37 replies to this topic

[Mike W]

Did you get the silver ring off?

[Procyon]

Did you get the silver ring off?

Yeah, that white plastic retainer ring I can remove also if you want. See pictures above.

Edited by Procyon, 15 April 2025 - 10:31 PM.

[Procyon]

I don't think it's going to work out by removing only the silver part...when I rack it back all the way up it still stops at the same spot, with the current vb (without the silver part)....hmmmm. 

 

People had issues reaching focus with a 2" diagonal back in 2005... https://www.cloudyni...0mm-f6-is-here/

 

https://www.cloudyni...mm-f6-flocking/

Edited by Procyon, 16 April 2025 - 12:13 AM.

[T1R2]

I have a ES AR127 and every EP I have comes to focus, from a 4mm plossl to a 38mm/70*, maybe you might want to sell it and look into trading up to a AR102. 

I've never heard of focus problems with those scopes. I know its just more hassle but it may be the best bet in the long run, doesn't have to be an AR though maybe you can find another brand that doesn't have those issues, I think some of the Orion ST series also had some focus issues so you might want to research whether or not those issues are valid before you buy, but Im not certain if that was for just using as a guide camera like the ST80 with certain cameras, or if that also included EP's for visual use. 

[Procyon]

I think it's best I just get an eyepiece that has been proven to come into focus like a 35mm TV Panaoptic and be done with it. I bought this scope just to see some Asterisms the rest I can see with my 11" SCT. Thanks all, thanks Mike, it was worth a try. If anyone knows of other eyepieces that have their focal plane near the flange or others like the T6's that require focusing outward instead of inward let me know thanks!
 

Another thing about eyepieces to consider when trying to detemine what length of M48 spacer will be needed is that the spacing recommendations are the distance from the output of the TSFLAT2 to the focal plane of the eyepiece.  The actual location of the focal plane for eyepieces varies all over the place in relation to the flange, or lip, on the outside that rests against the top of the 2" diagonal.  For example, the 2" 30mm UFF and Astro-Tech 28mm UWA have their focal planes almost parfocal with one another, about 1 mm above the flange.  Other 2" eyepieces like the Orion 38mm Q70 (same as the Agena 38mm SWA) have their focal planes way up inside, about 10-11 mm above the flange.  That, along with the actual optical path of your particular 2" diagonal,  has to be taken into account as a starting point for figuring out what size M48 spacer, if any, will be needed.  Even then, you will often find that varying the spacing a few millimeters one way or the other results in the best overall performance from a given eyepiece.

 

 

I had no problem with the 17mm T4 coming to focus in a the Orion 100mm f/6 . This was about 2.5-3 years ago. I no longer own that scope. I also recall no problems with a 27mm Panoptic, 20mm T5,26mm T5 and all T6's .
I used the WO 2" enhanced and 2" SV dielectric.

The eyepieces that did not work for me were the 2" 30mm GSO Superview and SWA 5-8mm zoom eyepiece.

The one's that did come into focus in the low power range were the TV 35mm Panoptic, 24mm Panoptic , 20mm TV Nagler T5 and 42mm GSO Superview.

 

Last Fall I did a test with all my 2" eyepieces, and some 1.25" eyepieces as well, in order to figure out exactly where their focal planes were relative to the flange on the barrel that rests against the diagonal.  When I do these eyepiece comparisons I start with a simple 1.25" eyepiece, a 12mm Plossl, whose focal plane is even with the flange on the barrel.  I then focus on a distant target over a mile away.  When, after switching eyepieces, I have to rack the focuser farther outward to regain focus on the same distant target, that means the focal plane of that eyepiece is down lower than the flange.  The Tele Vue Nagler Type 6 eyepieces are a prime example, where I usually need to rack the focuser outward another 8-9 mm compared to a lot of other 1.25" eyepieces.  Long focal length 1.25" eyepieces like 25mm and 32mm Plossls need the focuser to be racked farther inward, which means their focal planes are up above the flange.

 

 

Otherwise I think it's time to take out the saw.

Edited by Procyon, 16 April 2025 - 08:43 AM.

[ABQJeff]

OP: you hit on one of the biggest reasons I think refractors are much more complicated than SCTs. Backfocus is only published for some refractors, optical path length only for some diagonals (namely Baader), and for eyepieces the field stop location is only published for Televue, Pentax XW (at the shoulder), Morpheus and a couple others.

Meaning if you are missing optical path length numbers, you have to self test all your gear with another working refractor to measure to get the focus distance.

Why refractors don’t all carry at least 150mm back focus as standard, I have no idea. AP types can use extensions and reducers (which consumes optical path lengrh), us visual folks are stuck with a less than useful scope.

If your focuser attaches with three screws, shortening is easy. But if it is threaded on that would require a machine shop, lathe, etc.

Edited by ABQJeff, 16 April 2025 - 09:23 AM.

[Procyon]

So true. Surprised no one has created like a master list for many eyepieces.

 

Wandering around the boards and with the help of Oldfracguy we kind of created a mini list with some notes, maybe it'll help someone in the future...

 

yepiece Focal Plane position measurements by Oldfracguy

Results of testing to see where the position of the eyepiece focal plane is relative to the flange, or lip, on the barrel that rests against the diagonal (average of several tests rounded off to the nearest mm):

2” Arcturus Ebony 22mm                     5 mm above flange
2” TS ED35                                              6 mm above flange
2” Agena 38mm SWA                          11 mm above flange
2” Astro-Tech 40mm Titan II                9 mm above flange
2” 30mm UFF                                          1 mm above flange
2” Agena 26mm SWA                            1 mm above flange
2” Orion 38mm Q70                             10 mm above flange

With a TSFLAT2 installed ahead of the 2” diagonal, all these 2” eyepieces came to focus with the telescope’s focuser racked between 7 and 8 mm farther inward than where it was without the TSFLAT2 installed.  Therefore, the TSFLAT2 itself consumes between 7 and 8 mm of the optical path.

I used to have the 2” 28mm UWA eyepiece (Astro-Tech, Stellarvue, etc.), and it was parfocal with the 30mm UFF within a millimeter or so, so I would estimate its focal plane to be close to 1 mm above the flange as well.

Results of testing some 1.25” eyepieces:

1.25” 10mm UFF                                     4 mm below the flange
1.25” 15mm UFF                                     3 mm below the flange
1.25” 18mm UFF                                     2 mm below the flange
Nagler 7mm Type 6                                7 mm below the flange
Nagler 3.5mm Type 6                             7 mm below the flange

Some tests of 2” diagonals using the 2” Celestron Ultima Edge 30mm UFF (clone of the APM 30mm UFF) looking at a distant target in focus with an Astro-Tech AT115EDT refractor:

                     Diagonal                                     AT115EDT Focuser Scale Reading    

2” Baader BBHS Silver Sitall Mirror                                 44.2 mm

2” Astro-Tech Dielectric Mirror                                        52.2 mm
   (2” William Optics Dura-Bright clone)

2” Baader BBHS Zeiss-spec Prism                                     61.5 mm

 

My notes (Looks like a mess, did it this morning)

 

Can achieve focus (Proven on board to work with an Orion 100mm Astroview 600mm f/6 refractor)  Notes
 
42mm GSO Superview 
41 mm Panoptic 
35mm Panoptic   The 35mm Panoptic is parfocal with the 30mm UFF, the 32mm Q70 and the 28 mm UWA, 4.0mm forward requires racking the focuser outward
32mm Q70 
30mm UFF   about 1 mm above the flange
Astro-Tech 28mm UWA   about 1 mm above the flange
27mm Panoptic 
26mm T5 
24mm Panoptic 
20mm T5 
17mm T4 
all T6's    rack the focuser outward another 8-9 mm

 

Can't achieve focus (in Orion 100mm Astroview 600mm f/6 refractor)  Notes
  
Orion 38mm Q70   about 10-11 mm above the flange
Agena 38mm SWA   about 10-11 mm above the flange
30mm GSO Superview  
SWA 5-8mm zoom  
Astro-Tech 40mm Titan II TMB Paragon clone 68°         
TS ED35 35mm TMB Paragon clone 69°                       
31 mm Nagler

 

TSFLAT2 adds 7-8mm to lightpath

 

2" eyepieces

 

Make FL Model Focus Distance (mm)

  
Leica  ASPH Zoom -16 1/2
Nikon 17 NAV-HW -16 1/2
Leica  ASPH Zoom -16
Zeiss/Optik 30 BW -14
Takahashi 50 LE -11 1/2
Tele Vue 17 Ethos -9
Tele Vue 22 NaglerT4 -7
TMB 40 Paragon -2
Vixen 42 LVW -1 1/2
Pentax 40 XW -1
TMB 40 Paragon -0.5
TMB 30 Paragon 0
Pentax 30 XW  1/2
Pentax 40 XW  1/2
Nikon 12.5 NAV-HW 1 1/2
William Optics 28 UWAN 3 1/2
Tele Vue 13 Ethos 7 1/2
Tele Vue 3.7 Ethos-SX 8
Tele Vue 8 Ethos 19 1/2

 

The new T7's should be easy to achieve focus but not the new 24mm Delos.

Edited by Procyon, 16 April 2025 - 03:07 PM.

[vrodriguez2324]

What type of focuser do you have?

 

I have the ST120. I found that the 31mm Nagler barely reached focus with the Stock Focuser. I had maybe 1mm left. My solution was to get the GSO Focuser which is about 25mm shorter.

 

With the shorter focuser I can comfortably reach focus with all my eyepieces. 

 

Here is a thread I started. Post 17 shows the difference in focusers.

 

https://www.cloudyni...out/?p=12452469

[Procyon]

What type of focuser do you have?

 

I have the ST120. I found that the 31mm Nagler barely reached focus with the Stock Focuser. I had maybe 1mm left. My solution was to get the GSO Focuser which is about 25mm shorter.

 

With the shorter focuser I can comfortably reach focus with all my eyepieces. 

 

Here is a thread I started. Post 17 shows the difference in focusers.

 

https://www.cloudyni...out/?p=12452469

Yeah, Oldfracguy mentionned that, though I don't feel like going through all that atm, but good info to add to the thread maybe it'll help someone out in the future. I'll probably just get a Panoptic 35mm and call it a day for now. 

[Procyon]

Someone correct me if I'm wrong, but isn't the parfocality (listed in mm) mentioned in the link below (once you click on an eyepiece) the measurement of how far down or over the focal point (or focal plane) distance from the eyepiece's flange is? 

 

https://astro--talks...hl=en-US#p41976

 

* 1 distance from the eyepiece support flange to its front focus, "+" when the focal plane is shifted from the end of the focuser towards the lens, "-" when the focal plane is shifted towards the eye

 

And Televue has them in inches (Column F) https://www.televue....page.asp?id=214

Edited by Procyon, 17 April 2025 - 11:27 AM.

[Eddgie]

Why refractors don’t all carry at least 150mm back focus as standard, I have no idea. AP types can use extensions and reducers (which consumes optical path lengrh), us visual folks are stuck with a less than useful scope.
 

There is an explanation to everything, and I can provide one for this.

 

One of the simplest and most useful things to remember is what focal ratio really means. Focal ratio is the ratio convergence of the light cone for the distance traveled.

 

Let's take actual examples.   First, let's say a scope is f/10 and 127mm in diameter.  As the light cone enters the system, it is 127mm in diameter.  As it travels toward the focal plane, for every 10mm that the cone travels, it reduces in diameter 1mm.

 

For a 127mm f/6 system, for every 6mm, the cone reduces in aperture by  1mm. 

 

So, the ratio of distance traveled to the size of the diameter of the cone is 1:10 for the f/10 system, and 1:6 in the f/6 system. This is what focal ratio means. When we write this, we use f/10, and f/6. 

 

What is not intuitive to many is that this works not just from the objective to the focal plane, but it also works in reverse from the focal plane to the primary, and this is the important part to answering your question. A characteristic of telescopes is their fully illuminated circle. This is the size of the circle that receives 100% of the light that enters the objective even at the edge of that circle. This circle size is chosen by the designer based on what they believe the intended us of the instrument will be. For visual use, a scope may have a very small fully illuminated field. For example, an 8mm SCT has a fully illuminated field of about 8mm.  This field is fine for visual use, but would vignette the corners of a 35mm diagonal sensor.

 

 Let's say for explanation that the designer has a goal to fully illuminate a 35mm sensor and lets take our f/10 and f/6 scopes as design subjects.  Remember, the focal ratio expresses the change in diameter of the light cone for a distance traveled, but this time, we are working from the focal plane towards the objective.

 

For the f/10 scope, for every millimeter we move from the focal plane towards the objective, the desired 35mm circle will grow 1mm in diameter.  Let's way that we have a focuser that is 52mm of inner diameter. Remembering that the 35mm cone gets bigger as you move toward the objective and at some point, it has to pass through the circular aperture of the focuser tube.  If it starts at 35mm, it can expand grow an additional 17mm in diameter to reach a diameter of 52mm,  which is what it needs to be to pass though the opening of the focuser tube nearest the primary.  We know that the change is 10mm for 1mm of focuser travel, so th.at means that it will travel 170mm before it grow to 52mm, and the focuser tube can be 170mm long, assuming the focal plane is at the back of the focuser tube. For now, we are going to ignore the camera flange to focal plane distance.  

 

What happens at f/6 though?  Now the cone expands 1mm for every 6mm of light travel, so in this case, to grow from 35mm to 52mm, so at 1mm of diameter change for 6mm of travel, then in 6 x 17mm, or 102mm of travel, the light cone will expand enough to just barely fit through the front opening of a 2" focuser tube that is 4" long. 

 

Now, let's say we add a filter wheel and a camera, and we increase the amount of distance the cone travels by 30mm for the filter wheel and 44mm for a Canon EF-S mount.  We could cut the focuser tube by 7.4mm, and still have a focuser tube that was 162.6mm long. 

 

With the f/6 system though, adding 76mm of light path would mean that we would have to cut the focuser tube by 12.66mm to get the full light cone to fit, or, we could simply make the focuser 2.5". 

 

This is why you see modern, fast apos coming now with 2.5" and 3" focusers. You have one of two choices when you are dealing with such a system.  These systems often are used with focal reducers and these decrease the focal ration, so you need a bit more inward travel and this means in effect that the front of the focuser tube is a bit closer to the focal plane.  

 

There are four options then for these fast scopes. Make the focuser tube larger, make the focuser tube shorter, use a combination of these, or make the focuser tube with removeable sections (Explore Scientific used to do this). 

 

The OP is using his scope with a 112 light path diagonal, so if the focuser has a 4" long tube, the light has 212 of travel. At f/6, a light cone that travels 212mm will have converged from the 54mm that it enters into the focuser to about 17mm, and this is OK for visual.  With a full frame camera though, there might be some vignetting, but for the OPs scope, I am sure the designer considered that it would likely be used only for visual, so a smaller than 35mm circle is fine. If you followed the math, you would see that it matches pretty closely to what the case is with the OPs instrument. The focuser tube is probably 4", and since most diagonals are 100m of light path, for a total of 203mm, most developers would likely add a little to be safe, then the scope has the right amount of back focus and the right focuser. If the OP was using a more standard 2" diagonal, my bet is that all of his eyepieces would reach focus. 

 

This is the answer to your question though. The designer has to be concerned about the size of the fully illuminated field and this is why some scopes have shorter focusers than others or bigger focuser tube diameters, and why scopes with 2.5 and even 3" have become mainstream.  With cameras and focal reducers, which add length and shorten the light cone, these focusers become very necessary in today's fast refractors to be able to accommodate the various different configurations required by users. 

 

This is why it is important to fully understand what focal ratio really means. Focal ratio is the size cone diameter for distance traveled, and knowing this you can easily calculate fully illuminated circle. 

Edited by Eddgie, 17 April 2025 - 12:45 PM.

[Procyon]

 

The OP is using his scope with a 112 light path diagonal, so if the focuser has a 4" long tube, the light has 212 of travel. At f/6, a light cone that travels 212mm will have converged from the 54mm that it enters into the focuser to about 17mm, and this is OK for visual.  With a full frame camera though, there might be some vignetting, but for the OPs scope, I am sure the designer considered that it would likely be used only for visual, so a smaller than 35mm circle is fine. 

 

This is the answer to your question though. The designer has to be concerned about the size of the fully illuminated field and this is why some scopes have shorter focusers than others or bigger focuser tube diameters, and why scopes with 2.5 and even 3" have become mainstream.  With cameras and focal reducers, which add length and shorten the light cone, these focusers become very necessary in today's fast Apos. 

 

This is why it is important to fully understand what focal ratio really means. Focal ratio is the size cone diameter for distance traveled, and knowing this you can easily calculate fully illuminated circle. 

Eddgie, thanks for the knowledge. Great to know. 

 

Can you guess my telescope's focusing length simply by looking at that picture I attached with the measuring tape. I've extended the focuser to it's limit. Is it only 75mm? Is there a way to know beforehand, or before buying, a refractor's focusing range tube length? Some kind of stat somewhere. Or does that not even matter when it comes to knowing which eyepiece will come into focus with a certain 2" diagonal. Or is it just my little crappy achro that has a little tube? lol Actually, I like the scope, the views, when focus is achieved are great to my eyes. No issues there. Just need the right 2" eyepieces that can match the scope and diagonal I guess. I have so much outfocus left with every eyepiece I insert, best to look for those. I looked into the 31mm Nagler, it won't work with my scope, but the 35 and 41mm Pans will...

Edited by Procyon, 17 April 2025 - 11:42 AM.

[Eddgie]

Not all manufacturers really bother to tell us what the working back focus is, but it is becoming much more common. 

 

Very fast refractors typically used to have relatively short focuser travel, and this is to make sure it has well illuminated field over a decently wide diameter circle,  but as you can see today, a 2.5" focuser now seems to be the "standard" focuser for most ED and Apo scopes, as they will often be used for imaging, and as scopes get ever faster, even 3" is now becoming common. The 3" focuser allows for use of large, fast focal reducers and still have enough range for visual and other uses. 

The goal of my earlier post though, was to answer the question as to why all telescopes don't have 150mm of back focus. For modern fast telescopes used for imaging, you can do it, but not with a 2" focuser. By using the 3" focuser, you can have a focuser with a very long tube.