode. Sorry, quite long and waffley but hopefully helpful!

I've just done a few measurements, and some of the results are... odd. I'll take a couple of mins to write up plus what I think is likely to cause some of the weird outliers.

Nice modification Victor. One thing worth noting, after a previous thread regarding clear aperture of 2" diagonals we discovered that quite a few have less than 46.5mm clear aperture - i.e. max field stop of 2" EPs - so I replaced mine with a GSO manufactured 2" diagonal. Mine was TS but it's also available as Revelation and others. As a bonus (and relevant to this thread!), I found that the light path is quite a lot shorter with the GSO/TS than my existing Skywatcher one. The SW appear to be the same design as the WO ones. I'll measure it later today for you, but as I recall it was in the region of 10mm.

Excellent, thanks for digging those out Ed! No mention of the K18mm so maybe they only sold it with the C90 or it's earlier/later than the catalogue. I also have that Frank's book - I got it when researching my CF about ten years ago! Great book.

Also looking forward to this, almost feels like I've bought a new scope!

Haha excellent, enjoy!

Sorry I can't offer any imaging advice! But on the subject of the starfields in/around Lyra, I recently was in darker skies (Bortle 5 instead of 7/8) with a basic 70mm scope showing max TFOV of about 4° and was also surprised at the richness of that area. Loads of stars I had never really seen properly before, I ended up spending ages panning around there when I took in the various multiples as well.

Thanks - and no worries, don't put yourself to any trouble.

Reasonably sure that the EP in question is only a single coating, which I would tend to expect from older EPs. I normally do the 'reflection roll' test - i.e. move it around under a light and observe the different reflections - when a get a new one but will double check it again.

All hail the brave pioneer! I look forward to your thoughts John!

I guess we'll never really know for sure unless we can dkrectly speak to manufacturers or staff, or if there's any written records. Still, I find all this stuff quite fascinating. For example, I had found out that Charles Frank used a lot of ex-military, and I later discovered after some digging that the finderscope for my particular Charles Frank was a WW2 Aldis illuminated sighting scope from an AA gun, originally with a 24v connection. There's no way I'd have found that by any other way than research, and joining the dots of several auctions, historical records and catalogues.

So the hype says that it has an upgraded focuser as well as being much sturdier than the Evostar range, but I'm with you on the excellent R&P of your Tecnosky - the rotating focuser alone is a huge win.

It's hard job but somebody has to do it, John 😂

I'm waiting with bated breath (and secretly hoping you go for the Evolux 😂)

My first ever view of M13 was through a 12" under pretty dark skies. I was blown away 🤯

Thanks so much for all the help! Now off to dig into Celestron C90 reviews 🙂

Amazing! Great find. The hive mind of SGL never ceases to amaze! 💫

Ah, apologies I think I misread the description - he suggests that only the K40mm is from that era. https://astromart.com/auctions/astromart-auctions/eyepieces/show/rare-set-of-vixen-double-circle-v-eyepieces-12204

Good sleuthing! I must have missed this, thanks! Another search to add to the list 🙂

Thanks @Franklin- this makes sense and fits with another lead I have - the below photos from a random 5 year old auction of a set of Celestron / Vixen EPs featuring a 30mm Kellner which is clearly the same era/case Design. I can also see your 20mm and 5mm in the last picture too. The seller mentions that the majority of EPs in the set are from the '50s/'60s but that the 30mm Kellner is "later". He also suggests that the double-circle-v denotes, at least for the early EPs, that they were made in a different factory and to a higher standard, info he gleaned from a Japanese amateur astronomer who worked for Vixen in the '50s.