8 replies to this topic

[Profguy]

I have a deforked ETX 70 that I’d like to use as a finder on a larger scope. I’d like to use the #933 45* diagonal with it to achieve a correct image. It is my understanding that the #933 diagonal has a built in 2.4x Barlow to bring it to focus on the ETX 70. Would this raise the effective power of a 30 or 40mm eyepiece so high as to make the ETX 70 useless as a finder? Thanks in advance for any help!

Gary

[MisterDan]

There is no Barlow in the 933.

 

Best wishes.

Dan

[photoracer18]

Pretty much, not to mention that diagonal does not support 2" eyepieces so even a 30-40mm eyepieces is not going to have a a wide enough field at 1.25" to be useful as a finder. Divide the multiplication of the Barlow into the FL of the eyepiece to get the actual FL. A better finder would be an 80mm F5 compact scope then 40mm would be 10x a good power for a large finder. Or it could double as a guidescope for imaging. I assume you are collecting binos as no sane person would need that many. Even I don't have that many although I claim to be collecting mini-binos.

[Redbetter]

There is no Barlow in the 933.

 

Best wishes.

Dan

Are you sure about that?  What I have read is that the 933 has a Barlow and the 932 does not.  That is what three different sources say,  but I don't have either and have never compared them.

 

If it has the Barlow the internal diameter with the Barlow optics is narrower; and of course the resultant TFOV will be narrower by the Barlow factor because the operating focal length will be much greater.  Depending on how narrow the build in Barlow makes the ID, with the eyepiece focal plane close to that restriction it might not handle 27mm field stops well either.

 

TFOV = 57.3 * field stop diameter (mm) / operating focal length.  

[Profguy]

Regarding the 933 having a Barlow, I was going by this thread on CN:

https://www.cloudyni...,ETX 60/70/80. 
There are even pics purporting to show the difference between the two, where a Barlow lens can be seen in the diagonal that is identified as a 933. I have a 932, and it will not come to focus on the ETX 70. However, when I insert a 2x Barlow between the 70 and the 932 diagonal, it will come to focus. It sounds like I’ll be using the flip mirror and a low power eyepiece in the 90* viewing port to see how that works. I’ll just have to deal with the reversed image. Thanks!

 

Gary

[MisterDan]

Well, dang.  I stand corrected!  I apologize for my mistake and the "false lead," Gary.

 

As you've likely noticed, most item descriptions of the 933 don't mention an internal Barlow/magnifier at all.  Most "933" photos appear to show the 932 (or fail to show the collar side) and are not helpful.  Other photos appear to show the 933's collar to be a different size (smaller) than that of the 932.  However, Tom Duncan posted a photo of his Meade Amici diagonal (apparently a 933) and notes that his fits both ETX series (Maksutovs and/or refractors).  This is from January 2019:

https://www.cloudyni...932-933-prisms/

 

I checked my own ETX-70 user manual, and - lo and behold - it notes that the 933 does indeed utilize an integrated 2.4x "relay" (Barlow-type negative lens) magnifier.

 

Well, poo.  The 933 would "change" an f/5 ETX-70 into an effective f/12 refractor - DEFINITELY not "finder territory."

 

It would still work as an f/5 finder using the integral (and switch-able) diagonal mirror, but - obviously - it would yield a "backwards" image (left-right flipped).  This may be fine for a larger refractor or cassegrain, as the main scope (with a standard star diagonal) would yield the same image orientation, but that's typically a "personal taste" proposition.

 

Thanks for asking me the question, sir: "Are you sure about that?" 

 

Best wishes and kind regards.

Dan

[Max Coe]

I use my ancient ETX70 as a finder with the original flip mirror and a 28mm Pentax wide. 12.5x, but no correct view, of course. #

[Profguy]

Thanks for all the info. CN folks are great!

 

Gary

[MisterDan]

Now that my ETX-70 is more accessible (after checking the manual about the 933), maybe I'll take it for a spin and see how the "old timer" is doing, these days.  It's been a couple years...

 

-Hard to believe the ETX refractors were introduced in 2000.  -Almost a quarter century! 

 

Cheers.