Evoguide 50 questions

[Ags]

I am thinking of replacing my Askar FMA 135 with an Evoguide 50. I think the extra focal length will help with guiding my RC (fl = 926mm) and the 50 mm aperture will be useful as an optional imaging scope  - I find the FMA 135 is a bit too widefield... I would be using the Starizona FF with 55 mm back focus, so I am wondering about using this a travel refractor. I know many have tried and failed to get Evoguides to work visually though.

Given the Evoguide has 55 mm backfocus, adding a Baader T2 amici prism with 47.5 mm optical path would leave 7.5 mm path on the eyepiece side? I have a short T2-1.25"" adapter, so would this be enough optical path for most eyepieces?

[vlaiv]

8 minutes ago, Ags said:

Given the Evoguide has 55 mm backfocus, adding a Baader T2 amici prism with 47.5 mm optical path would leave 7.5 mm path on the eyepiece side? I have a short T2-1.25"" adapter, so would this be enough optical path for most eyepieces?

I don't have experience with Evoguide per se, but keep in mind that focal position of eyepiece would be around it's shoulder (ideally at the shoulder).

This means that you need to sink in about 20 or so mm of nose piece of EP somewhere - but with only 7.5mm of path you won't be able to do that and protruding nose piece of EP will hit the prism.

Your best bet is to "convert" eyepiece by unscrewing the nose piece and making suitable adapter for it to attach directly to T2 thread.

I've already done something like that for a friend to convert spotter scope for use with astronomical eyepieces.

[vlaiv]

Alternatively - don't get Evoguide, get this instead:

https://www.teleskop-express.de/en/binoculars-spotting-scopes-microscopes-range-finders-31/spotting-scopes-216/spotting-scopes-for-astronomy-26/ts-optics-50-mm-f-4-ed-travel-refractor-spotting-scope-and-guiding-scope-with-crayford-focuser-perfect-optics-13750

[Stuart1971]

16 minutes ago, Ags said:

I am thinking of replacing my Askar FMA 135 with an Evoguide 50. I think the extra focal length will help with guiding my RC (fl = 926mm) and the 50 mm aperture will be useful as an optional imaging scope  - I find the FMA 135 is a bit too widefield... I would be using the Starizona FF with 55 mm back focus, so I am wondering about using this a travel refractor. I know many have tried and failed to get Evoguides to work visually though.

Given the Evoguide has 55 mm backfocus, adding a Baader T2 amici prism with 47.5 mm optical path would leave 7.5 mm path on the eyepiece side? I have a short T2-1.25"" adapter, so would this be enough optical path for most eyepieces?

I had the evoguide as a guide scope and wide field imaging scope, and as a guider it was perfect, BUT as an imaging scope it was very poor, due to the very poor helical focuser, it was a real pain to get good focus as the threads on the helical focuser are just not well meshed, and when you change focus direction it moved half a turn before the focus even changed, the backlash was terrible, so I got rid of it and bought the Askar FMA180 pro, and it’s superb as a dual guide and Imaging scope…

[Ags]

26 minutes ago, vlaiv said:

I don't have experience with Evoguide per se, but keep in mind that focal position of eyepiece would be around it's shoulder (ideally at the shoulder).

This means that you need to sink in about 20 or so mm of nose piece of EP somewhere - but with only 7.5mm of path you won't be able to do that and protruding nose piece of EP will hit the prism.

Your best bet is to "convert" eyepiece by unscrewing the nose piece and making suitable adapter for it to attach directly to T2 thread.

I've already done something like that for a friend to convert spotter scope for use with astronomical eyepieces.

Not sure that's always true. I think for a typical 30 mm plossl, the focal plane is at the bottom of the barrel.

[Elp]

1 hour ago, vlaiv said:

get this instead

My good old dinky scope:

[Ags]

27 minutes ago, Elp said:

My good old dinky scope:

 

Tell me more!  

[vlaiv]

1 hour ago, Ags said:

Not sure that's always true. I think for a typical 30 mm plossl, the focal plane is at the bottom of the barrel.

Focal plane is usually at the field stop of the eyepiece - at least for "simpler" types.

If you can see the edge of field stop sharply when looking thru the eyepiece  - it is at the focal plane where the sharp image forms.

Look at the plossl eyepiece from the bottom and you will see that the field stop is roughly at the shoulder level.

 

[Ags]

I was thinking of (at least) the TV 40 mm plossl.

[vlaiv]

4 minutes ago, Ags said:

I was thinking of (at least) the TV 40 mm plossl.

Why?

That is a bit of a waste of light. Most such small scopes are F/4, perhaps even a bit faster. With such F/ratio - longest focal length that you could use would be around 25-28mm, depending on how dilated your pupil becomes (for 6mm you get 4*6 = 24mm).

In the same price range you could get something like

https://www.firstlightoptics.com/stellalyra-eyepieces/stellalyra-24mm-ultra-flat-field-125-eyepiece.html

or (for a bit more)

https://www.firstlightoptics.com/explore-scientific-eyepieces/explore-scientific-68-degree-series-eyepieces.html

24mm version

[Ags]

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.

Edited Sunday at 12:39 by Ags

[Elp]

27 minutes ago, Ags said:

Tell me more!  

In my never ending search for astro items I wanted a scope that I could use for full disk solar ha imaging with my quark. I believe I can do it with this and my 183 if I use a desktop version of software (the asiair has a 1080 resolution limit in video mode so is no good). The focuser is a bonus as most small finders have that screw in/out front objective type focusing method which is a chore.

The benefit of this scope is that the rear tube can be unscrewed (with a bit of faffing around loosening the tube rings and the rail it's mounted on) to make it shorter for visual use. 

Note mine is the none ED version (from Ali, much much cheaper) as for solar it doesn't matter (for those interested the spec of mine is Z4 + K9 glass type doublet FMC coating). My first test was a visual one, I was dreading what I would see but the stars were crisp dots. I didn't test on the moon or Jupiter but I'd suspect blue fringing may be present, I'll see, the front tube is also threaded so I can always use a step down ring and an m48 filter to help any fringing...hmmm I could also do some narrowband imaging with it...

The focuser being a bit "sticky" I thought would present a challenge for solar imaging focusing but I managed it fine.

The following is a bit of a mish-mash (I had to mount a finder bracket for my solar finder/asiair) but I had a small hard case lying around, and the WO DSD dovetail I mounted the scope onto just about fits within (so I can use it directly in my mounts or flip it over and use it on my camera rigs Arca clamps, it needs to be long to accommodate the quark imaging train behind), the hard shell zip case measures approx 300 x 230 x 100mm with a top carry handle and short shoulder strap so truly portable and as you can see room to store a 1.25 diagonal and zoom eyepiece.

I'm happy with it, though I had enough guidescopes already prior.

[Ratlet]

1 hour ago, Elp said:

In my never ending search for astro items I wanted a scope that I could use for full disk solar ha imaging with my quark. I believe I can do it with this and my 183 if I use a desktop version of software (the asiair has a 1080 resolution limit in video mode so is no good). The focuser is a bonus as most small finders have that screw in/out front objective type focusing method which is a chore.

The benefit of this scope is that the rear tube can be unscrewed (with a bit of faffing around loosening the tube rings and the rail it's mounted on) to make it shorter for visual use. 

Note mine is the none ED version (from Ali, much much cheaper) as for solar it doesn't matter (for those interested the spec of mine is Z4 + K9 glass type doublet FMC coating). My first test was a visual one, I was dreading what I would see but the stars were crisp dots. I didn't test on the moon or Jupiter but I'd suspect blue fringing may be present, I'll see, the front tube is also threaded so I can always use a step down ring and an m48 filter to help any fringing...hmmm I could also do some narrowband imaging with it...

The focuser being a bit "sticky" I thought would present a challenge for solar imaging focusing but I managed it fine.

The following is a bit of a mish-mash (I had to mount a finder bracket for my solar finder/asiair) but I had a small hard case lying around, and the WO DSD dovetail I mounted the scope onto just about fits within (so I can use it directly in my mounts or flip it over and use it on my camera rigs Arca clamps, it needs to be long to accommodate the quark imaging train behind), the hard shell zip case measures approx 300 x 230 x 100mm with a top carry handle and short shoulder strap so truly portable and as you can see room to store a 1.25 diagonal and zoom eyepiece.

I'm happy with it, though I had enough guidescopes already prior.

I had one of those sat in my basket for months.  A really interesting concept and potentially a great idea.

[Ratlet]

3 hours ago, vlaiv said:

Alternatively - don't get Evoguide, get this instead:

https://www.teleskop-express.de/en/binoculars-spotting-scopes-microscopes-range-finders-31/spotting-scopes-216/spotting-scopes-for-astronomy-26/ts-optics-50-mm-f-4-ed-travel-refractor-spotting-scope-and-guiding-scope-with-crayford-focuser-perfect-optics-13750

I wonder how that would do for imaging?  At F4 it'll be a challenge I'd imagine and they don't specify the quality of glass?  Would probably be verging on too good to be true if it did work for imaging for that price.

[Ags]

TS don't advertise it as an imaging scope.

Regarding the focal plane of eyepieces and my amici plan, adding a barlow element to the eyepiece does give a few more mm in my experience of similar setups. So if a 30 mm plossl doesn't quite reach focus, it likely will adjusted to 20 mm with a barlow element screwed in.

But I am abandoning the plan. I have thought about the FMA 180 and it is attractive as an imaging option but wouldn't be much of a visual option except under really dak skies.

[vlaiv]

30 minutes ago, Ratlet said:

I wonder how that would do for imaging?

Maybe with small sensor - anything larger would instantly suffer from field curvature and I'm not sure if any field flattener could be adopted to work with such scope.

[vlaiv]

@Ags

What's your exact goal here?

Small scope that would work as both visual, guide scope and imaging scope?

How about 60mm class scope?

https://www.teleskop-express.de/en/telescopes-4/apochromatic-refractor-55/ed-refractor-222/ts-optics-60-mm-f-6-ed-refractor-finder-and-guide-scope-extendable-for-astrophotography-15631

 

[Ags]

Yes, that’s the goal… I have looked at that 60 mm a few times but I think it is a bit large/long. I may just see if there is someone interested in swapping an FMA 180 for my FMA 135. It won’t be ideal visually but it ticks the guiding, imaging and size boxes.

[Stuart1971]

15 minutes ago, Ags said:

Yes, that’s the goal… I have looked at that 60 mm a few times but I think it is a bit large/long. I may just see if there is someone interested in swapping an FMA 180 for my FMA 135. It won’t be ideal visually but it ticks the guiding, imaging and size boxes.

Good choice..👍🏻

[PeterC65]

It's worth bearing in mind that the back end of the FMA135 is M42 whereas the FMA180 is M48, so if, like me, you are setup for 55mm of back focus from an M42 thread then the FMA180 would need an adapter that would mess up the back focus. It's the main reason I went for the FMA135 rather than the FMA180.

 

[Stuart1971]

4 minutes ago, PeterC65 said:

It's worth bearing in mind that the back end of the FMA135 is M42 whereas the FMA180 is M48, so if, like me, you are setup for 55mm of back focus from an M42 thread then the FMA180 would need an adapter that would mess up the back focus. It's the main reason I went for the FMA135 rather than the FMA180.

 

You can remove the M48 adapter and it leaves an M54 thread, to which you can add an M54 to M42 adapter if needed..👍🏻

[Elp]

1 hour ago, Ags said:

It won’t be ideal visually but it ticks the guiding, imaging and size boxes.

Below 60mm, the Sharpstar 50 edph is the only one I've seen that still functions/features like a refractor. It is a tiny scope so I believe you can only use 1.25 diagonals (60mm can use 2 inch), also you can get a reducer for it, it is also a triplet, it is also relatively cheap.

[Stuart1971]

Just now, Elp said:

Below 60mm, the Sharpstar 50 edph is the only one I've seen that still functions/features like a refractor. It is a tiny scope so I believe you can only use 1.25 diagonals (60mm can use 2 inch), also you can get a reducer for it, it is also a triplet, it is also relatively cheap.

What do you mean “functions” like a refractor, the FMA180 pro, is a 5 (hmm actually may be six) element refractor with built in reducer / flattener, I looked at buying the 50 EDPH, and the one thing that lets it down altogether is no proper camera only rotator, you have to rotate the whole focuser, which is no good for many imaging set ups..

[vlaiv]

14 minutes ago, Stuart1971 said:

What do you mean “functions” like a refractor

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

[Elp]

10 minutes ago, Stuart1971 said:

What do you mean “functions” like a refractor

I thought those small Askars were astrographs only, but a quick check on the specs show otherwise. I stated in terms of out the box it's more like a traditional refractor, focuser, multiple points for mounting saddles which is important for AP, works for visual out of the box.

The FMA and in particular the FRAs are Askars best I believe, but for the prices the 71F maybe the more tempting choice, especially with the reducer on the way.

The FMAs, for FL and size, you have a lot of camera lens options for AP, though can't use them for visual (or very difficult to adapt).