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Advice regarding eyepieces and barlows\powermates

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#51 NHRob

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Posted 12 January 2015 - 09:37 AM

Thanks all.  That is what I was hoping.  I have tried barlows but never a telecentric/Powermate.

I want a 2X 1.25" to use with my Konigs and was considering the ES focal extender.

 

Rob



#52 BillP

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Posted 12 January 2015 - 10:27 AM

In a Barlow, the magnification keeps increasing the farther you are away from the lens.

In a telecentric lens, the magnification would be the same at all distances.

 

In practice, many commercial "telecentric" lenses are not pure and do have a very slightly varying magnification with distance from the lens,

but they are closer to that than any standard Barlow lens.

 

As in all things, the consumer really can't go by the name of the device to determine if they are getting what they expect.  So as Don points out, some "Telecentrics" behave just like Barlows in several respects so any anticipated "advantage" that one is looking for may not be there.  Case-in-point are both TeleVue and Siebert.  I have a Siebert Telecentric and it behaves just like a Barlow giving good magnification increase with distance.  Of the TV Powermates, the 5x Powermate is similar (http://www.televue.c...werIncrease.jpg).  So if one wants parallel rays, one won't always get that from an amplifier.  So the consumer needs to dig deeper.  Cudos to TV for publishing these facts on their equipment so at least not much digging is needed.

 

Now, as far as is a Powermate/Telecentric an advantage or not...that just depends.  I have found them to be advantageous and disadvantageous, depending on the application.  IMO, their best visual observing attribute is with longer focal length eyepieces *only*.  Barlows many times vignette the FOV with long focal length wide fields (not always but a lot of times) and they can increase the eye relief so if it is overly long for the eyepiece can make it even longer and more difficult to maintain eye position.  However, with shorter focal length eyepieces (usually 20mm and shorter), I find the Powermate/Telecentric a disadvantage.  For shorter focal length eyepieces, adding eye relief is often a good thing, so a Barlow is an advantage over an Amplifier here.  Also, I've never had a shorter focal length EP's FOV vignette with a Barlow to date.  So then the extra glass and extra cost of the Amplifier does nothing for me in these circumstances.  Finally, depending on how scatter-free your optical chain is, you may notice the extra scatter induced by the extra glass in an Amplifier over a Barlow.  It is not a whole lot, but it is easy enough for me to see when I experiment and switch between Barlow and Amplifiers of same power.  So for observing applications where one wants to keep scatter minimized, a same quality Barlow will produce less than a same quality Amplifier with twice the glass in it.

 

So the real answer to the question is that sometimes a Barlow is better, and sometimes an Amplifier is better, just depending on what you are doing and what you want to optimize.



#53 REC

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Posted 12 January 2015 - 10:47 AM

Not sure if this has been brought up in this post, but maybe the OP might be interested as I see he has a DSLR camera.

 

The 2.5x Powermate screws apart and TelVue makes a "T" adapter that to connects the optics part to the camera mount.  This way you are able to take an image at prime focus with additional magnification that a standard barlow won't do.  I have a basic 2x shorty that works ok for the eyepice magnification, but I wanted to shoot the moon and planets with my DSLR.  Here is a shot of the moon using the Powermate in my 8" scope.

Attached Thumbnails

  • PM.jpg


#54 David Knisely

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Posted 12 January 2015 - 02:11 PM

One nice thing about my 2.5x Tele Vue Powermate is that it worked in my 100mm f/6 refractor when placed just ahead of the eyepiece and in the back end of my scope's star diagonal.  My Barlow would not come to focus in that scope, so the Powermate fit the bill nicely.  One thing to note for those who want to use the Powermates to get past f/30 for solar H-alpha: the 2x, 2.5x, and 4x models are at least somewhat telecentric, but the 5x is not, so if you want full-field H-alpha with the near-focus H-alpha filters, don't use the 5x model.  Clear skies to you. 



#55 Procyon

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Posted 22 December 2016 - 11:21 PM

What kind of powermate should one use on an SCT combined with a 22mm or 26mm Nagler, with or without an edge focal reducer?

Edited by Procyon, 23 December 2016 - 05:19 AM.


#56 Starman1

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Posted 23 December 2016 - 12:41 AM

2x, 2".




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