Owners of TV diagonal everbrite 1.25"

[Piero]

Could you tell me how long (in mm) is the eyepiece barrel holder inside, please? 

mm.. a bit tricky to explain.. 

in other words, how long can the longest eyepiece barrel be to fully enters in the diagonal eyepiece holder?

Thanks

Piero

[alan potts]

They have a machined stop before the mirror so there is no chance of any eyepiece no matter how long going any further than the stop. I don't have a 1.25 any longer all mine are 2 inch now but they are the same design.

Alan

[Piero]

Thanks Alan. I am aware they have a machined stop before the mirror. My Revelation diagonal 1.25" also has it and thankfully they have it!

However, my question is how long can be a barrel to fully enter in the diagonal holder. 

The reason for this question is because the barrel of my Bresser 2x SA Barlow does not completely enter in the diagonal eyepiece holder, whereas all my eyepieces do. As consequence, I have inward focus if I use diag+barlow+eyepiece. Unfortunately, on my TV60, this inward shift is longer than the fine focuser, obliging me to move the coarse focus too. 

The problem would be solved if I use:

1. eyepieces instead of barlow (expensive!) 

2. a powermate instead of a barlow (my barlow is not parfocal).

3. if a diagonal eyepiece holder is 'deep' enough for holding my barlow (while the eyepieces would not reach the machined stop), barlow and eyepieces would reach focus at the same point more or less. 

That's why I wonder how 'deep' is the eyepiece holder for a TV diagonal everbrite.

ps 

the solution barlow+diagonal+eyepiece does not work on the TV60. You need an extension tube, which in this case, would make things even more complicated.

Thanks

[russ.will]

I have the ES 2x Focal Extender which, like the Bresser, is a clone of the Meade Telextender and basically the same thing as a Powermate. Basically it's not a Barlow - It's a telecentric focal multiplier and it adds it's own optical length to the optical train, so it will take up some in-focus simply by being there. It's not like a Barlow where the exiting rays are diverging and therefore the point of focus pushed out.

For instance, if I place it in the 2-1.25" adapter in the top of my 2" diagonal (which it definitely does insert all the way into) then add the EP I was using, then it requires significant inward focusing to return the EP to approximately where it was prior to the insertion of the ES FE. Make sense? Yes, the fact the long nose doesn't go all the way in to your diagonal takes up an amount of in-focus, but not as much as the SA would anyway.

On the other hand, because of the telecentric design where the parallel rays entering the Barlow also leave parallel, it's magnification is insensitive to the EP to to 'Barlow' distance. This means you could insert it in the telescope focuser draw tube (which it will go all the way into) followed by your diagonal and then the EP and still end up only getting a 2x multiplication, but you will have gained back the extra gap you lost in the top of the diagonal. That may be enough.

Russell

[Piero]

Thanks Russell. Your explanation makes sense and I agree completely with it, except the fact that I don't think the Bresser SA is a real Telexender. Apart from the fact that it is called Barlow and not Telextender, I can assure you that you cannot put it between the diagonal and the telescope. It does not focus and if you try to 'extending it manually' (basically without locking, leaving the entire coarse tube out and eyepiece out) until you reach focus, you will see that the magnification is much higher. 

Also, if it were a proper telextender / powermate, I think you should have much less inward focus in the case that you put the barlow between the diagonal and eyepiece. I have more than 1inch inward focus. 

It is a very nice optical tool. It just do the job perfectly. However, I think it is suitable with a newton, not really with telescopes having diagonals. 

I haven't ever tried the ES telextender, but judging from the photos, it seems its design is different from the bresser SA. Possibly that is a real telextender, whereas the bresser just an excellent barlow? 

[russ.will]

It is this one, right? Looks a lot like this? Can be had under the Omegon brand too? They're all the same and ES just make theirs look different. It's a standard JOC design, who of course own ES, Bresser, Maxvision, etc.

If it is that one, then it's definitely a four element negative doublet followed by a positive doublet and it is a telecentric focal multiplier. There is no other reason to use 4 elements and even Televue, who LOVE a bit of extra glass, don't cram more than two elements into a true Barlow! GSO/Revelation do a 3-element Apo Barlow (got one of those too) but 3 is the maximum number of elements I've seen in a Barlow.

To be fair you do get a bit of extra amplification with increasing distance with the FE/SA/TE/PM, but it's minimal compared to a Barlow - TV show how it varies here, although one might assume there will be a slight difference from manufacturing house to manufacturing house. But fundamentally, if it's absolutely telecentric, the magnification is fixed regardless of distance.

An inch of turns sounds about right - It's a good couple of turns on my focuser with the FE and would equate roughly to the length of the doublet pairs in the SA - Their optical length. I've not owned everything out there, but I can't think of a multiplier (for want of a better term) that will just drop in without requiring significant re-focusing in one direction or the other, regardless of where you place it.

Russell

[Piero]

Thanks for your help Russell. It was very useful. 

[John]

...... I've not owned everything out there, but I can't think of a multiplier (for want of a better term) that will just drop in without requiring significant re-focusing in one direction or the other, regardless of where you place it.

Russell

I've recently been using a 2.5x Tele Vue Powermate and that did not require any focuser adjustment when put between the eyepiece and the diagonal. I can't recall how the ES 2x Focal Extender that you kindly loaned me some time ago was in this respect, maybe it was the same ?. The ES unit was optically a great performer too, as we agreed at the time and somewhat less expensive than the Powermate.

[russ.will]

Definitely requires extra in-travel I'm afraid.

Russell

[Piero]

I noticed there is a new powermate which seems to have a shorter barrel 

http://cdn3.bigcommerce.com/s-3hzp5/products/268/images/615/powermate25__87770.1420315689.600.600.jpg?c=2

than the usual one

http://i.ebayimg.com/00/s/NjAwWDYwMA==/z/loYAAOxyRhBSq5m0/$_35.JPG

John, Russell, which of these two are you using? 

I might be wrong, but the *new* one has a barrel size which should enter in the diagonal completely, and therefore should not have any inward re-focusing...  

[Piero]

Apologise, the second link seems not to work. Here is another one: http://www.photohost.ru/pictures/659474.jpg

[John]

I was using the 1st one above. I believe both have the same barrel length but the more recent design now incorporates a 1.25" filter thread wheras the one that I had did not. Not that I would be putting a filter there but perhaps some do like to do that, hence the revised design.

Unfortunately I was using it with the 2" version of the Everbrite diagonal which won't answer your question about the 1.25" I'm afraid.

[SpaceWalker]

There is no other reason to use 4 elements and even Televue, who LOVE a bit of extra glass, don't cram more than two elements into a true Barlow! GSO/Revelation do a 3-element Apo Barlow (got one of those too) but 3 is the maximum number of elements I've seen in a Barlow.

FWIW, there is a 5-element Orion Highlight Barlow. They say Orion HighLight Barlows employ a sophisticated 5-element lens configuration engineered to render optically flatter, sharper images than standard Barlows can render. But I'm just puzzled why they needed 5 elements in a telenegative Barlow?

[russ.will]

Blimey! Good spot. Barlow with field flattener is all I can think of, but thinking isn't my strong point...

Russell

[Piero]

Thanks everybody for your help! 

Piero