12 mm Delos or 12.5 mm Morpheus

[Louis D]

6 hours ago, michael.h.f.wilkinson said:

it focused at the same point as my XWs, something the Delos 14 won't do

Yep, you'll need to invest in a Tele Vue In-Travel Adapter which will run you about another £43 or so to parfocalize it somewhat.

[michael.h.f.wilkinson]

17 minutes ago, Louis D said:

Yep, you'll need to invest in a Tele Vue In-Travel Adapter which will run you about another £43 or so to parfocalize it somewhat.

Not really necessary, I feel. The 14mm is part of my planetary and smaller DSO line-up, together with the T4 Naglers, whereas the shorter Delos and the XWs are my planetary bunch. In the latter, parfocality is far more important, as you want to adapt quickly to changing seeing conditions. For DSOs that is not as important

[John]

The shorter Delos are about 8mm from being par-focal with the XW's aren't they ?

I went for the 17.3 and 14 Delos because they reach focus pretty close (but not exactly) to where the XW's do because the two types play together in my 1.25 inch set.

I need parfocal rings for my Nagler 2-4 zoom and Panoptic 24 though which normally are part of the same par-focal group as the shorter Delos's.

 

[michael.h.f.wilkinson]

3 minutes ago, John said:

The shorter Delos are about 8mm from being par-focal with the XW's aren't they ?

I went for the 17.3 and 14 Delos because they reach focus pretty close (but not exactly) to where the XW's do because the two types play together in my 1.25 inch set.

I need parfocal rings for my Nagler 2-4 zoom and Panoptic 24 though which normally are part of the same par-focal group as the shorter Delos's.

 

The shorter focus 0.25" further out to be exact, whereas the 14mm focuses 0.25" further inwards, compared to the XWs.

[John]

10 minutes ago, michael.h.f.wilkinson said:

The shorter focus 0.25" further out to be exact, whereas the 14mm focuses 0.25" further inwards, compared to the XWs.

The 14mm and 17.3mm seem closer than that to my XW's

Not that it matters too much though, with those focal lengths.

[Louis D]

Agreed, most Pentax, Morpheus, and ES (JOC) eyepieces focus very close to the shoulder.  All Tele Vue eyepieces focus further in or out relative to the shoulder except for the 17mm NT4 which also focuses right at the shoulder.  I really notice swapping in my 10mm Delos because I have to rack it 1/4" out to reach focus.  The 12mm NT4 was so annoying at being 20mm below the shoulder that I added five 4mm thick O-rings and a 20mm 2" extension ring to the bottom to nearly parfocalize it.

Edited August 3, 2020 by Louis D

[steveex2003]

10 hours ago, Louis D said:

Agreed, most Pentax, Morpheus, and ES (JOC) eyepieces focus very close to the shoulder.  All Tele Vue eyepieces focus further in or out relative to the shoulder except for the 17mm NT4 which also focuses right at the shoulder.  I really notice swapping in my 10mm Delos because I have to rack it 1/4" out to reach focus.  The 12mm NT4 was so annoying at being 20mm below the shoulder that I added five 4mm thick O-rings and a 20mm 2" extension ring to the bottom to nearly parfocalize it.

Apologies for being dim. What does the term; close to the shoulder mean?
Thanks.

[John]

1 hour ago, steveex2003 said:

Apologies for being dim. What does the term; close to the shoulder mean?
Thanks.

The shoulder of an eyepiece is where the (usually) black body section meets the 1.25 inch or 2 inch barrel. It usually is where the eyepiece rests when it is inserted into a diagonal or focuser drawtube.

Field stop position is usually expressed in terms of mm above or below the shoulder.

Tele Vue have a useful page of data on their eyepieces which includes a diagram to explain the various measurements. They refer to the shoulder as the "reference surface":

http://www.televue.com/engine/TV3b_page.asp?id=214&plain=TRUE

[steveex2003]

Thank you. 

[Louis D]

5 hours ago, steveex2003 said:

Apologies for being dim. What does the term; close to the shoulder mean?
Thanks.

Sorry about that.  How about we hold a contest for a more intuitive name for the "reference surface" than either that or "the shoulder"?  Neither is very good I'll admit.

[Don Pensack]

How about "Eyepiece Rest"?

Nope, too graveyard.

OK, how about "Eyepiece Waist"?

Given how porky some eyepieces are, it might be apropos.

How about "Eyepiece reference surface"?

That might need some explanation, however.

What about "Eyepiece barrel stop"?

 

[steveex2003]

Once understood, I quite like Reference Surface. It has a Datum Level ring of no nonsense about it.

Edited August 4, 2020 by steveex2003

[Louis D]

How about "Insertion Barrel Termination Plane"?

Or "The point or plane of the eyepiece where the part you stick in the focuser suddenly gets wider and stops the eyepiece from going any further in" to be totally long-winded? 🤪

[John]

On my SLR and now DSLR cameras position of the the focal plane of the camera is marked on the body with a symbol:

I wonder if it would be useful to have the field stop position (which has to be at the focal plane of the eyepiece) marked similarly on the body or barrel of the eyepiece ?

 

 

 

[Louis D]

1 hour ago, John said:

On my SLR and now DSLR cameras position of the the focal plane of the camera is marked on the body with a symbol:

I wonder if it would be useful to have the field stop position (which has to be at the focal plane of the eyepiece) marked similarly on the body or barrel of the eyepiece ?

It might be useful.  In photography, one use is for panoramic stitching.  You need to pivot directly below the focal plane to avoid stitching issues with near/far objects.

For eyepieces, such a marking would need to be at the virtual field stop for eyepieces with lens elements between the objective and the physical field stop because those lenses generally move the focus position relative to the physical field stop just as a Barlow or focal reducer would.  For example, most positive-negative eyepiece designs have the physical field stop located above the focal plane (virtual field stop).

Another issue would be that it would have to be marked on the insertion barrel for eyepieces with a focal plane below the "reference surface".  A stamped marking on chrome or printing on Morpheus style kerfs might be difficult to read.

[Don Pensack]

If the manufacturers just told us it would help, like Baader, TeleVue, Explore Scientific (though recently I discovered some of the ES data is in error), Pentax.