Suitable prism diagonal for Takahashi FS-60CB / FC-76DCU

[Andrew_B]

I'm looking to buy a prism diagonal for my Tak FS-60CB / FC-76DCU (I also have the 1.7x extender module in case that makes a difference) after finding that my existing StellaMira mirror diagonal has an optical length just a bit too big to allow my eyepieces to come to focus.

One option would be Takahashi's own prism diagonal which seems to be good quality and reasonably priced, but I saw @mikeDnight singing the praises of a Baader T-2 prism with helical microfocuser which looked like a better option still.

The prism set is out of stock everywhere I've looked but I could buy the prism then add a focuser / clamp (I already have a T-2 nosepiece lying around). There are two eyepiece clamps I could use: one with a basic retention screws and another with a ClickLock adaptor which have optical path lengths of 29.0-35.5mm and 32.0-38.0mm respectively. Since the basic prism set has a path length of 64mm, I think this would mean it would increase to 67mm if I used the ClickLock clamp.

Would this potentially cause focusing problems or is the difference going to be too small to notice?

Thanks in advance

[Deadlake]

9 hours ago, Andrew_B said:

I'm looking to buy a prism diagonal for my Tak FS-60CB / FC-76DCU (I also have the 1.7x extender module in case that makes a difference) after finding that my existing StellaMira mirror diagonal has an optical length just a bit too big to allow my eyepieces to come to focus.

One option would be Takahashi's own prism diagonal which seems to be good quality and reasonably priced, but I saw @mikeDnight singing the praises of a Baader T-2 prism with helical microfocuser which looked like a better option still.

The prism set is out of stock everywhere I've looked but I could buy the prism then add a focuser / clamp (I already have a T-2 nosepiece lying around). There are two eyepiece clamps I could use: one with a basic retention screws and another with a ClickLock adaptor which have optical path lengths of 29.0-35.5mm and 32.0-38.0mm respectively. Since the basic prism set has a path length of 64mm, I think this would mean it would increase to 67mm if I used the ClickLock clamp.

Would this potentially cause focusing problems or is the difference going to be too small to notice?

Thanks in advance

The Baader BBHS mirror and prism are very close in performance. However I would get the prism as it has a shorter optical path and more versatility via providing more infocus for binos, etc..

365 have two in stock here

https://www.365astronomy.com/Baader-T-2-Star-Diagonal-Prism-Zeiss-Prism-with-male-female-T-2-thread

I use a 2” nose to on the telescope end and a click lock the EP side.

The T2 prism still has the shortest optical path of any diagonal I’m aware of.

 

Edited August 14, 2021 by Deadlake

[jadcx]

I have the Tak prism with the same scope setup and it's very nice.  Small, short, and with excellent optics.  I went with this because I was focussing on portability, so the compactness was the deciding factor.

[Andrew_B]

7 hours ago, Deadlake said:

The Baader BBHS mirror and prism are very close in performance. However I would get the prism as it has a shorter optical path and more versatility via providing more infocus for binos, etc..

365 have two in stock here

https://www.365astronomy.com/Baader-T-2-Star-Diagonal-Prism-Zeiss-Prism-with-male-female-T-2-thread

I use a 2” nose to on the telescope end and a click lock the EP side.

The T2 prism still has the shortest optical path of any diagonal I’m aware of.

 

Thanks for the info. That prism seems to be the better version of the one I'd been looking at which was the standard T-2 prism diagonal that came in at about 100 quid less.

On the Baader site it lists the optical length of the BBHS Zeiss T-2 prism as being 3.5mm longer than that of the basic T-2 prism so if that works for you then it should be no problem for me to use either the standard eyepiece clamp or the longer ClickLock model.

[Andrew_B]

6 hours ago, jadcx said:

I have the Tak prism with the same scope setup and it's very nice.  Small, short, and with excellent optics.  I went with this because I was focussing on portability, so the compactness was the deciding factor.

That's good to know and the Tak prism is definitely an option. I was leaning towards the Baader setup because I could have the microfocuser which I do miss when using higher magnifications.

Do you have much in-focus left when using the Tak prism or is the focuser pretty much racked all the way in when you're focused at infinity?

[jadcx]

28 minutes ago, Andrew_B said:

That's good to know and the Tak prism is definitely an option. I was leaning towards the Baader setup because I could have the microfocuser which I do miss when using higher magnifications.

Do you have much in-focus left when using the Tak prism or is the focuser pretty much racked all the way in when you're focused at infinity?

No there's definitely a bit left, but I can see the appeal of the micro focuser anyway as the FS-60 stock focuser is quite course for such a narrow focus plane.  I'm considering upgrading it somehow, maybe a 10:1 unit, maybe splashing on a Feathertouch, not sure yet.

Edited August 14, 2021 by jadcx

[JeremyS]

I use the Tak prism diagonal for my smaller Taks: FS 60 Q and FC 76 DCU. Great performer. It’s also light and compact.

Note that eyepieces with the dreaded undercuts are slightly tricky as harder to grip and to extract. But I still struggle on with my TV Naglers. The Baader twist lock would be better (which I use with my larger Taks). Non undercuts (and Baader eyepieces with safety smurfs) are fine

[jadcx]

1 minute ago, JeremyS said:

I use the Tak prism diagonal for my smaller Taks: FS 60 Q and FC 76 DCU. Great performer. It’s also light and compact.

Note that eyepieces with the dreaded undercuts are slightly tricky as harder to grip and to extract. But I still struggle on with my TV Naglers. The Baader twist lock would be better (which I use with my larger Taks). Non undercuts (and Baader eyepieces with safety smurfs) are fine

Yes, that is an annoyance with the Tak prism.  I butchered mine 😮 by cutting down the plastic retaining ring by a few mm so now it tightens 'properly' on any eyepiece, undercut or not.

[Andrew_B]

20 minutes ago, jadcx said:

No there's definitely a bit left, but I can see the appeal of the micro focuser anyway as the FS-60 stock focuser is quite course for such a narrow focus plane.  I'm considering upgrading it somehow, maybe a 10:1 unit, maybe splashing on a Featherlite, not sure yet.

Thanks again, that's good to know. I got used to having a microfocuser with my ZenithStar61 that's mainly going to be used for imaging from now on. The Tak focuser is fine at lower magnifications, if not better than the WO focuser but when I push the magnification I could do with being able to adjust the focus point with more precision.

The Baader helical focuser in the eyepiece clamp is a fraction of the cost of any of the other options from Tak, More Blue, or Starlight Instruments so I figured it's got to be worth a go, especially if other much more experienced observers rate it highly.

[JeremyS]

25 minutes ago, jadcx said:

Yes, that is an annoyance with the Tak prism.  I butchered mine 😮 by cutting down the plastic retaining ring by a few mm so now it tightens 'properly' on any eyepiece, undercut or not.

Great idea. Does it catch on the undercuts? And does it still work ok with non undercuts?

[jadcx]

Just now, JeremyS said:

Great idea. Does it catch on the undercuts? And does it still work ok with non undercuts?

No it works really smoothly, doesn't catch on undercuts (although you have to loosen it slightly more than you would otherwise, maybe ½ a turn extra?) and holds straight barrels perfectly well.

If you're going to try it, just trim very conservatively and test as you go.  You don't need to remove very much.

[Andrew_B]

25 minutes ago, JeremyS said:

I use the Tak prism diagonal for my smaller Taks: FS 60 Q and FC 76 DCU. Great performer. It’s also light and compact.

Note that eyepieces with the dreaded undercuts are slightly tricky as harder to grip and to extract. But I still struggle on with my TV Naglers. The Baader twist lock would be better (which I use with my larger Taks). Non undercuts (and Baader eyepieces with safety smurfs) are fine

I've ended up with the same two telescopes so I'm really looking forward to exploring their capabilities!

My plan was originally to follow @DirkSteele's example and put together an FC-76DCU(Q) from parts including a Feather Touch focuser so I ordered everything from FLO and it duly arrived apart from the focuser which is on quite a long lead time for orders. I then had 90% of a telescope sitting in boxes and was itching to use it so in the end I gave in and bought an ex-demo FS-60CB from FLO so that I could use its focuser and try out the various combinations of both scopes in normal and Q-modes. The Feather Touch is still on order so when it does arrive I'll have two scopes to play with.

Seeing hasn't been great here and the wind has been non-stop but I still managed to see the GRS and a shadow transit on Jupiter for the first time ever on Thursday night with the FC-76DCU. Even without being able to really push their capabilities I can tell that these scopes are punching well above their weight.

I encountered that eyepiece issue but fortunately I was anticipating it and none of my eyepieces currently have particularly big undercuts so it's not been a big problem. I think I'd be happy with either prism diagonal but I am leaning towards the Baader one for its flexibility and better compatibility with eyepieces, even if they're designed by lunatics who love undercuts! 🤣

[JeremyS]

1 minute ago, Andrew_B said:

I've ended up with the same two telescopes so I'm really looking forward to exploring their capabilities!

My plan was originally to follow @DirkSteele's example and put together an FC-76DCU(Q) from parts including a Feather Touch focuser so I ordered everything from FLO and it duly arrived apart from the focuser which is on quite a long lead time for orders. I then had 90% of a telescope sitting in boxes and was itching to use it so in the end I gave in and bought an ex-demo FS-60CB from FLO so that I could use its focuser and try out the various combinations of both scopes in normal and Q-modes. The Feather Touch is still on order so when it does arrive I'll have two scopes to play with.

Seeing hasn't been great here and the wind has been non-stop but I still managed to see the GRS and a shadow transit on Jupiter for the first time ever on Thursday night with the FC-76DCU. Even without being able to really push their capabilities I can tell that these scopes are punching well above their weight.

I encountered that eyepiece issue but fortunately I was anticipating it and none of my eyepieces currently have particularly big undercuts so it's not been a big problem. I think I'd be happy with either prism diagonal but I am leaning towards the Baader one for its flexibility and better compatibility with eyepieces, even if they're designed by lunatics who love undercuts! 🤣

Great plan, ending up with two scopes. Well 4 actually: FS 60, FS 60 Q, FC 76 and FC 76 Q. The FC is actually quite a capable planetary scope as you have found 👍🏻

[jadcx]

1 minute ago, Andrew_B said:

I've ended up with the same two telescopes so I'm really looking forward to exploring their capabilities!

My plan was originally to follow @DirkSteele's example and put together an FC-76DCU(Q) from parts including a Feather Touch focuser so I ordered everything from FLO and it duly arrived apart from the focuser which is on quite a long lead time for orders. I then had 90% of a telescope sitting in boxes and was itching to use it so in the end I gave in and bought an ex-demo FS-60CB from FLO so that I could use its focuser and try out the various combinations of both scopes in normal and Q-modes. The Feather Touch is still on order so when it does arrive I'll have two scopes to play with.

Seeing hasn't been great here and the wind has been non-stop but I still managed to see the GRS and a shadow transit on Jupiter for the first time ever on Thursday night with the FC-76DCU. Even without being able to really push their capabilities I can tell that these scopes are punching well above their weight.

I encountered that eyepiece issue but fortunately I was anticipating it and none of my eyepieces currently have particularly big undercuts so it's not been a big problem. I think I'd be happy with either prism diagonal but I am leaning towards the Baader one for its flexibility and better compatibility with eyepieces, even if they're designed by lunatics who love undercuts! 🤣

They are phenomenal little scopes  Although not quite so little by the time you've constructed the 76Q variety, but still, I'm very happy with it.

I can feel my bank balance slumping as my Feathertouch envy grows.  

[Andrew_B]

4 minutes ago, JeremyS said:

Great plan, ending up with two scopes. Well 4 actually: FS 60, FS 60 Q, FC 76 and FC 76 Q. The FC is actually quite a capable planetary scope as you have found 👍🏻

When you put it like that it's an absolute bargain!

I was surprised at how good the view was through the FS-60Q as well and it seems that the longer focal ratios do a better job of cutting through bad seeing. I've tried it in FC-76Q mode as well but not really had the seeing to take advantage of magnifications over about 150x so I'm looking forward to taking it out under dark skies as well as viewing the Moon when it's higher in the sky.

The standard FC is incredibly good and so lightweight. I'm not an experienced observer and conditions haven't been great but I was still able to see colour and detail in the GRS. There's a lot to be said for a small scope - last night I was viewing the Moon from out of my kitchen window because it was too windy to take the scope outside. I wouldn't have been able to do that with a big dob!

[Andrew_B]

17 minutes ago, jadcx said:

They are phenomenal little scopes  Although not quite so little by the time you've constructed the 76Q variety, but still, I'm very happy with it.

I can feel my bank balance slumping as my Feathertouch envy grows.  

The 76Q is like a proper old school long refractor but even in that mode it's not at all heavy.

I don't want to think how much I've spent 🤣. I'd budgeted a certain amount for a scope plus some accessories like a good diagonal and a widefield eyepiece but I've definitely gone over that amount. Scopes like these are the kind of things you can own for a lifetime and they're small enough that no matter where I end up living I won't have a problem storing or using them.

I've also used my Z61 as a spotting scope quite a few times and I'm eager to do some birdwatching with the FS-60Q once I've got a suitable diagonal.

[jadcx]

Just now, Andrew_B said:

The 76Q is like a proper old school long refractor but even in that mode it's not at all heavy.

I don't want to think how much I've spent 🤣. I'd budgeted a certain amount for a scope plus some accessories like a good diagonal and a widefield eyepiece but I've definitely gone over that amount. Scopes like these are the kind of things you can own for a lifetime and they're small enough that no matter where I end up living I won't have a problem storing or using them.

I've also used my Z61 as a spotting scope quite a few times and I'm eager to do some birdwatching with the FS-60Q once I've got a suitable diagonal.

It's generally best not to add these things up... 🤫

[JeremyS]

2 hours ago, Andrew_B said:

The 76Q is like a proper old school long refractor but even in that mode it's not at all heavy.

I don't want to think how much I've spent 🤣. I'd budgeted a certain amount for a scope plus some accessories like a good diagonal and a widefield eyepiece but I've definitely gone over that amount. Scopes like these are the kind of things you can own for a lifetime and they're small enough that no matter where I end up living I won't have a problem storing or using them.

I've also used my Z61 as a spotting scope quite a few times and I'm eager to do some birdwatching with the FS-60Q once I've got a suitable diagonal.

Sounds like you need to consider a Tak FOA 60 Q at some stage 🤣👍🏻

Edited August 14, 2021 by JeremyS

[Andrew_B]

1 hour ago, JeremyS said:

Sound like you need to consider a Tak FOA 60 Q at some stage 🤣👍🏻

Get thee behind me satan! 🤣

I don't need anymore temptation and that FOA 60 Q looks like a very nice bit of kit, both optically and mechanically. You should get one and let us know what it's like!

[HollyHound]

17 hours ago, JeremyS said:

Sounds like you need to consider a Tak FOA 60 Q at some stage 🤣👍🏻

Don’t listen to him, he’s a bad man… before you know it you‘ll have no money left and a room full of Taks 🤣

Seriously though the FOA-60(Q) is an absolutely exquisite little gem of a scope 😃

Edited August 15, 2021 by HollyHound

[HollyHound]

16 hours ago, Andrew_B said:

You should get one and let us know what it's like!

I can confirm it’s just sublime… if you like double stars, get this…simply the most perfect clean view I’ve had 😃

Oh and it’s great on lunar and planets too… seems to just keep taking magnification and although things get darker, still show detail 😮

Edited August 15, 2021 by HollyHound

[HollyHound]

In answer to your original question, I have both the small and larger Baader T2 prisms on all my Taks and they work superbly 👍

There isn’t too much difference in light path length between them I don’t think, but yes the smaller are much cheaper… unfortunately I haven’t seen them in stock anywhere for a good while… I think I might have bought the last one FLO had😬

Edited August 15, 2021 by HollyHound

[Andrew_B]

3 hours ago, HollyHound said:

In answer to your original question, I have both the small and larger Baader T2 prisms on all my Taks and they work superbly 👍

There isn’t too much difference in light path length between them I don’t think, but yes the smaller are much cheaper… unfortunately I haven’t seen them in stock anywhere for a good while… I think I might have bought the last one FLO had😬

That's a very neat setup. What's the connector between the prism and the rearmost Takahashi adapter?

I managed to find one of the little prisms in stock at RVO so I've ordered that plus the ClickLock T-2 eyepiece clamp with the built in helical focuser. I've already got a T-2 nosepiece so I should be all set when it arrives.

[HollyHound]

2 hours ago, Andrew_B said:

That's a very neat setup. What's the connector between the prism and the rearmost Takahashi adapter?

I managed to find one of the little prisms in stock at RVO so I've ordered that plus the ClickLock T-2 eyepiece clamp with the built in helical focuser. I've already got a T-2 nosepiece so I should be all set when it arrives.

That’s just a Baader T2 extension ring (15mm).

Nice, this will be perfect for your scope 👍

[Andrew_B]

4 hours ago, HollyHound said:

That’s just a Baader T2 extension ring (15mm).

Nice, this will be perfect for your scope 👍

It looks very securely fixed, presumably threaded onto the scope rather than using the eyepiece clamp.

Do you use a binoviewer at all? It's something I might get in due course and I'd imagine I'd need a more secure attachment than a compression fitting.