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Brandon Dakin 2" Barlow inTeleVue-85: Will it work?

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#1 Thomas_M44

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Posted 18 February 2020 - 02:03 PM

Hello Folks

 

I'm seeking comments, advice, thoughts on the following prospective equipment pairing:

 

TeleVue TV-85 refractor together with APM 2-inch Amici prism 90-deg diagonal, *possibly* using a Vernonscope Brandon Dakin Barlow lens (positioned between the telescope and the prism diagonal).

 

I presently own the APM 2-inch Amici prism diagonal, which is specified as:

--46mm clear aperture  --113mm focal length

 

I'm interested in possibly buying the 2-inch 2.4X Dakin Barlow most specifically to extend the utility/magnification options for three new 2-inch Masuyama eyepieces I have obtained. all are 85 degree AFOV:

 

32mm (20mm eye-relief), 26mm (15mm eye-relief), 20mm (12mm eye-relief)

 

I have considered simply utilizing a TeleVue 2-inch 2X Powermate between the diagonal and the eyepieces, but  like the idea of going "minimal glass" with a simple 2-element Barlow such as the Dakin, and also like the idea of having the Barlow between the scope and the diagonal, thereby not having a big Powermate and 2" eyepiece towering up vertically from the diagonal.

 

Here are my concerns and questions:

 

(1) might the TV-85 have trouble mechanically reaching focus with the 2-inch Dakin Barlow between the scope and the prism diagonal?

 

(2) should the 2-inch Dakin Barlow work adequately

/properly feeding into my 2" APM Amici diagonal?

 

(3) Will the Dakin Barlow likely have adequate clear aperture to avoid Vignetting in my longer focal length 85-Degree AFOV 2-inch Masuyama eyepieces?

 

(4) I'm assuming that the expected increase in eye-relief via Barlow usage will likely not be a problem, given that my longest focal length Masuyama eyepiece is 32mm (with a 20mm nominal eye relief specification). Am I on track here?

 

(5) I assume the Brandon 2-inch 2.4X Dakin Barlow is rated at 2.4X when used in the position between the scope and diagonal? ( The documentation for this Barlow online is minimal.)

 

 

So...  any input or thoughts would be much appreciated, particularly from anyone who has experience with using a Brandon 2-inch 2.4X Dakin Barlow in a TV-85 (F/7) or similar focal F-Ratio small APO scope.

 

If the Dakin appears likely problematic for my application,  I may just end up going with a 2" 2X TV Powermate, as I'm fairly confident I'd have no trouble reaching focus, or avoiding vignetting with my 85 degree AFOV 2-inch eyepieces.


Edited by Thomas_M44, 18 February 2020 - 02:11 PM.


#2 russell23

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Posted 18 February 2020 - 08:48 PM

The 2" Dakin is an outstanding barlow - the best I have ever used optically.  It is designed to be placed in front of a star diagonal with a refractor for approximately 2.4x magnification.  With my Astrophysics Maxbright diagonal it was closer to 2.6x magnification. 

 

I'm not sure what the magnification factor would be with a 2" Amici prism.  It would depend upon the light path of the prism. 

 

I used the 2" Dakin with a number of long focal length eyepieces and do not recall any vignetting with the eyepieces I used but vignetting is one of those things that varies with the specific barlow-eyepiece combination.



#3 Thomas_M44

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Posted 18 February 2020 - 09:29 PM

The 2" Dakin is an outstanding barlow - the best I have ever used optically.  It is designed to be placed in front of a star diagonal with a refractor for approximately 2.4x magnification.  With my Astrophysics Maxbright diagonal it was closer to 2.6x magnification. 

 

I'm not sure what the magnification factor would be with a 2" Amici prism.  It would depend upon the light path of the prism. 

 

I used the 2" Dakin with a number of long focal length eyepieces and do not recall any vignetting with the eyepieces I used but vignetting is one of those things that varies with the specific barlow-eyepiece combination.

Do you recall having any issues with running out of focus when using the Dakin?

 

I'm just now getting my brand new TV-85 and associated gear cobbled together. I have yet to do any experimentation with the new equipment I've recently obtained.

 

The 2-inch Dakin Barlow looks promising.  Every bit of input certainly helps me in my deliberations. I'm cautious in my purchases, and seldom impulse buy anything over $20.


Edited by Thomas_M44, 18 February 2020 - 09:30 PM.


#4 russell23

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Posted 18 February 2020 - 09:34 PM

Do you recall having any issues with running out of focus when using the Dakin?

 

I'm just now getting my brand new TV-85 and associated gear cobbled together. I have yet to do any experimentation with the new equipment I've recently obtained.

 

The 2-inch Dakin Barlow looks promising.  Every bit of input certainly helps me in my deliberations. I'm cautious in my purchases, and seldom impulse buy anything over $20.

No problems with focus as long as you put it before the diagonal.



#5 alnitak22

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Posted 18 February 2020 - 09:36 PM

Hello Folks

 

I'm seeking comments, advice, thoughts on the following prospective equipment pairing:

 

TeleVue TV-85 refractor together with APM 2-inch Amici prism 90-deg diagonal, *possibly* using a Vernonscope Brandon Dakin Barlow lens (positioned between the telescope and the prism diagonal).

 

I presently own the APM 2-inch Amici prism diagonal, which is specified as:

--46mm clear aperture  --113mm focal length

 

I'm interested in possibly buying the 2-inch 2.4X Dakin Barlow most specifically to extend the utility/magnification options for three new 2-inch Masuyama eyepieces I have obtained. all are 85 degree AFOV:

 

32mm (20mm eye-relief), 26mm (15mm eye-relief), 20mm (12mm eye-relief)

 

I have considered simply utilizing a TeleVue 2-inch 2X Powermate between the diagonal and the eyepieces, but  like the idea of going "minimal glass" with a simple 2-element Barlow such as the Dakin, and also like the idea of having the Barlow between the scope and the diagonal, thereby not having a big Powermate and 2" eyepiece towering up vertically from the diagonal.

 

Here are my concerns and questions:

 

(1) might the TV-85 have trouble mechanically reaching focus with the 2-inch Dakin Barlow between the scope and the prism diagonal?

 

(2) should the 2-inch Dakin Barlow work adequately

/properly feeding into my 2" APM Amici diagonal?

 

(3) Will the Dakin Barlow likely have adequate clear aperture to avoid Vignetting in my longer focal length 85-Degree AFOV 2-inch Masuyama eyepieces?

 

(4) I'm assuming that the expected increase in eye-relief via Barlow usage will likely not be a problem, given that my longest focal length Masuyama eyepiece is 32mm (with a 20mm nominal eye relief specification). Am I on track here?

 

(5) I assume the Brandon 2-inch 2.4X Dakin Barlow is rated at 2.4X when used in the position between the scope and diagonal? ( The documentation for this Barlow online is minimal.)

 

 

So...  any input or thoughts would be much appreciated, particularly from anyone who has experience with using a Brandon 2-inch 2.4X Dakin Barlow in a TV-85 (F/7) or similar focal F-Ratio small APO scope.

 

If the Dakin appears likely problematic for my application,  I may just end up going with a 2" 2X TV Powermate, as I'm fairly confident I'd have no trouble reaching focus, or avoiding vignetting with my 85 degree AFOV 2-inch eyepieces.

I’d give TeleVue a call. You’ll get Al or David on the line and they will be able to answer any questions fully. Not that the folks here aren’t helpful and knowledgeable, but being able to call the designer is a nice plus.



#6 Thomas_M44

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Posted 18 February 2020 - 10:32 PM

No problems with focus as long as you put it before the diagonal.

This is good to hear. 

 

I'm assuming you were using it with intermediate F/ratio smaller APO's?

 

I’d give TeleVue a call. You’ll get Al or David on the line and they will be able to answer any questions fully. Not that the folks here aren’t helpful and knowledgeable, but being able to call the designer is a nice plus.

This is a really good point.

 

I do wonder. however, if they might tend to simply steer me towards the TV 2X TV Powermate or 2X TV BIG Barlow?

 

There's one way to find out, I suppose.



#7 alnitak22

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Posted 18 February 2020 - 11:44 PM

This is good to hear. 

 

I'm assuming you were using it with intermediate F/ratio smaller APO's?

 

This is a really good point.

 

I do wonder. however, if they might tend to simply steer me towards the TV 2X TV Powermate or 2X TV BIG Barlow?

 

There's one way to find out, I suppose.

Well yeah...they might steer you towards TV...and why not? And to be fair...if you could call up the designer of the Dakin and talk to him, he might steer you towards it. Ah....but you can’t! Call TeleVue. Btw...congrats on your TV85. I’ve had one since 2001 and have used it day and night since then and it’s just as good as the day I got it.



#8 Tyson M

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Posted 18 February 2020 - 11:47 PM

Do you recall having any issues with running out of focus when using the Dakin?

 

I'm just now getting my brand new TV-85 and associated gear cobbled together. I have yet to do any experimentation with the new equipment I've recently obtained.

 

The 2-inch Dakin Barlow looks promising.  Every bit of input certainly helps me in my deliberations. I'm cautious in my purchases, and seldom impulse buy anything over $20.

The TV85 is an amazing scope.  Small for its size, packs a large punch on a small mount and exudes build quality.  You will love it! 



#9 Tyson M

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Posted 18 February 2020 - 11:49 PM

The 2" Dakin is an outstanding barlow - the best I have ever used optically.  It is designed to be placed in front of a star diagonal with a refractor for approximately 2.4x magnification.  With my Astrophysics Maxbright diagonal it was closer to 2.6x magnification. 

 

I'm not sure what the magnification factor would be with a 2" Amici prism.  It would depend upon the light path of the prism. 

 

I used the 2" Dakin with a number of long focal length eyepieces and do not recall any vignetting with the eyepieces I used but vignetting is one of those things that varies with the specific barlow-eyepiece combination.

So if I bought the 2" Vernonscope 2.4x barlow and put it before the diagonal- I would have something like 2.5x or 2.6x with my long light path BBHS?



#10 russell23

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Posted 19 February 2020 - 06:39 AM

This is good to hear. 

 

I'm assuming you were using it with intermediate F/ratio smaller APO's?

 

This is a really good point.

 

I do wonder. however, if they might tend to simply steer me towards the TV 2X TV Powermate or 2X TV BIG Barlow?

 

There's one way to find out, I suppose.

I was using it with a 120mm f/7.5 APO and a 140mm f5.7 Vixen Neo-achro.   The 2" Dakin has better optical performance than the 2" Big barlow.  It was also better than the 2.5x Powermate.  The Dakin provided cleaner sky background and sharper views to the edge than both.



#11 russell23

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Posted 19 February 2020 - 06:41 AM

So if I bought the 2" Vernonscope 2.4x barlow and put it before the diagonal- I would have something like 2.5x or 2.6x with my long light path BBHS?

Probably.  I measured it typically at about 2.6x with my AP maxbright.  That was directly measured using a ruler on a wall 30 ft from my scope to get relative FOV with and without the barlow.  If the BBHS is even longer it could be a little higher.


Edited by russell23, 19 February 2020 - 06:41 AM.

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#12 Thomas_M44

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Posted 19 February 2020 - 01:21 PM

I was using it with a 120mm f/7.5 APO and a 140mm f5.7 Vixen Neo-achro.   The 2" Dakin has better optical performance than the 2" Big barlow.  It was also better than the 2.5x Powermate.  The Dakin provided cleaner sky background and sharper views to the edge than both.

Helpful information, thank you.

 

I'm curious, however, did you end up retiring the 2.4X Dakin and move on to something else?



#13 russell23

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Posted 19 February 2020 - 05:08 PM

Helpful information, thank you.

 

I'm curious, however, did you end up retiring the 2.4X Dakin and move on to something else?

I just found it a bit inconvenient to pull the diagonal and slip the Dakin in front.  Then pull it out when I was not using the barlow.  Part of that issue was really the focuser on the SW120ED.  It had tiny screws to loosen and tighten and no compression ring so it was a pain.   Now I have a Moonlite on my SW120 and then the focusers on the SV102 Access an AT102ED are excellent. 

 

The other issue was that the SW120ED is a little undermounted on my Vixen SP mount.  So the Dakin barlow extended the length another 5-6" which added vibrations at the same time I was jacking up magnification.

 

But setting those non-optical issues aside, I would say that optically the 2" Dakin is the finest barlow I've used.  Also be aware that the 2" Dakin it is a little better than the 1.25" Dakin.  Really the only barlow that matches the 2" Dakin that I have used is the 1.25" 1.6x Nikon barlow. 

 

Dave



#14 Thomas_M44

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Posted 20 February 2020 - 02:41 AM

I just found it a bit inconvenient to pull the diagonal and slip the Dakin in front.  Then pull it out when I was not using the barlow.  Part of that issue was really the focuser on the SW120ED.  It had tiny screws to loosen and tighten and no compression ring so it was a pain.   Now I have a Moonlite on my SW120 and then the focusers on the SV102 Access an AT102ED are excellent. 

 

The other issue was that the SW120ED is a little undermounted on my Vixen SP mount.  So the Dakin barlow extended the length another 5-6" which added vibrations at the same time I was jacking up magnification.

 

But setting those non-optical issues aside, I would say that optically the 2" Dakin is the finest barlow I've used.  Also be aware that the 2" Dakin it is a little better than the 1.25" Dakin.  Really the only barlow that matches the 2" Dakin that I have used is the 1.25" 1.6x Nikon barlow. 

 

Dave

Thanks Dave

 

This information is interesting, and helpful in my deliberations.



#15 Thomas_M44

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Posted 20 February 2020 - 05:48 PM

I just found it a bit inconvenient to pull the diagonal and slip the Dakin in front.  Then pull it out when I was not using the barlow.  Part of that issue was really the focuser on the SW120ED.  It had tiny screws to loosen and tighten and no compression ring so it was a pain.   Now I have a Moonlite on my SW120 and then the focusers on the SV102 Access an AT102ED are excellent. 

 

The other issue was that the SW120ED is a little undermounted on my Vixen SP mount.  So the Dakin barlow extended the length another 5-6" which added vibrations at the same time I was jacking up magnification.

 

But setting those non-optical issues aside, I would say that optically the 2" Dakin is the finest barlow I've used.  Also be aware that the 2" Dakin it is a little better than the 1.25" Dakin.  Really the only barlow that matches the 2" Dakin that I have used is the 1.25" 1.6x Nikon barlow. 

 

Dave

I'm now possibly considering the Astro-Physics BARADV Convertible Barlow unit.  Evidently I could detach the compact doublet lens assembly from the tube body, and simply screw it into the M48 threaded nosepiece of my prism diagonal.

 

Apparently, this should give about 2X, perhaps a bit more.

 

This might be a viable alternative to using a 2.4X 2-inch Dakin Barlow which has a relatively long body.


Edited by Thomas_M44, 21 February 2020 - 02:36 AM.


#16 Thomas_M44

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Posted 21 February 2020 - 02:37 PM

I just found it a bit inconvenient to pull the diagonal and slip the Dakin in front.  Then pull it out when I was not using the barlow.  Part of that issue was really the focuser on the SW120ED.  It had tiny screws to loosen and tighten and no compression ring so it was a pain.   Now I have a Moonlite on my SW120 and then the focusers on the SV102 Access an AT102ED are excellent. 

 

The other issue was that the SW120ED is a little undermounted on my Vixen SP mount.  So the Dakin barlow extended the length another 5-6" which added vibrations at the same time I was jacking up magnification.

 

But setting those non-optical issues aside, I would say that optically the 2" Dakin is the finest barlow I've used.  Also be aware that the 2" Dakin it is a little better than the 1.25" Dakin.  Really the only barlow that matches the 2" Dakin that I have used is the 1.25" 1.6x Nikon barlow. 

 

Dave

I just looked into that Nikon EiC 1.6X 1.25" Barlow you mentioned.

 

It appears to be a very high quality unit.  I see Agena Astro offers it for around $220, but it's out of stock.

 

I wonder if it has been discontinued?

 

Of course, being a 1.25" Barlow, the Nikon 1.6X unit doesn't help me with my present issue of needing a Barlow for use with my 2" Masuyama EP's, but nonetheless might be a good lower-power Barlow for my 1.25" eyepieces. I'll have to perhaps investigate that prospect separately.




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