TOTT finder- SW Evoguide 50ED

[markse68]

Totally over the top this but I managed to pick up a second hand 50ed for a reasonable price so why not. The classic SW 9x50 RACI is surprising good for the money- nice and light and compact, bright and sharp but having looked through the superb Tal  9x50 straight-through finder I really fancied something like that but with a right angled eyepiece. The SW RACI is advertised as 45 AFOV, the Tal 55deg and it really makes a difference to the viewing experience.

Trouble is the 50ed has quite a short back focus even with its extension tube removed which doesn't leave much space for a star diagonal to still reach focus. 

I had a Baader T2 prism diagonal already and wasn't using it much- it's the cheaper one (not the Zeiss) and spec'd at 35mm physical "length" and 32mm glass path which I think means its effective length is 32mm as it shifts the focal plane backward by 3mm.

Using a Baader ultra short 1¼" eyepiece-clamp I tried a few eps and struggled finding the right one that would reach focus. My 19mm TV Widefield does and is nice but would give a magnification of ~13x a bit too high. None of my 24-25mm plossls would reach focus.

Then I remembered I had a rather peculiar Tal 25mm plossl from a Tal-M scope. It's the same optics but much shorter than the standard Tal 1.25" plossl putting its field stop close to the end of its barrel. But the barrel is 32mm diameter so wouldn't fit the Baader clamp. As it happens I think it's the same plossl optics in the lovely Tal finder too.

Couple of weeks later and a couple of ebay T2 eyepiece holders arrived direct from china- a bargain at £3.99 each. I skimmed the bore to 32mm and had to shorten it by about 1mm but now I could fit the Tal plossl to the new finder.

The peculiar plossl had a 21mm field stop but the one in the Tal finder and the 1.25" plossls I have are 24mm. This would have meant an AFOV of only 48deg vs the Tal finders' 55deg so I ordered some o-rings the correct size to replace the metal field stop with.

The view through this finder is very similar to the Tal now- identical eyepiece and all. Same nice big 55deg AFOV, slightly higher magnification at ~9.7x. They both have a similar amount of edge of field distortion but strangely it is different. Stars stretch into arcs at the field stop in the Tal finder but are stretched to radial lines in the 50ed. Nothing too bad though and the larger AFOV is just nicer to look at than the Skywatcher. The ED glass should give better colour I guess and less chromatic aberration but the Tal is really very sharp and the colour pretty good at its low magnification.

It's not RACI though. I might look into fitting an Amici prism at some point but it would involve seriously shortening the tube by removing the nice helical focuser of the 50ED and would need to be quite a large prism anyway to cover the larger field stop. The SW wins here.

When I bought the 50ed it came with a Starizona Evo-FF2 field flattener meant to improve it as an imaging scope. I need to play with it some more but am not seeing a great improvement visually but then it's probably swamped by the edge of field distortion anyway. A Panoptic 24mm would be a better eyepiece- larger field of view, better edge correction and there maybe the field flattener would be appreciated but I don't have one and buying one for a finder really would be a bit OTT!

Is it worth it and will I use it? Not sure yet- it is nicer to look through than the SW but it's still just a finder...  It's bigger and heavier than the others- twice the weight of the SW! It doesn't fit in the case I use currently for the SW finder and bracketry either... Time will tell

 

 

 

Mark

 

 

Edited December 16, 2022 by markse68

[happy-kat]

Interesting read. The EvoFFv2 covers from memory 27mm camera chip and not the full width, I've only tried it out with a camera not thought to use with an eyepiece but I think my eyepieces are too long in the neck.

[markse68]

Some rubber retained spotting scope caps arrived today- much better than the silly metal cap supplied with the 50ed for an active finder scope I think

Mark

 

[markse68]

Been playing with other eps that reach focus and this Nagler 7mm has I think a pretty flat field. Enough magnification to see banding on Jupiter and rings of Saturn quite clear and sharp. Without the FF there was slight distortion of Jupiters disk at the field stop and the banding was smeared to unobservable. With the FF there was same slight egg shaped distortion but the banding remained quite clear. I think the disk might have been a tiny bit sharper in the centre of fov without the FF though but not much in it. 

So i think with a good enough ep with itself a flat field the FF is probably worthwhile though the view was perfectly acceptable without.

Mark

Edited November 12, 2022 by markse68

[markse68]

Now it’s really TOTT. Found a 16mm Nagler so now i have 5deg fov at 15x. Had to shorten the tube by 1mm to reach focus but glad i did- I found the end of the tube was not square to the tube! I have the cell screwed hard up against the tube end so that was forcing the lenses to tilt slightly- not ideal.

The Nagler has a pretty flat field and combined with the ff gives an acceptably sharp image right to the field stop, with a bit of distortion as to be expected, and strangely slight vignetting which i don’t understand- it shouldn’t according to the specs. But it’s fine.

I picked up some ADM tube rings- bit bulky but they are so nice to adjust- really lovely.

Mark