Which 2.5mm?

[Louis D]

That's why I measure each independently via photographing the AFOV and TFOV simultaneously with the rulers.

[Louis D]

I finally compared my new 2.5mm TMB Planetary clone to my 3.5mm Pentax XW and 2x Barlowed 5.2mm Pentax XL on the moon last weekend.  I used my sharpest Barlow, a 1990s 1.25" Orion Deluxe 2x from Japan in my 6" f/5 Newtonian.  It's about 6 inches long, so it very gently diverges the light cone limiting aberrations.  That, and it has at least one baffle to limit stray light.

Sorry, I forgot to mention I was using a new to me GSO 6" f/5 Newtonian with their dual speed focuser.  It was mounted on my DSV-2B.  I kept the coma corrector out of the optical train to limit introduced spherical aberration on axis.

The views were probably best with the 5.2mm XL by itself, followed by the 3.5mm XW just due to the ever tinier exit pupils involved.  However, at about 2.5mm, the Barlowed Pentax XL was a bit sharper and contrastier, but not by much.  The bigger difference was the 5.2mm XL's wider AFOV and much longer eye relief that made using it even without eyeglasses much more enjoyable and relaxing than using the TMB clone.  Of course, the Barlowed XL was way fatter, longer, heavier, and more expensive than the TMB clone.

For the money, the 2.5mm TMB is a really good deal for a rarely used focal length.  I seriously doubt I'll use it much going forward, but it made for an interesting learning experience.  It reinforced my view that 1mm is about the smallest exit pupil I can comfortably use for any significant length of time.

Edited November 8, 2022 by Louis D
Added telescope used

[NGC 1502]

Interesting comparison 👍……..as you say, a rarely used focal length. I occasionally use my Vixen LV 2.5mm on tight double stars in rare stable atmosphere.  I love seeing the airy disc and diffraction rings, and of course it’s a delight when you finally split that double or triple after many tries!!

[F15Rules]

I don't know what scope you're using..would 3.2mm get you close enough? ( and using a 1.3x Barlow nosepiece such as the Baader 2.25x Q Turret one would take 3.2mm down to 2.46x).

If so, the original Burgess TMB Planetary 3.2mm could work for you. A nice, light, well made eyepiece and the nosepiece mentioned above would only add 1" or less to your "stack".

I have a 5mm and it's a nice, sharp eyepiece.

There's a 3.2mm for sale on UKABS..

https://www.astrobuysell.com/uk/propview.php?view=192782

Dave

[JeremyS]

I’ve always found my Pentax XL 5.2 a super eyepiece with my FS 102, @Louis D. But I rarely use it these days. I should do.
Maybe we could generate a cult following of the XL 5.2 🤔

[F15Rules]

1 hour ago, JeremyS said:

I’ve always found my Pentax XL 5.2 a super eyepiece with my FS 102, @Louis D. But I rarely use it these days. I should do.
Maybe we could generate a cult following of the XL 5.2 🤔

I'm in..even though I don't have one now..I do have a 10.5mm XL in great condition that Barlows really well with my Baader 2.25x barlow down to 4.66mm🙂

Dave

[Louis D]

3 hours ago, JeremyS said:

I’ve always found my Pentax XL 5.2 a super eyepiece with my FS 102, @Louis D. But I rarely use it these days. I should do.
Maybe we could generate a cult following of the XL 5.2 🤔

I know 25585 (Richard?) really likes his as well.  He's only active over on CN now.

[NGC 1502]

4 hours ago, Louis D said:

I know 25585 (Richard?) really likes his as well.  He's only active over on CN now.

indeed 25585 is Richard, lives in the UK.  I’ve bought items from him, he’s reliable, items accurately described and posted promptly.

[Second Time Around]

1 hour ago, NGC 1502 said:

indeed 25585 is Richard, lives in the UK.  I’ve bought items from him, he’s reliable, items accurately described and posted promptly.

Ditto.