70° 27mm 1.25" eyepiece

[Ags]

Saw this thing on aliexpress - not sure 27mm 70 degrees is possible but who can resist a yellow eyepiece?

€ 48,35  29%OFF | Astronomical telescope eyepiece 70° 27mm Super wide angle High magnification achromatic eyepieces 1.25 inch all metal
https://a.aliexpress.com/_mrLm022

 

[cajen2]

High magnification! 🤣🤣🤣 In what?

[Ags]

Well, with the optional 5x barlow 😀

Or maybe it is high magnification versus the sadly nonexistent 36mm 52° plossl that shows the same true field.

Edited December 21, 2022 by Ags

[Don Pensack]

Sorry to burst your bubble, but that 27mm focal length has a 53° apparent field in all 8 labels (maybe more) it's sold under.

The whole line is also sold in colors if you look up "AngelEyes".

https://www.aliexpress.us/item/2251832657095693.html?spm=a2g0o.productlist.main.3.74a32ed8hivC3Y&algo_pvid=9c214683-03aa-4896-a23e-04b5f0916fb0&algo_exp_id=9c214683-03aa-4896-a23e-04b5f0916fb0-1&pdp_ext_f={"sku_id"%3A"65093639638"}&pdp_npi=2%40dis!USD!69.9!48.93!!!!!%4021021d7b16716629313185371d0793!65093639638!sea&curPageLogUid=RhVSeCCVfg2F

8, 12, and 16 are 60°.  The 19mm is 65°.  And the 27mm is 53°.  When dealing directly with China, beware false advertising.  There's a lot of it.

Likewise, a 36mm 1.25" Plössl would have at most a 47° field, and likely smaller.

[Ags]

53° sounds a lot more credible but also a lot less interesting...

[Louis D]

I wonder if the 28mm Pentax XL which has a 55° AFOV has more distortion to account for its wider AFOV.  Has anyone ever done a shootout between these two to see if they have the same TFOV?

[Louis D]

15 hours ago, cajen2 said:

High magnification! 🤣🤣🤣 In what?

In a 14" Celestron SCT?  Given its 3910mm focal length, a 27mm eyepiece would yield 145x.  Not super high power, but decently high power.

[Don Pensack]

4 hours ago, Louis D said:

I wonder if the 28mm Pentax XL which has a 55° AFOV has more distortion to account for its wider AFOV.  Has anyone ever done a shootout between these two to see if they have the same TFOV?

That's true.  Distortion can make a difference.  But are you sure the XL28 was really 55°?

[Louis D]

9 hours ago, Don Pensack said:

That's true.  Distortion can make a difference.  But are you sure the XL28 was really 55°?

Based on my measurements of my Pentax XL, XW, and XF eyepieces (6 in total), Pentax has always been dead on accurate with their quoted AFOV numbers.  Thus, I have no reason to doubt their number for the 28mm XL.

[Don Pensack]

Well, not quite dead on:

XW:

40  68°

30  68°

20  70°

14  69.4°

10  68.4°

7  69.3°

5  69.3°

3.5  68.5°

XL

40  63.3°

21  61°

 

The point is that without some serious pincushion distortion (not Pentax' general rule), a 28mm is more likely to be ~52° in a 1.25" barrel.

[Highburymark]

Think Lunt sells a version of these. And probably many other brands.

[Don Pensack]

5 hours ago, Highburymark said:

Think Lunt sells a version of these. And probably many other brands.

Many brands

Astromania

BST (Barsta)

Lunt

Omegon

Orion

OVL (First Light Optics)

Skywatcher

Telescope Service

[Louis D]

On 23/12/2022 at 08:57, Don Pensack said:

Well, not quite dead on:

XW:

40  68°

30  68°

20  70°

14  69.4°

10  68.4°

7  69.3°

5  69.3°

3.5  68.5°

XL

40  63.3°

21  61°

 

The point is that without some serious pincushion distortion (not Pentax' general rule), a 28mm is more likely to be ~52° in a 1.25" barrel.

Well, my AFOV measurements via flashlight method and photographic method are below:

3.5XW : 70° and 70.2°

5.2XL : 66° and 64.8°

7XW : 70° and 70.0°

12XF : 60° and 61.1°

14XL : 64° and 64.9°

40XW-R : 70° and 69.7°

What methods did you use to measure your values?

Photographically, the more accurate and precise method, these are all range between 0° and 1.1° off, with 0.3° being the average including the XF and 0.16° excluding it.  I'm sticking with Pentax providing bang-on values for XL and XW eyepieces to within 0.24% error using 68° for the denominator.  That would make the 55° claim for the 28XL to be somewhere between 54.9° and 55.1°.

[vlaiv]

I only had opportunity to use one eyepiece labeled AngelEyes - it is 7-21mm zoom and it is a horror show of an eyepiece, so be warned

 

[Marvin Jenkins]

I personally have never used one but just judging from the name Angel eyes is enough for me to run away screaming.

[Don Pensack]

2 hours ago, Louis D said:

Well, my AFOV measurements via flashlight method and photographic method are below:

3.5XW : 70° and 70.2°

5.2XL : 66° and 64.8°

7XW : 70° and 70.0°

12XF : 60° and 61.1°

14XL : 64° and 64.9°

40XW-R : 70° and 69.7°

What methods did you use to measure your values?

Photographically, the more accurate and precise method, these are all range between 0° and 1.1° off, with 0.3° being the average including the XF and 0.16° excluding it.  I'm sticking with Pentax providing bang-on values for XL and XW eyepieces to within 0.24% error using 68° for the denominator.  That would make the 55° claim for the 28XL to be somewhere between 54.9° and 55.1°.

http://astro-talks.ru/forum/viewtopic.php?f=32&t=1483#p41976

Go down the page to the second chart, which has hyperlinks to each individual review.

Personally, I use the flashlight test to measure apparent fields, but the best I can do is +/- 0.2°.

Ernest is a designer, so likely has more sophisticated instruments to measure the apparent field, like:

http://www.southastrodel.com/Page209.htm#:~:text=A dynameter is an astronomical,particular eyepiece through a telescope

https://adsabs.harvard.edu/full/1970JBAA...81...24C

https://trioptics.com/us/products/optitest-visual-measurement-instruments/

https://www.cloudynights.com/topic/574401-an-easy-way-to-measure-apparent-field-of-view/

S&T uses a pivoting magnifier similar to a theodolite that can measure apparent field to <0.1°

[Louis D]

2 hours ago, Don Pensack said:

S&T uses a pivoting magnifier similar to a theodolite that can measure apparent field to <0.1°

Unless the pivot point is exactly at the exit pupil, which itself can be poorly defined due to SAEP (see 26mm Meade MWA), rotating the measurement device can make the AFOV appear larger than it appears from the exit pupil point with eyepieces that either lack a field stop or have it misplaced relative to the eyepiece focal plane.  I see this with Rini eyepieces all the time.  I can "look under" the edge if I back away from the exit pupil and roll my gaze over to the opposite side above the eyepiece.  With flashlight projection, the projected circle has a gradient field stop instead of a sharp field stop, again making it difficult to determine the AFOV.  Below is a composite showing three Rini eyepieces, none of which have a physical field stop.  Note how fuzzy they get near the edge even photographically making it difficult to judge what the AFOV is:

My point is, some eyepieces can defy even the best measurement efforts.

I also like my method for eyepieces plagued by SAEP like the aforementioned Meade MWA 26mm.  I back off the camera to where SAEP is not much of an issue and take an image and go from there to figure out the "easy to view" AFOV.  I suppose it's the same point as the top of the cylinder of uncertainly for its exit pupil.  If the rotating device could pivot there, it should be able to get an accurate AFOV, plus or minus the effect of a field stop defined by SAEP rather than a physical ring.

[Don Pensack]

I've discovered a lot of eyepieces have internal vignetting, which can make the field stop somewhat vague in the flashlight test.

It's usually pretty easy to see where light goes to zero, though.

The more difficult part is determining where the exit pupil is because if it's large and there is SAEP, its position becomes quite vague.

Though, this doesn't make a large difference in the calculation of apparent field if the circle measured is at least a meter from the scope.

I discovered that this measurement can be improved by stopping the scope down, making the f/ratio longer and the exit pupil much smaller.

It is pretty easy to determine the exit pupil position when the exit pupil is small.

 

Some day when I have time, I'll do this with a bunch of eyepieces to determine the accuracy of their eye relief figures.  At this point, I don't have enough data

to determine how accurate that figure is for most eyepieces.

I propose an index card with mm markings on it that can be held up to the eyepiece to measure the effective eye relief, added to a measurement of the lens depth.

The advantage would be a determination of both design and effective eye reliefs at the same time.