Do your eyepieces or filters hit your diagonal mirrors and prisms?

[Jim L]

Continuing the astronomy gear theme of never standardizing anything, neither eyepiece nose length, Barlow nose length, filter length, nor diagonal depth can be counted upon to protect your diagonal’s mirror or prism from damage if your eyepiece or filter hits the diagonal reflective surface when inserted. Further, while some diagonals protect themselves with an internal excursion lip many, or even most, do not.

A look at one of my Takahashi diagonals next to a Nagler zoom, followed with a couple of quick measurements with a plastic caliper, made me realize there’s a problem, and further investigation showed it wasn’t limited to the aforementioned items and was actually quite common. Fortunately, diagonal strikes are easily prevented with a few simple calculations and readily available Astro accessories.

I took a systematic approach by first recording whether each of my diagonals had a functional protective lip and, if not, the distance from the top of the eyepiece holder to the diagonal mirror or prism.

I next recorded all of my eyepiece and Barlow barrel lengths to determine which ones would contact the diagonal reflective surfaces.

If you install filters at the ends of your eyepieces or Barlows you’ll want to measure them and include them in your calculations as well.

A simple spreadsheet later and I had a quick and accurate means of identifying eyepieces that weren’t a problem under any circumstances (blue font), those that were a problem with filters installed (magenta), and those that were always a problem in unprotected diagonals (red). If your numbers don’t quite match with mine, it’s probably that I added a “safety factor” to most of my measurements to err on the safe side.

The worst offenders were made safe with parafocal rings, readily available from FLO in the UK and Europe, and Agena Astro and even Amazon here in the States.

To make the grub screws that secure the parafocal rings on your precious eyepieces and Barlows less bitey you can remove the sharp tip surface with a sharpening stone and a drop of oil. Doing so makes a tremendous difference and I blunt all of my grub screws on all of my telescope gear.

By checking combinations of eyepieces, Barlows, filters and diagonals it’s simple enough to prevent strikes on your diagonals by taking precautions before your observing sessions. For example, if I’m using eyepieces that can strike a diagonal with a filter installed, I install the filter on the diagonal instead of the eyepiece. Takahashi diagonals are a special case since their nosepieces do not appear to be threaded to accept filters, but other remedies are possible. Good luck and happy observing.

 

 

 

 

Edited December 1, 2023 by Jim L

[Roy Challen]

I've never noticed this before with my eyepiece and diagonal combos until I got my SvBony zoom. On unpacking it, I thought the barrel looked longer than average, and sure enough, it does contact the mirror in my Tal diagonal. Lifting it up a few mm solves that problem but a parfocal ring would be better.

[John]

I've been aware that this could happen and a couple of my 1.25 inch eyepieces do have longer than average 1.25 inch barrels. Being aware of the potential the only 1.25 inch diagonal that I use (a Baader T2 zeiss prism) has a T2 15mm extension below the eyepiece holder so eliminating the possibility that an eyepiece barrel or barrel + filter could come in contact with the prism.

My other diagonals are of the 2 inch variety and when the 1.25 inch adapter is fitted the clearance (from the mirrors) is comfortable.

I guess the other instance when this could happen is if one of the eyepieces with a 2 inch / 1.25 inch hybrid barrel is used in 2 inch mode in a diagonal 🤔

Edited December 1, 2023 by John

[Mr Spock]

7 minutes ago, John said:

I guess the other instance when this could happen is if one of the eyepieces with a 2 inch / 1.25 inch hybrid barrel is used in 2 inch mode in a diagonal 

I've just tried this with a Baader 2" Clickstop diagonal and an LVW - there's miles of clearance. There would still be clearance if it were a prism. I'll update if it's the same with a Baader Zeiss T2 when it arrives in a few days.

[PeterC65]

I've been concerned about this, but on closer inspection the Baader 32mm T2 Prism diagonal that I mostly use has a collar just in front of the prism that is slightly smaller than 1.25" so while it looks like the eyepiece barrel is touching the prism in fact it is stopped by the collar.

 

[Jim L]

5 minutes ago, PeterC65 said:

I've been concerned about this, but on closer inspection the Baader 32mm T2 Prism diagonal that I mostly use has a collar just in front of the prism that is slightly smaller than 1.25" so while it looks like the eyepiece barrel is touching the prism in fact it is stopped by the collar.

 

Be careful Peter; some filters and Barlows may have a smaller outer diameter than a standard 1.25” eyepiece barrel and sneak by the exclusionary collar. 

[John]

8 minutes ago, PeterC65 said:

I've been concerned about this, but on closer inspection the Baader 32mm T2 Prism diagonal that I mostly use has a collar just in front of the prism that is slightly smaller than 1.25" so while it looks like the eyepiece barrel is touching the prism in fact it is stopped by the collar.

 

That collar is not used on the T2 zeiss prism. I guess that is how it has the larger clear aperture ?

[Mr Spock]

Just tested the Tak prism - it has a small lip inside. My (very long barrelled) x2.5 Powermate sticks out a fair bit - and sticks out a huge amount when I put a variable polariser on it.

[Mr Spock]

I've tested the T2 Zeiss prism now. x2.5 Powermate plus filters doesn't even come close to the prism. 2" eyepieces hit the lip inside the holder.

LVWs in 2" mode will contact the prism - don't try it! I guess any other 1.25" eyepiece with a 2" option will do the same. In these cases stick to 1.25"!

[John]

22 hours ago, Mr Spock said:

I've tested the T2 Zeiss prism now. x2.5 Powermate plus filters doesn't even come close to the prism. 2" eyepieces hit the lip inside the holder.

 

 That's interesting - my Baader T2 zeiss prism diagonal does not have that lip. Must be a design change 🤔 

So caution is still advised in case you have an older Baader T2 zeiss prism (mine was purchased new in 2016).

 

Edited December 12, 2023 by John

[John]

1 minute ago, John said:

 That's interesting - my Baader T2 zeiss prism diagonal does not have that lip. Must be a design change 🤔 

So caution is still advised in case you have an older Baader T2 zeiss prism (mine was purchased new in 2016).

 

Just realised that you are referring to a lip in the 2" to 1.25" adapter rather than in the diagonal itself.

 

[Mr Spock]

27 minutes ago, John said:

Just realised that you are referring to a lip in the 2" to 1.25" adapter rather than in the diagonal itself.

No, in the diagonal. 2" eyepieces can't make contact with the prism. 1.25" eyepieces, plus filters etc, when used in the adapter aren't long enough to reach.

The only eyepiece I have long enough to reach is the LVW when used in 2" mode as the 1.25" barrel extends too far down - with a filter attached; without it doesn't reach.

[John]

4 minutes ago, Mr Spock said:

No, in the diagonal. 2" eyepieces can't make contact with the prism. 1.25" eyepieces, plus filters etc, when used in the adapter aren't long enough to reach.

The only eyepiece I have long enough to reach is the LVW when used in 2" mode as the 1.25" barrel extends too far down.

This is with a 2 inch Baader eyepiece holder and a 2" to 1.25" adapter when required ?

If the diagonal is used with a 1.25 inch eyepiece fitting eyepiece holder, the risk still exists I think.

 

 

Edited December 12, 2023 by John

[Mr Spock]

Some images as they speak better than words.

It's not possible for 2" eyepieces to reach.

 

LVW in 2" mode is about 5mm or so short. You could get away with a filter but I wouldn't advise trying!

 

The long barrel x2.5 Powermate is a long way short.

I don't have the 1.25" fitting so I can't test that one 

 

[John]

15 minutes ago, Mr Spock said:

Some images as they speak better than words.

It's not possible for 2" eyepieces to reach.

 

LVW in 2" mode is about 5mm or so short. You could get away with a filter but I wouldn't advise trying!

 

The long barrel x2.5 Powermate is a long way short.

I don't have the 1.25" fitting so I can't test that one 

 

That's great if you use a 2 inch eyepiece holder with that diagonal 🙂

I used to use a very similar arrangement.

If you use a 1.25 inch one (as I do now) the risk is there because there is no lip to stop a long eyepiece barrel, at least with the ones I've tried, including my current Baader click stop 1.25 inch holder. I use a T2 extension between the eyepiece holder and the diagonal to remove that risk.

My arrangement is this for 1.25 inch eyepieces:

 

 

Edited December 12, 2023 by John

[johnturley]

12 minutes ago, John said:

That's great if you use a 2 inch eyepiece holder with that diagonal 🙂

I used to use a very similar arrangement.

If you use a 1.25 inch one (as I do now) the risk is there because there is no lip to stop a long eyepiece barrel, at least with the ones I've tried, including my current Baader click stop 1.25 inch holder. I use a T2 extension between the eyepiece holder and the diagonal to remove that risk.

My arrangement is this for 1.25 inch eyepieces:

 

 

John

On a slightly different note, I notice that you have fitted a micro-focuser to your Tak 100 DL, I just wondered which version that was, and how you rate it.

John 

[John]

2 hours ago, johnturley said:

John

On a slightly different note, I notice that you have fitted a micro-focuser to your Tak 100 DL, I just wondered which version that was, and how you rate it.

John 

Hi John,

It is the Feathertouch Micro Pinion MPA-TAK2.5. 

I was quite happy with the stock single speed R&P on the scope and had it adjusted quite nicely but then this very lightly used FT unit came up for sale at a good price so I thought it worth giving a try.

It took me a few attempts to get the feel of the focuser right (there is plenty of room for adjustment / fine tuning when fitting it) but now I'm pleased with it and feel that it is a nice addition to the scope. I used the full FT focuser on my 130mm triplet as a target to aim for when adjusting the micro unit and have now got thing working in a satisfying manner. The feel is different from the single speed R&P of course but I'm now used to it and would miss it if I had to remove the unit for any reason.

I have not tried either the Tak or More Blue units so I don't know how they compare.

Hope that helps,

John