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20mm XW with Dakin barlow

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#1 russell23

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Posted 18 November 2012 - 11:59 PM

I just have to say after a few months of using these together that the 20mm XW with the Vernonscope Dakin barlow is a tremendous combination in my Vixen 140mm neo-achro refractor. Without the Dakin there is some of the often commented on field curvature with the 20mm XW. This is a minor issue in the Vixen neo-achro. But there is something about the Dakin combined with the 20mm XW. The stars are just tack sharp right to the edge of the field with this combination - really reminds me of the 10mm Delos.

The pair gives 96x with the Vixen 140 and provides outstanding views of any deep sky objects you want to attack. I don't know what it is about this combination because I don't like the Dakin with some of my eyepieces, but it seems like through no intent of any designers that the 20mm XW and the 2.4 Dakin are a perfect match.

I can't say the same thing when I use the 20mm XW with a 2.5x TV powermate. The 20mm XW/2.5 powermate combination is just not as good as the 20mm XW/2.4x Dakin barlow combination. And I consider the TV powermate to be excellent with certain eyepieces. But with the Dakin/20XW the field is cleaner, the stars are sharper pinpoints, the sky background is blacker and more evenly illuminated. The 20mm XW has some very mild edge brightening unbarlowed and with the Dakin this goes away almost completely whereas with the powermate the edge brightening seems enhanced.

I liked the 10mm Delos so much I was thinking about getting an 8mm Delos, but the 20mm XW with Dakin is so good that now I'm not sure an 8mm Delos makes sense.

I thought I'd share this. If you have access to both the Dakin barlow and the 20mm XW you should give them a try together.

Dave

#2 Mike B

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Posted 19 November 2012 - 12:38 AM

If you have access to both the Dakin barlow and the 20mm XW you should give them a try together.



I've had the Dakin Barlow ("2.4x") for some time now, and really like how it plays with *many* of my EPs. Tho it'll be quite a bit of horsie in my Dob, i'll have to try my new (to me) 10mm XW with it... an effective ~4mm FL!

Glad to hear you like yours- thanks for the suggestion.
:grin:

#3 Scanning4Comets

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Posted 19 November 2012 - 08:27 AM

I used to own a 14mm Pentax XW. I see more FC in the 14mm XW than the 20mm XW in my 10" reflector and the same was for my 12" F/5 reflector, so I got rid of the 14mm XW right away as the FC in the 14mm XW to me is intolerable.

However, when I *did* own the 14mm XW, I used it with a barlow plus an extension tube in the 12" F/5 I had in Oct 2005, and it cleaned up the FC in the 14mm XW really nicely. I saw Mars @ 261x with this configuration and the detail was excellent.

Cheers,

#4 russell23

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Posted 19 November 2012 - 09:21 AM

If you have access to both the Dakin barlow and the 20mm XW you should give them a try together.



I've had the Dakin Barlow ("2.4x") for some time now, and really like how it plays with *many* of my EPs. Tho it'll be quite a bit of horsie in my Dob, i'll have to try my new (to me) 10mm XW with it... an effective ~4mm FL!

Glad to hear you like yours- thanks for the suggestion.
:grin:


I'll be interested to hear how it works with the 10mm XW. I know the FC is different in the 10mm than the 20mm so I wonder if that will affect the way the two optics interact?

Dave

#5 russell23

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Posted 19 November 2012 - 09:24 AM

I used to own a 14mm Pentax XW. I see more FC in the 14mm XW than the 20mm XW in my 10" reflector and the same was for my 12" F/5 reflector, so I got rid of the 14mm XW right away as the FC in the 14mm XW to me is intolerable.

However, when I *did* own the 14mm XW, I used it with a barlow plus an extension tube in the 12" F/5 I had in Oct 2005, and it cleaned up the FC in the 14mm XW really nicely. I saw Mars @ 261x with this configuration and the detail was excellent.

Cheers,


The 20mm XW works really nicely in my refractor. I'm not sure if I would say the same about the 14XW. At the time when I had gone through purchasing 40mm, 30mm, 20mm, 14mm, 10mm, and 7mm XW's the 20mm XW was my clear favorite with the 40mm my next favorite. The 14mm was my least favorite.

Dave

#6 Starman81

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Posted 19 November 2012 - 11:48 AM

I did some daytime testing with the Pentax XW's (20/14/10/7) in my f/3.9 Starblast 4.5 to test for field curvature. I used some detailed features on an adjacent apartment building and viewed from indoors. I did this quick-and-dirty testing because I could never really 'see' the FC as reported in the XW 20 & 14 in night time use. I gave each eyepiece many turns in the focuser and it took about 60 minutes to do the testing. I would focus on a nail protruding from the wooden veneer of the apartment building which cast a nice shadow. I focused the nail on the center of the FOV, at 1/2 way out, 3/4 the way out and at the extreme edge. My results were:

- 20mm had moderate FC
- 14mm had minimal FC (less than the 20mm)
- 10mm had no FC
- 7mm had no FC

I also threw two more eyepieces into the mix:

- 10 Delos had no FC
- 10 Hyperion had moderate+ FC (worse than XW 20)

Take this with a grain of salt as I am new to 'seeing FC'. Also, I am not sure what additional aberrations may have been added due to the fast focal ratio in play (coma) or due to the eyepieces themselves (astigmatism?) and with my limited experience would not know how to ferret these out. But, since all were tested in the same system, the results are all relative.

Night time testing the next night with the XW 20 and 14 on a few DSO's (M42, M35, NGC 2264) in my f/6 dob. Any FC that might have been there was not objectionable per se, and I saw it more in the 20 than in the 14. Since these results are contrary to the conventional wisdom, more testing is needed.

Also, I have a Vixen LVW 22 coming in soon that I will be pitting against the XW 20.

#7 Mike B

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Posted 19 November 2012 - 11:58 AM

Nice work, Starman81! But if your eyes are fairly young, they may have been sabotaging your fine efforts... the eye naturally adjusts itself for focus, automatically- i doubt we even have control over this function! :lol: Younger eyes naturally have a greater range of focus & flexibility in achieving it than do older eyes.

So the very thing you were attempting to chase *may* have eluded your grasp because your eyes are still TOO good at doin' their thing. Us older dogs can see the FC quite well, as our eyes can't quite keep up, our lenses having too much mileage on them, and no longer being flexible enough for the task.

Maybe try the same testing with another observer alongside?... someone into their 50's or 60's? They may very well see & measure things differently?

It's also possible the scope used for the test provided its own FC, and such FC could be either same direction as the EPs tested, or else opposite- same direction compounding the effect, opposite canceling.

Generally, when such tests are performed, a scope of known FC (ideally NONE!) is used, to remove that factor.


#8 dscarpa

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Posted 19 November 2012 - 03:27 PM

My 10 and 7 XWs are excellent with a 2.5X Powermate in my WO ZS-110. The XW's clarity, cool tone and contrast are unaffected. The entire FOV is dark and views are razor sharp. David

#9 russell23

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Posted 20 November 2012 - 12:16 PM

My 10 and 7 XWs are excellent with a 2.5X Powermate in my WO ZS-110. The XW's clarity, cool tone and contrast are unaffected. The entire FOV is dark and views are razor sharp. David


That's why I am very interested to see what is reported about the 10mm XW with the Dakin barlow. Since the field curvature of the 10mm and 20mm XW's is different I wouldn't be surprised if the interaction between the 10mm XW and Dakin is different than the 20mm and Dakin which is just outstanding. Your findings with the 10mm XW and 2.5x Powermate would seem to confirm the variation in interaction between different designs.

Dave

#10 Mike B

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Posted 20 November 2012 - 01:08 PM

That's why I am very interested to see what is reported about the 10mm XW with the Dakin barlow.



As soon as i get a clear nite, will give it a try. Also have a 2.8x 'Klee' Barlow which seems very nice on many EPs... will try that combo, too.
:grin:

#11 russell23

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Posted 20 November 2012 - 01:33 PM

That's why I am very interested to see what is reported about the 10mm XW with the Dakin barlow.



As soon as i get a clear nite, will give it a try. Also have a 2.8x 'Klee' Barlow which seems very nice on many EPs... will try that combo, too.
:grin:


Cool! Can't wait to hear what you find!

Dave

#12 Dr Morbius

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Posted 20 November 2012 - 01:57 PM

You should try that Dakin with a Brandon eyepiece. Classic!

#13 Mike B

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Posted 20 November 2012 - 03:28 PM

Oh yeah, huh. I *do* have a 16mm Brandon... will give that a shot, too.
;)

The 16mm in a 2.8x ought to, theoretically, yield about the same magnification as a 10mm EP in my 1.7x Siebert Barlow. Ought to, theoretically anyway, be very interesting!

#14 Mike B

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Posted 21 November 2012 - 10:32 PM

You should try that Dakin with a Brandon eyepiece. Classic!



Okay, i finally did! Tho our nights have been drizzly & overcast... so this run was my standard test jig- a tape measure hung on a street-lite pole about 80 feet away, full daylight. :grin: The tape edge was viewed so as to bisect the FoV, and tape readings to 1/16" were carefully noted top & bottom. That measure (net) was divided by the distance to the tape, arctan taken, etc... FL of EPs were assumed face-value, so all Barlow measurements taken will be "relative" to the "base" measure; the whole exercise was *primarily* to calculate Barlow factors, anyway, so the amplification factors should be pretty close.

The scope was a 6" F~8 classic Newtonian, 1168mm FL. The oculars were my new (to me ;)) Pentax XW 10mm, a Brandon Ortho 16mm, and a typical VT Ortho 12.5mm ("EdsCorp")... and the Barlows as listed. Sometimes a short extension tube was required to reach focus. A final measurement of the focuser hilt to EP shoulder was noted, as a relative in/out focus position.

Will see if i can string this data here, in some semblance of order:
EP/FL... Native X... Focus @... AFoV
XW 10mm... 117x..... +4.75".... 67.2*.. sharp to edges!
@2.4x Dakin.. 2.46x... +7.0".............sharp!
@2.8x Klee.... 2.77x... +6.125"..........sharp, but dark at 323x!
--------
VT 12.5mm.. 94x..... +5.0"...... 36.1*.. sharp to edges
@2.4x Dakin.. 2.42x.... +7.125".............sharp to edges
@2.8x Klee.... 2.67x... +6.0"..........sharp, but dark fuzzy ring around extreme edge
--------
Brndn 16mm.. 73x..... +5.0"..... 39.3*.. sharp almost to edges, slight outfield softening focusing can't 'fix'
@2.4x Dakin.. 2.38x.... +7.125".............sharp to edges
@2.8x Klee.... 2.42x... +6.0"...........sharp to edges

Things i found strange:
1) The Klee's amplification with the Brandon fell waay short compared to its higher (closer to label'd) factor for the other EPs,
2) The VT Ortho's AFoV was really tiny. I knew this, as i've measured it thus before... yet we commonly seen them "advertised" as 40-45* AFoV. Maybe was unique to the older "EdsCorp" manuf. specs? I dunno...
3) Both Ortho's had almost identical focal positions! And the XW focused a tad closer native & with the Dakin, yet a skosh farther out with the Klee,
4) The odd ring of fuzzy-darkness at the extreme edge of the VT Ortho with the Klee... wasn't repeated at all with the Brandon. Never seen this on any EP and/or Barlow before... a very strange form of vignetting, if that's what it was. :shrug:

While i'm at it, i may as well post some more data- obtained with the identical jig, different EPs, different day. Focus position wasn't noted, unfortunately:
CZJ 10mm... 117x..... 37.3*.. (Carl Zeiss Jena) sharp to edges
@2.4x Dakin.. 2.61x.... sharp to edges
@2.8x Klee.... 3.07x... sharp to edges
--------
Oly 12.5mm.. 93.5x..... 51*.. (Olympus Ortho, Gerd Neumann) sharp mostly to edges, outfield soft
@2.4x Dakin... 2.23x.... sharp almost to edges
@2.8x Klee..... 3.33x... sharp almost to edges

Hope this info is helpful
:grin: mike b

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#15 mvw

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Posted 03 May 2013 - 10:12 PM

Glad for the review, I'm thinking of getting the Dakin Barlow.






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