6 replies to this topic

[kmgr1100rt]

I have a Skywatcher Star Travel 120.  I like the scope, but I would like some upgrades. I have a Celestron twist lock 2in diagonal and have collected most of Explore Scientific's 82 degree eyepieces. They all work with the scope, but the stock visual back seems really cheap to me and I have some issues with how secure the two grub screws secure the diagonal (or not).  The focus travel is almost to the minimum with the larger eyepieces.

 

I would really prefer a compression ring visual back but am having a bit of trouble.  I got a Baader 4-in1 back https://agenaastro.c...ck-2458190.html which seems really well made and holds things more securely than the stock, but the problem is that it is about 10mm or so longer than the stock back. This seems to mean that the larger eyepieces cannot achieve focus (not sure if it's all, or just the 14mm and larger).

 

I need to somehow shorten the optical path, either with a shorter, more compact visual back, or with a different diagonal.

 

I haven't had much luck finding a visual back, and the few that might be around don't give info about length or are no longer available.  There also doesn't seem to be any info about optical path length given for diagonals. 

 

Any suggestions? I'd prefer to stick with 2 inch.

[sevenofnine]

Welcome to C/N!  

 

You've described the problem very well! I recommend sending a similar description of the problem in an email to Agena Astro. They are very good to deal with IME. Best of luck to you!  

 

 https://agenaastro.com/contact/.

[kmgr1100rt]

Seems the only option at this time is to replace the entire focuser with one that comes with the compression ring clamps.

[vtornado]

A two inch prism will have a shorter optical path than a mirror but $$$ and heavy.

 

Are you near sided?   On one of my refractors with a 2 inch diagonal and 2 inch eyepiece forces me to wear my glasses to get that extra few millimeters.

[kmgr1100rt]

I ended up replacing the focuser with a GSO Linear bearing dual speed. It seems well made and works great. I still had focus issues with my Explore Scientific 82 deg 18 and 24 mm eyepieces with certain diagonals. They worked OK with the Celestron twist lock, which is good but I found changing 1.25 in eyepieces difficult. I have an Explore Scientific 2 inch diagonal that I like because of the compression rings and it is a little lighter than the Celestron, but it is slightly shorter. I solved this with a spacer ring I found on Amazon for 2 in eyepieces made by Astromania intended to make eyepieces parfocal. One ring adds just enough to bring the 2in eyepieces into focus range without affecting the 1.25 eyepieces.

[Oldfracguy]

I have a Skywatcher Star Travel 120.  I like the scope, but I would like some upgrades. I have a Celestron twist lock 2in diagonal and have collected most of Explore Scientific's 82 degree eyepieces. They all work with the scope, but the stock visual back seems really cheap to me and I have some issues with how secure the two grub screws secure the diagonal (or not).  The focus travel is almost to the minimum with the larger eyepieces.

 

I would really prefer a compression ring visual back but am having a bit of trouble.  I got a Baader 4-in1 back https://agenaastro.c...ck-2458190.html which seems really well made and holds things more securely than the stock, but the problem is that it is about 10mm or so longer than the stock back. This seems to mean that the larger eyepieces cannot achieve focus (not sure if it's all, or just the 14mm and larger).

 

I need to somehow shorten the optical path, either with a shorter, more compact visual back, or with a different diagonal.

 

I haven't had much luck finding a visual back, and the few that might be around don't give info about length or are no longer available.  There also doesn't seem to be any info about optical path length given for diagonals. 

 

Any suggestions? I'd prefer to stick with 2 inch.

The typical 2" mirror diagonals have optical paths from between about 105 mm for the 2" William Optics Dura-Bright Carbon Fiber Dielectric to as much as 114 mm for the Baader 2" BBHS Silver Sitall Mirror.  The only 2" diagonal I have used, and whose optical path I have measured, that is significantly shorter, and might work in your situation, is the Baader 2" Zeiss-spec Prism diagonal.  I measured its optical path to be 96 mm.  The problem is that diagonal not only costs about as much as your StarTravel 120, it will very likely compound the already significant amount of CA you see in that scope.

 

The other thing to try will be to only use those 2" eyepieces whose focal planes are at, or very near, the flange, or lip, on the outside of the eyepiece barrel that rests atop the diagonal's 2" receptacle.  Examples of such 2" eyepieces are the 30mm UFF (APM, Celestron Ultima Edge, etc.) and the 28mm UWA (Astro-Tech, Stellarvue, etc.).  Some 2" eyepieces to avoid in your situation, ones whose focal planes are way up inside above the level of the flange, and require the focuser to be racked even farther inward, are the 70° SWA eyepieces like the Agena and old Orion Q70 models.

Edited by Oldfracguy, 23 March 2025 - 09:02 PM.

[kmgr1100rt]

I did replace the focuser with a GSO linear bearing, which I like.  I also found a work-around for the 2-inch eyepiece focus issue - Astromania 2-in eyepiece parfocal rings found on Amazon for $15.  They add about 5 mm of space between the eyepiece and diagonal or the diagonal and the focuser (although, they could be clamped on the barrel to add whatever distance needed).  I found that adding one ring ahead of the diagonal worked just fine.