TV Powermate as focal extender

[badhex]

Hello all, 

I was recently using my ZS73 with a 2" Powermate but the resulting configuration, especially with a large EP, is a bit cumbersome. 

It occurred to me that actually, I might be able to use the PM before the diagonal which would lead to a more stable configuration, mass distribution wise, giving me a sort of focal extender configuration and making the scope F11.8. Obviously this is no different to using the PM as intended. 

I'm of the understanding that the PM should not change magnification (well, not enough to worry about) depending on the distance to the field lens of the EP, so should function exactly the same as long as I can reach focus etc.

Am I missing something here or is it really this simple? 

[Mandy D]

I'm confused as to why you have not simply tried this. My experience with an Explore Scientific focal extender on an RC6 is that I run out of focuser travel with it inserted directly into the focuser and have to add more extension pieces between it and the scope. Your diagonal will be performing this function in it's normal position in the optical train, will it not? So,I'm guessing, you might find it is not that simple, but I really don't know. Try it and see.

[badhex]

17 minutes ago, Mandy D said:

I'm confused as to why you have not simply tried this. My experience with an Explore Scientific focal extender on an RC6 is that I run out of focuser travel with it inserted directly into the focuser and have to add more extension pieces between it and the scope. Your diagonal will be performing this function in it's normal position in the optical train, will it not? So,I'm guessing, you might find it is not that simple, but I really don't know. Try it and see.

Haha the only reason I haven't tried it is a literally just thought of it 😂

The one potential issue that I thought of is that the slightly smaller clear aperture of the field lens on the PM might intersect the light cone since it is closer to the objective, and cause some vignetting, but I'm not sure that's true. I read something ages ago on the nead for large clear aperture on barlows etc. being a bit of a myth. 

Edited September 28, 2023 by badhex

[Mr Spock]

The 2x 2" Powermate doesn't alter it's magnification with focus position https://www.televue.com/engine/TV3b_page.asp?id=53&Tab=_app

This suggests it can be used before a diagonal, however, I suspect the focus point would move outwards so you may not have enough back focus without an extension tube. Best way is to try it of course!

[Mandy D]

4 minutes ago, Mr Spock said:

The 2x 2" Powermate doesn't alter it's magnification with focus position https://www.televue.com/engine/TV3b_page.asp?id=53&Tab=_app

This suggests it can be used before a diagonal, however, I suspect the focus point would move outwards so you may not have enough back focus without an extension tube. Best way is to try it of course!

That's pretty much what I found and in line with my advice, above.

[josefk]

hi Joe - i think you'll find focus moves out more than 50mm when inserted before the diagonal - this is my pet peeve re. an otherwise optically brilliant and very useful accessory. i put some pics in this thread :- 

https://stargazerslounge.com/topic/409800-extender-q-16x-or-powermate/#comment-4380381

😞 

[badhex]

Ah damn. Just tried it on some houses across road and I ran out of focuser travel before it got to focus. Oh well, that's the end of that very short saga! 

What started me on this was wondering whether a focal extender is available for the ZS73, since it performs surprisingly well on planets when you bump the magnification up. Sadly it seems not, however such an accessory would turn this scope from a widefield wonder to a solar system slayer.

I guess shall just have to stick to the tried and tested BCO + barlow/PM method for now! 

[josefk]

The optical rewards (planet slayer) could be well worth you looking out for a simple extension tube though - its a pet peeve of mine for sure ergonomically but optically its super - and though i have no clue what i'm talking about maybe that optical superbness is more usefully leveraged in a fast scope when the PM is slowing the light cone of the scope down before the image is formed rather than doubling up the magnification of the image as part of the eyepiece (i only have a fingertip grasp of what i am actually on about and could be unbelievably mixed up on the topic 🙂 )

[Mr Spock]

Whether it's with, or without the diagonal, the PM will intercept the light cone at the same place  Which is why you need an extension tube. All you are doing is replacing the light path of the diagonal with an extension, and moving the eyepiece farther away (with the diagonal) from the PM.

If you do this with a Barlow instead of a PM it will increase the magnification.

[badhex]

Cheers. Yes, I had presumed that's what was happening. Unfortunately I think it makes the scope a bit too unwieldy, and in the end this scope is primarily what I use for grab and go or travel, so it sort of defeats the object a little when I could just use a 2.5x PM or Barlow with an ortho and effectively end up with the same approx result with much less weight. 

Anyway, always good to have a little play around with these things! 

[DirkSteele]

5 hours ago, badhex said:

Ah damn. Just tried it on some houses across road and I ran out of focuser travel before it got to focus. Oh well, that's the end of that very short saga! 

What started me on this was wondering whether a focal extender is available for the ZS73, since it performs surprisingly well on planets when you bump the magnification up. Sadly it seems not, however such an accessory would turn this scope from a widefield wonder to a solar system slayer.

I guess shall just have to stick to the tried and tested BCO + barlow/PM method for now! 

Don’t forget that you need more out focus to bring nearby objects to focus so you should still try it at infinity with the stars just in case it works.

[badhex]

1 hour ago, DirkSteele said:

Don’t forget that you need more out focus to bring nearby objects to focus so you should still try it at infinity with the stars just in case it works.

Cheers, yes the thought had occurred, but the focus travel on the ZS73 is massive and it was all the way out, so wouldn't be ideal even when focused at infinity. Might be worth a go though nonetheless, like you say.