Explore Scientific 28mm AR 68 degree ep

[Carl Au]

Does anyone have any experience of these eyepieces? Can anyone compare them to the APM UFF 30mm

Edited March 11, 2022 by Carl Au
Spelling mistake

[Louis D]

I did a write-up here comparing the 30mm APM UFF to the Tele Vue equivalent 27mm Panoptic along with the 30mm ES-82 and a 30mm generic 80 degree Wide Scan III clone.  Reportedly, the 28mm ES-68 has better eye relief than the Panoptic, but has poorer edge correction beyond the 80% point.  I do have the 40mm Meade 5000 SWA which is the same as the 40mm ES-68, except for eye relief, and really like it a lot.  I actually prefer it to my 40mm Pentax XW except its ergonomics  being broadly flat topped.

[Carl Au]

Thank you, I will take a look. I have been waiting for 2 months for a APM UFF so far and I don’t hold out much luck for the next month either. I was thinking about changing my order for the ES. What do we think 🤔 

[Louis D]

There's also the optically-same Meade UHD, Celestron Ultima Edge, Tecnosky Ultraflat, and Altair Ultra Flat versions.  Perhaps one of them might be in stock.

[Carl Au]

The Altair Version is, I might ask for a refund and go with Altair 

[Carl Au]

On 11/03/2022 at 19:59, Louis D said:

There's also the optically-same Meade UHD, Celestron Ultima Edge, Tecnosky Ultraflat, and Altair Ultra Flat versions.  Perhaps one of them might be in stock.

Are you sure they are the same, the Altair version only has 8 elements not the 9 APM claim to have. The Tecnosky also claims to be 75 degrees and not 70. While the Meade has 9 elements and is 70 degrees. I honestly don’t know what to think anymore, all very confusing 

Edited March 12, 2022 by Carl Au

[Don Pensack]

44 minutes ago, Carl Au said:

Are you sure they are the same, the Altair version only has 8 elements not the 9 APM claim to have. The Tecnosky also claims to be 75 degrees and not 70. I don’t know what to think anymore, all very confusing 

I have no idea where people get their information.

Prior to an eyepiece's arrival the "specs" are all over the place.

The 30mm Ultra Flat Field from KunMing United optics is a 9 element design and 70.1° apparent field (actual measurement), and is now sold to 8 different companies.

 

Altair has a cross-sectional diagram:

https://www.altairastro.com/altair-30mm-ultraflat-eyepiece---precision-barrel-stainless-steel-238-p.asp

It shows 9 elements, which is correct.  If their website page for the eyepiece shows 8 elements, it is a simple typo.

 

Note: the field stop diameters listed on the chart are the iris measurements in the eyepieces and do NOT apply to a calculation of true fields because that is not where the focal plane in the eyepieces are.

 

The field size dimension in the 30mm UFF is 36.4mm, which is the number you can use for field size calculation.  Likewise, the 24mm has a 27.3mm field diameter, regardless of the field stop shown in the diagram.

In negative/positive eyepiece designs, the field size is NOT the same as the internal field stop dimension, so if you take such an eyepiece apart and measure the iris dimension, you are not measuring the field diameter that relates to true field.

 

If you want to look at any manufacturer's field stop claim for rationality, try this:

[Eyepiece Apparent Field / 57.296] X eyepiece focal length = field stop diameter  or [AF/57.296]*FL

It does assume the manufacturer is not lying or mistaken about the apparent field, and it can be off by a couple tenths of a mm.

The 30mm APM, at 70.1° translates to a calculated field stop of 36.7mm, but the actual measurement is 36.3-36.4, so you can see the formula can be off a bit.

However, the chart from Altair shows a 30.4mm field stop.  The field diameter calculation shows this cannot be the case, which is where the utility of the field stop calculation can show how plausible the figures are.

In this case, the manufacturer does not specify the field diameters we can use for true field calculations, so we have to measure the eyepieces with a star timing or some other method that is consistent.

 

 

 

 

[Carl Au]

Well thank you for taking the time to answer that question so fully, much appreciated.  I am a fan of KUO, I own one of their rather wonderful 4 inch refractors. Which ever version I end up with I am sure it will go well in my scope. Seems to have the edge over the ES 28 mm 2 inch