Which 2.5mm?

[Sunshine]

Just finished looking through eyepiece choices online for a good 2.5mm with as wide a field as possible considering I mostly use manual equatorial or Alt-az mounts. To my surprise I found one!  I repeat, one…the Tak TOE but with a FOV of 52 degrees and a price of $500 I was quickly turned off. Am I missing something? surely there are more choices in the 2.5mm range, I have no problem justifying the money but not for 52 degrees, not for a super sharp straw. Some may ask if I have a barlow which I do, a 2.5x Powermate but I’d rather not use it for visual as I hate fumbling eyepieces into Powermate and he whole appendage into the diagonal to end up with a foot long trunk sticking out of my diagonal which to make things worse, throws my alt az out of balance, suddenly. 
 

Anyone know of a 2.5mm fitting my needs? Thanks.

Edited September 22, 2022 by Sunshine

[Roy Challen]

Not wide field but much cheaper than the TOE : Vixen SLV 2.5mm

[Mr Spock]

I use an SLV 2.5mm. Very high quality image but not wide field.

[Stu]

There used to be a 2.5mm type 7 Nagler ie 82 degree afov. Very nice eyepiece, had one for a while. Picking one up used would be the only possibility now though I think.

[markse68]

Type 1 4.8mm nagler barlowed? It’s very sharp and wide but only about the size of a 20mm plossl. If you don’t mind tight eye relief… I’ve not tried mine barlowed before but will give it a try next time. 

Mark

[NGC 1502]

Ok, you’ve said you don’t like a barlow, and I do understand your objections.

However, everything is a compromise in one way or another.  I do have an original Vixen LV 2.5mm with 45 deg apparent field, the modern replacement SLV has a slightly wider 50 deg field.

If it were me I’d accept a compromise, use a barlow or live with a narrow field.

Long shot- a used 2.5mm Nagler. But that’s a compromise too with 12mm eye relief, or a used 2-4mm Nagler zoom, or…whatever…

Ed.

[Sunshine]

1 hour ago, NGC 1502 said:

Ok, you’ve said you don’t like a barlow, and I do understand your objections.

However, everything is a compromise in one way or another.  I do have an original Vixen LV 2.5mm with 45 deg apparent field, the modern replacement SLV has a slightly wider 50 deg field.

If it were me I’d accept a compromise, use a barlow or live with a narrow field.

Long shot- a used 2.5mm Nagler. But that’s a compromise too with 12mm eye relief, or a used 2-4mm Nagler zoom, or…whatever…

Ed.

In that case I’d rather stick with my 5mm XW with my 2.5 Powermate as it maintain the 70 degrees of the XW, as much as I dislike a foot long appendage on the diagonal. It is more power than a prime 2.5 but I don’t think they make a 2x Powermate which would be closer to what a 2.5 prime would give.

Edited September 22, 2022 by Sunshine

[Ricochet]

1 hour ago, Sunshine said:

In that case I’d rather stick with my 5mm XW with my 2.5 Powermate as it maintain the 70 degrees of the XW, as much as I dislike a foot long appendage on the diagonal. It is more power than a prime 2.5 but I don’t think they make a 2x Powermate which would be closer to what a 2.5 prime would give.

There is a 1.25" 2x ES telextender but the transmission won't be as good as the powermate and you also need to flock between the lens groups to avoid reflections. 

[Stu]

1 hour ago, Sunshine said:

In that case I’d rather stick with my 5mm XW with my 2.5 Powermate as it maintain the 70 degrees of the XW, as much as I dislike a foot long appendage on the diagonal. It is more power than a prime 2.5 but I don’t think they make a 2x Powermate which would be closer to what a 2.5 prime would give.

There is a 2” x2 Powermate…..

[markse68]

what advantage does a power mate give over a barlow at short fl?

 

[Louis D]

I've got a 2.5mm Planetary TMB II Eyepiece on order from China via ebay.  For $35+tax I thought it would be worth a try.  Once I get it, I'll compare it to my 3.5mm Pentax XW and my 5.2mm Pentax XL Barlowed.  It might end up being a project eyepiece where I have to blacken or flock the interior to control stray light if the optics pan out.  If the optics are bad, I'll do an exposé on it here on SGL and write it off as a learning experience.  If I like it, I might get the 4mm version as well.

[Stu]

18 minutes ago, markse68 said:

what advantage does a power mate give over a barlow at short fl?

 

Main benefit would be that it doesn’t affect the eye relief by pushing it out further. That can be useful if barlowing short f/l length orthos and Plossls, but with an XW that has 20mm already it could be a pain as it might get harder with eye placement.

[Deadlake]

I get very good performance when using a UFF 10 mm barlowed, compared with a TOE 4 mm. The Barlow is placed ahead of the diagonal.
However the stack does become a little long,  but the contrast is worth it…

[Sunshine]

9 hours ago, Stu said:

There is a 2” x2 Powermate…..

Thanks for the heads up, unfortunately my diagonal is 1.25”, that one looks like a 2” 

[Paz]

I would add to the posts mentioning the 2.5mm SLV which I think is good.

[Louis D]

Well, I had a quick first light with the  2.5mm Planetary TMB II eyepiece in my AT72ED.  Here are my recollections:

• Its field of view appears to be flat, sharp, and color free edge to edge after looking at Luna at nearly full phase, Jupiter, and Saturn.
• I didn't notice any imparted color tone on Luna.
• Craters, NEB/SEB, and rings all looked clean and well defined.
• Stars and Galilean moons appeared pinpoint edge to edge with no refocusing.
• I didn't see any flaring, glare, scatter, ghosting, etc.
• I didn't see any indication of bright objects just outside the field of view casting light into the FOV.
• There is no thin blue line at the field stop nor any false color on the limb of Luna.
• There is just barely enough eye relief to take in the entire FOV with my eyeglasses pressed to the retracted and folded eyecup, so I'd estimate about 15mm of ER.  However, at that tiny exit pupil, my astigmatism was a non-issue without eyeglasses.
• The AFOV appears to be on the order of 60 degrees.  I'll have to run it through my testing measurements to get an exact number.
• I did not try to check for edge astigmatism on a bright star because none were handy.  However, it would seem to have very low levels if it is present.
• The eye cup tube twists up and down smoothly through maybe 1cm of travel.
• The eye cup itself flips up and down and seems just the right stiffness to stay in place.
• I didn't notice any kidney beaning (SAEP), ring of fire (CAEP), general blackouts, or edge of field brightening (EOFB).
• It is labelled AQUILA, not TMB, as in the stock image below.  I'll take and upload images of my eyepiece when I get the chance.
• The matte black color, rubber grips, and chrome bands make for a handsome looking eyepiece.
• The tapered undercut is very shallow and caused zero issues inserting or removing the eyepiece into/from a compression ring eyepiece holder.
• I saw no debris in the FOV on the face of Luna.

Overall, a very impressive debut by a $35 eyepiece.  So far, I highly recommend it.  I'll have to do eyepiece/Barlow comparisons sometime when I have more time, and I'm less exhausted from a busy Saturday.

Aquila brand:

instead of TMB Optical

which is probably best since I'm sure TMB's estate isn't getting any royalties on the sale of these eyepieces.

Edited October 9, 2022 by Louis D

[Orlando Gonzalez]

On 22/09/2022 at 01:38, Stu said:

There used to be a 2.5mm type 7 Nagler ie 82 degree afov. Very nice eyepiece, had one for a while. Picking one up used would be the only possibility now though I think.

I recently located one in mint condition on Astromart. A wonderful eyepiece. It's a type 6 Nagler.

Edited October 9, 2022 by Orlando Gonzalez

[Louis D]

Quick test report on the Aquila (TMB) Planetary II SW 2.5mm:

Measured actual apparent FOV (AFOV): 58° by flashlight method, 57.2° photographically

Measured fieldstop via photography: 2.5mm

Calculated effective AFOV (eAFOV) based on above measurements: 57.5°

Measured usable eye relief: 13mm

Eye lens diameter: 20mm

Measured axial focal length via photography: 2.5mm

Measured radial edge focal length via photography: 2.2mm

Edge magnification distortion: 13.9%

Weight: 162g (5.7oz)

With eyecup folded down and fully screwed down: 92mm long x 45mm wide

Overall, it lives up to its posted specifications very well.

The image appears sharp across the field without noticeable vignetting.

Eye relief is a bit tight for eyeglass wearers, but you can still see a good portion (~90%) of the FOV without pressing in too much.

The field stop is nice and sharp.  However, due to extreme magnification, you can tell the edge is bit ragged due to machining limitations.

I'm really reaching the resolving limit of my 72ED scope.  However, the image held up fairly well.

I've posted an updated 2.5mm to 6mm eyepiece AFOV photo collection including this eyepiece in the following thread:

[Ags]

On 09/10/2022 at 05:10, Louis D said:

Overall, a very impressive debut by a $35 eyepiece.  So far, I highly recommend it.

I have fond recollections of a 9 and 6 mm TS Planetary HR - same eyepiece, different branding. I thought they were both rather good, particularly the relaxed viewing experience.

[Stu]

On 09/10/2022 at 04:21, Orlando Gonzalez said:

I recently located one in mint condition on Astromart. A wonderful eyepiece. It's a type 6 Nagler.

Well done! That should be great. Oops, typo there, quite right it’s a type 6 👍👍

[Don Pensack]

17 hours ago, Louis D said:

Quick test report on the Aquila (TMB) Planetary II SW 2.5mm:

Measured actual apparent FOV (AFOV): 58° by flashlight method, 57.2° photographically

Measured fieldstop via photography: 2.5mm

Calculated effective AFOV (eAFOV) based on above measurements: 57.5°

Measured usable eye relief: 13mm

Eye lens diameter: 20mm

Measured axial focal length via photography: 2.5mm

Measured radial edge focal length via photography: 2.2mm

Edge magnification distortion: 13.9%

Weight: 162g (5.7oz)

With eyecup folded down and fully screwed down: 92mm long x 45mm wide

Overall, it lives up to its posted specifications very well.

The image appears sharp across the field without noticeable vignetting.

Eye relief is a bit tight for eyeglass wearers, but you can still see a good portion (~90%) of the FOV without pressing in too much.

The field stop is nice and sharp.  However, due to extreme magnification, you can tell the edge is bit ragged due to machining limitations.

I'm really reaching the resolving limit of my 72ED scope.  However, the image held up fairly well.

I've posted an updated 2.5mm to 6mm eyepiece AFOV photo collection including this eyepiece in the following thread:

That's in keeping with the old rule that field stop = focal length at 57.296° apparent field (1 radian).

That makes an easy way to calculate field stop information not provided by the mfr in eyepieces from 45-70°:

(Apparent field / 57.296) x focal length = field stop.  Or, FS = (AF/57.296) x FL.

For a 58° eyepiece in 2.5mm, field stop would be 2.5mm, just what you measured.

 

How well does that work?

11mm Delite, 62°, field stop 11.7mm

Calculated field stop = (62/57.296) x 11 = 11.9mm

That is ~0.5' of field in a 1200mm scope, an inconsequential difference.

 

 

[Louis D]

I noticed that FS=FL relationship, but I couldn't quite put my finger on the reason.  I simply plug my measurements into my spreadsheet that has linear curve fitting derived coefficients from my 70+ measurements.

[badhex]

2 hours ago, Don Pensack said:

That makes an easy way to calculate field stop information not provided by the mfr in eyepieces from 45-70°

Don, can you explain why the formula only works within this AFOV range? Is there an adjustment or different formula for larger AFOV EPs? 

[Don Pensack]

It can be applied to all apparent fields, but its accuracy depends on the nature of the distortion in the eyepiece.

For instance, an eyepiece with significant positive rectilinear distortion (pincushion) will have an apparent field that is larger than its field stop would indicate,

i.e. the apparent field would imply a larger field stop that the one actually in the eyepiece.  Example 24mm Panoptic: calculated field stop 28.5mm, actual 27.0

An eyepiece with significant negative rectilinear distortion (barrel) will have an apparent field that is smaller  than its field stop would indicate,

i.e. the apparent field would imply a smaller field stop than the one actually in the eyepiece.  Example 12.5mm Noblex: calculated field stop 18.3mm, actual 19.2mm

The calculated field stop depends on a low distortion level in the eyepiece, and since distortion increases with apparent field, the accuracy of the calculation diminishes with wider fields.

If you knew the % of distortion, and its type, at the edge of the field, you could apply a factor to the formula and calculate a figure closer to the actual eyepiece.

 

It's actually more complicated than that, but that is a simplified explanation of why a field stop formula based on apparent field doesn't work perfectly.

We can know the field stop accurately, and we can measure the apparent field accurately, but one does not necessarily follow the other.

[badhex]

30 minutes ago, Don Pensack said:

It can be applied to all apparent fields, but its accuracy depends on the nature of the distortion in the eyepiece.

For instance, an eyepiece with significant positive rectilinear distortion (pincushion) will have an apparent field that is larger than its field stop would indicate,

i.e. the apparent field would imply a larger field stop that the one actually in the eyepiece.  Example 24mm Panoptic: calculated field stop 28.5mm, actual 27.0

An eyepiece with significant negative rectilinear distortion (barrel) will have an apparent field that is smaller  than its field stop would indicate,

i.e. the apparent field would imply a smaller field stop than the one actually in the eyepiece.  Example 12.5mm Noblex: calculated field stop 18.3mm, actual 19.2mm

The calculated field stop depends on a low distortion level in the eyepiece, and since distortion increases with apparent field, the accuracy of the calculation diminishes with wider fields.

If you knew the % of distortion, and its type, at the edge of the field, you could apply a factor to the formula and calculate a figure closer to the actual eyepiece.

 

It's actually more complicated than that, but that is a simplified explanation of why a field stop formula based on apparent field doesn't work perfectly.

We can know the field stop accurately, and we can measure the apparent field accurately, but one does not necessarily follow the other.

Great explanation Don, thanks.