Evoguide 50 questions

[Elp]

1 minute ago, vlaiv said:

Scopes with built in flatteners are PITA for visual, especially if back focus is 55mm (again - no way to easily adopt diagonal and EP).

I always thought these designs weren't for visual, otherwise you'd have lots of Redcat owners waxing lyrical about visual use.

[vlaiv]

33 minutes ago, Elp said:

I always thought these designs weren't for visual, otherwise you'd have lots of Redcat owners waxing lyrical about visual use.

Well, in general - they are not.

Having flat field usually degrades performance of the scope - they are often no longer diffraction limited systems. It is also quite difficult to get eyepiece in exact position. People that use short focal length refractors with long FL wide field eyepieces need to correct for field curvature as well and most of them use TSFLAT2 because it has enough working distance (I think it is about 90mm or so) - so that they can put it in front of diagonal and get the eyepiece in the ballpark where it should be.

Exception to this rule is for example TeleVue NP101 - which is excellent imaging and visual scope - but it does not have separate field flattener and corrective lens is at exact distance to focal plane so there is no need to calculate backfocus.

These two scopes might be good enough for visual:

https://www.firstlightoptics.com/askar-telescopes/askar-fra300-60mm-f5-petzval-astrograph.html

https://www.teleskop-express.de/en/telescopes-4/apochromatic-refractor-55/all-apos-und-eds-223/ts-optics-60-mm-f-5-quadruplet-flatfield-apo-astrograph-with-fpl53-objective-15647

Both have integrated flatteners and a bit more working distance than 55mm (could possibly work with T2 short optical path diagonal).

[Elp]

Think this may also be the same scope as the latter suggestion:

https://www.firstlightoptics.com/starfield-telescopes/starfield-gear60-quadruplet-f5-petzval-apo.html

 

[Stuart1971]

59 minutes ago, Elp said:

Think this may also be the same scope as the latter suggestion:

https://www.firstlightoptics.com/starfield-telescopes/starfield-gear60-quadruplet-f5-petzval-apo.html

 

Now that is an excellent little scope, I had one for a while…👍🏻

[Ratlet]

1 hour ago, vlaiv said:

Well, in general - they are not.

Having flat field usually degrades performance of the scope - they are often no longer diffraction limited systems. It is also quite difficult to get eyepiece in exact position. People that use short focal length refractors with long FL wide field eyepieces need to correct for field curvature as well and most of them use TSFLAT2 because it has enough working distance (I think it is about 90mm or so) - so that they can put it in front of diagonal and get the eyepiece in the ballpark where it should be.

Exception to this rule is for example TeleVue NP101 - which is excellent imaging and visual scope - but it does not have separate field flattener and corrective lens is at exact distance to focal plane so there is no need to calculate backfocus.

These two scopes might be good enough for visual:

https://www.firstlightoptics.com/askar-telescopes/askar-fra300-60mm-f5-petzval-astrograph.html

https://www.teleskop-express.de/en/telescopes-4/apochromatic-refractor-55/all-apos-und-eds-223/ts-optics-60-mm-f-5-quadruplet-flatfield-apo-astrograph-with-fpl53-objective-15647

Both have integrated flatteners and a bit more working distance than 55mm (could possibly work with T2 short optical path diagonal).

Just out of curiosity, would this be why the askar 71f would work for visual and imagining?  The back focus for imagining requires an extension?

I'm currently in the throws of debate about a small scope, although 60-70mm and the askar 71f is one contender along with altair's 70mm fpl-53 doublet.

[vlaiv]

31 minutes ago, Ratlet said:

Just out of curiosity, would this be why the askar 71f would work for visual and imagining?  The back focus for imagining requires an extension?

Yep.

It is four lens design with integrated flattener. It also has quite a bit of back focus - 100mm from M54 thread.  

 

[happy-kat]

I've an ED50 and starizona field flattener. When focusing the ED50 finish the focusing on an upward movement then when focused use the focus screw to hold it. I find focus holds really well. With the flattener I use the T2 to 1.25 adapter for filters which goes in the ZWO camera well, this works for me as I don't use a filter wheel. I gather focus for visual use can be obtained by placing a shorty barlow on the diagonal, though I haven't got round to trying it. The focuser movement is very fine which made it slow to turn for daytime use but I'd think for night use that wouldn't matter as stars don't move.

[Ags]

13 hours ago, Elp said:

Sharpstar 50 edph

I think that might be the scope I am looking for.

[Elp]

2 hours ago, Ags said:

I think that might be the scope I am looking for.

It would make a Stormtroopers job easier...

[badhex]

21 hours ago, Ags said:

No I had no plan on using the 40 mm plossl, and wouldn't have gone above 20 mm eyepieces. I was just giving a source for my statement that the focal plan isn't always at the shoulder even for simple eyepieces. 

I would like to use an amici prism for such a small telescope, it would remove my dependence on a RACI finder. However if that was not possible, I could use the TS T2 prism, with optical path of 40 mm.

I can tell you from experience that the converting a 50ED for visual use has its challenges! I went through this process a couple of years ago but due to my astro hiatus I've never really tested it properly and got everything set up perfectly. I'll spend a bit of time later this year fiddling and reporting back.

It's probably a better move to go for something like the 50mm with a built in focuser as per the previous posts if you want to avoid the faff, but if you did go down the 50ED route your best bet might be a TV plossl due to the position of the focal plane, as you've already identified.

I go into a bit more detail in the below thread if you fancy reading:

https://stargazerslounge.com/topic/400295-another-ultra-light-travel-kit-project/

[Backyardscope]

I recently bought a Sharpstar 50EDPH with a matching reducer, not had a chance for 1st light yet. It came with a free cradle.

[Backyardscope]

It came with a free cradle so you could add another generic finder shoe. The only thing to be concerned about is the locking screw does not have enough thread to lock the rotating focuser when the cradle is fitted. You need to replace it with another knob with a longer thread. That's my only gripe with it.

The scope oozes quality though.

Also to mount a camera with reducer, you would also need to mount the scope with its shoe to a longer dovetail.

It comes with a couple of extension tubes should want to look through it with an Eyepiece at the end if not using a diagonal.