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Re: 20mm Nagler t5 and AP barcon

Started by wrose, 09/06/2005 03:55AM
Posted 09/06/2005 03:55AM Opening Post
Hi Nils,
I have both a 20mm T2 and 20mm T5 Nagler which I have used with a TV Big Barlow and 2" PowerMate. The quality of image, especially with the PowerMate, is excellent in an NP-127, C9.25, and 12.5" ZOC.
Unfortunately I've never used an AP Barcon with either of these eyepieces. I can only tell you that the PowerMate works well with both these eyepieces. I doubt you'll notice any degree of degradation unless you're using very high quality optics on a great night of seeing.

Clear Skies, Bill
Nils Schoultz said:

Has anyone ever tried this combination? I'm trying to decide between the barcon and a 2x powermate. The powermate will work, of course, but the lighter weight and lesser magnification of the barcon is appealing. Anyone have any experience here?
thanks,
Nils
Posted 09/06/2005 06:11PM | Edited 09/06/2005 06:12PM #1
Nils,
I have the Big Barlow and have no complaints with it. I have used it for extremely high powers with my Starmaster. It worked well at 1060x. I have heard that the PowerMate does this also and without an change in eye relief. This can be good or bad depending on whether you are trying to get more eye relief out of a Orthos or some other tight EP I guess.
I believe the reason the PowerMate works well wtih more glass is the great coatings used.

[SIZE="Large"][/SIZE][COLOR="Blue"][/COLOR] Floyd Blue grin
Amateur Imager
Posted 09/08/2005 01:23AM #2
Hi Nils,

I believe the complaint about vignetting comes from the use of the Big Barlow with Panoptic eyepieces. With some of the Panoptics (35mm & 27mm. 22mm I think also) it does slightly vignette. That's why TV makes the 'TeleVue Panoptic Barlow Interface'. Personally I don't have a problem with eye relief since I don't wear glasses. Can't say as I've ever tried comparing the Big Barlow and Powermate as far as eye relief.
I haven't noticed any vignetting with the Powermate and Naglers but then I can't remember that there's any vignetting with the Panoptic and the Powermate.
The TeleVue Panoptic Barlow Interface definately helps with the 35mm & 41mm Panoptic but then addition of a 17mm and 22mm eyepiece eliminate the need for either.

Clear Skies, Bill


Nils Schoultz said:

Thanks for the advice, Bill. It sounds as if you think the powermate would give a better image than a 2" barlow. I've never been clear whether or how a powermate is better than a barlow - if (big if) the barlow does not cause vignetting in an eyepiece, I don't understand why a 4-element powermate would give better star images than a 2-element barlow. But your real experience counts for more than speculation, so I think I will go with the powermate unless someone else convinces me otherwise.
thanks again,
Nils
Posted 09/08/2005 04:32PM #3
>>I've never been clear whether or how a powermate is better than a barlow - if (big if) the barlow does not cause vignetting in an eyepiece, I don't understand why a 4-element powermate would give better star images than a 2-element barlow.>>

There is much confusion about Barlows and their effect on eyepieces. Let me see if I can add to this confusion smile

A Barlow increases the size of the virtual image by redirecting the incoming convergent rays of light from the objective lens or mirror. A 2x barlow will double the size of the image, so that the original field of 1" diameter (to give an example) becomes an image of 2" diameter some distance further back from the original focus point.

In order to achieve this 1" to 2" size increase, the original rays must diverge outwardly. The amount of divergence is inversely proportional to the focal length of the Barlow. What this means is that the rays will diverge mildly with a long focal length Barlow (one with a fl = -5") versus a shorty Barlow (fl = -2") where the rays will diverge rather rapidly with a large angle. To see what I mean by divergence angle, draw a 2" field and a 1" field separated by 5" versus one separated by 2" and connect the outer parts of the field with straight lines (see attached picture).

Normally, Barlows have fairly short focal lengths ranging from -1" to -2". This allows the eyepiece to come to focus close to the original point and uses less focuser travel. However, a short focal length Barlow with its large diverging light cone means that the outer rays diverge far enough to miss the edge of a typical eyepiece field lens diameter, so the Barlow appears to cause vignetting. It is in fact the result of too small a field lens in the eyepiece, not some stopped down effect of the Barlow itself.

A 4 element Barlow uses a positive achromat lens near the final focal plane to bring the divergent rays back to parallel so that the field lens of a typical eyepiece can accept the ray bundle without vignetting. The 2 element Astro-Physics Barcon achieves a similar goal by having a long focal length (-5" or -127mm) to minimize the divergence of the outer ray bundle. Most eyepieces can tolerate a small divergence, but begin to show heavy vignetting with short FL Barlows. Plossls are the worst in this regard because they actually have field lenses that are too small even without a Barlow's divergent ray bundle.

Roland Christen.

Attached Image:

RolandC's attachment for post 95760