17 replies to this topic

[Happy_Camper]

There are quite a few topics on this site already regarding the combination of focal reducers/correctors with binoviewers on the Nexstar 8SE (here, here, here, here, and so on). There didn't seem to be a consensus on whether this will work, although comments lean towards claiming that it does. The main purpose of this post is to confirm that yes, indeed, it does work, but only without a diagonal.

 

A frigid night, terrible "seeing", trees and clouds blocking the moon... And somehow still enjoying it!

 

My exact setup: Celestron f/6.3 focal reducer/corrector + stock 2" to 1.25" visual back + Celestron binoviewers + Baader Hyperion Mark IV zoom eyepieces

 

To be clear, this was unsuccessful with the stock diagonal; it adds too much to the light path. Straight through, on the other hand, works just fine. Now, just because you can doesn't mean you should... There are plenty of drawbacks to this setup including but not limited to:

• Many claims that this reduces the aperture by 1-2" in diameter.
• The mount is even more off balance than when simply using binoviewers with a diagonal, because of the increased lever arm between where the diagonal would go and the center of mass of the binoviewer+eyepieces (although since these binoviewers arrived, I realized I'll need a dovetail rail extension anyways...)
• It is hard to get comfortable viewing positions without the diagonal
• Clearance between the eyepieces and the base of the mount (i.e. where the AA batteries go) is much more reduced.

Nevertheless, it was cool to get a low-power view of the moon with binoviewers! I will definitely use it here and there for when I just feel like a low-power view.

[Rutilus]

With my SCT, I had to build my own system so that I could use the f/6.3 reducer and a diagonal  together.

It has a shorter light path than the systems that you can buy and works well with my SCT.   

[Happy_Camper]

I see your post now - very cool that you were able to reduce the light path enough! How specifically did you modify your equipment?

[Rutilus]

I have refined the system that is posted in the link, with a shorter light-path.

P.M. sent.

[noisejammer]

This idea comes up from time to time, so you may find this thread of interest.

 

Edit - I found this post.  You can measure the reducer's focal length by using it to focus light from the Sun - 230mm is about 9". If your focal reducer is one of the "short" ones, I'd pretty much stop there.

 

Many years back, Ted Agos at Acorn Hollow Obs analysed the performance of the original f/6.3 reducer. He found that the f/6.3 reducer was usable from about f/7.5 to f/5.5, the reduction ratio being set by the distance between the reducer and the focal plane. Sadly, he's no longer with us.

 

 

It's important to realise that as you reduce he apparent focal length, you also reduce the diameter of the exit pupil. Most reducers have an entrance pupil that's 38 mm. If your binoviewers use (say) 26 mm prisms, you can tolerate 26/38 = 0.68x or reduction to f/ 6.8 before the reducer starts to introduce vignetting. Ted wrote some notes that are available here.

 

This isn't a calamity - all it means is that the binoviewer needs to be close-coupled with the reducer. It should be dead easy to replace the bulky standard housing with one that has T2 threads at each end. (The gent mentioned above did exactly this and offered them for a while. Optec Inc picked it up some time later.)

 

Since the OP is after the maximum field of view, he/she is unlikely to use high power eyepieces. so that the glass path correctors can be left out. (You would normally want them at f/7 or so.)

 

The most effective arrangement may then be SCT - T2 Diagonal - modified focal reducer - binoviewer.

 

Since a SCT usually operates natively at about f/10, the T2 diagonal can happily be a prism.

[Happy_Camper]

Very informative - thanks! I'm personally a planetary/lunar observer, so I'm usually at higher powers, but it is nice to have a big FOV as an option (although my SCT isn't the best tool for this). I'm in the midst of trying to figure out what the precise limits are for my 8SE and these are some useful references. I just picked up "Astronomical Optics" by Daniel Schroeder to educate myself a lot more thoroughly, largely triggered by this SCT+reducer+binoviewer rabbit hole....

 

A quick question, though: Why use a 2" instead of a 1.25" prism when going through all other efforts to minimize the light path? Isn't the light path in the prism simply the length of one side of the prism?

[noisejammer]

... A quick question, though: Why use a 2" instead of a 1.25" prism when going through all other efforts to minimize the light path? Isn't the light path in the prism simply the length of one side of the prism?

I specifically suggest a Baader T2 diagonal, not a 2" model. The reason is the modified reducer or binoviewer can screw directly onto the diagonal. You don't need a slip-in 1.25" or 2" nosepiece... depending on the design, these gobble up 30-45 mm.

 

This might not seem like a big deal but the spherical aberration of a SCT increases as you drive the focus away from the visual back. This means your image quality is optimised when you have the shortest optical path to the eyepiece. Secondly, the focal length increases which means you are sacrificing any benefit that the reducer will bring you.

 

The choice between a T2 mirror or T2 prism won't matter appreciably for a SCT but the prism version I linked to is the shortest optical path they offer. For completeness, you would still need a compact adapter to connect it to the visual back of your scope.

 

I think I have one of Ted's reducers connected to an old CCD camera. The optical section is about 30mm long. If there's interest, I'll dig it out and post some images.

[Happy_Camper]

I specifically suggest a Baader T2 diagonal, not a 2" model.

I was totally confused by this, because somehow I always automatically assumed that the T2 was 2"... I mean, what else could the "2" possibly mean!? Well, it seems that "T2" is simply a threading standard with a diameter of 42mm≈1.65" (although still not sure what the "2" means), and it's used in astronomy because:

  a) it has larger field stop than 1.25"

  b) it appears to allow for other tricks to minimize light path, for example:

...the modified reducer or binoviewer can screw directly onto the diagonal. You don't need a slip-in 1.25" or 2" nosepiece... depending on the design, these gobble up 30-45 mm.

  c) maybe some other reasons

As a side note: ain't it neat that the T2 standard is an M42 x 0.75 metric thread which is a nice shout out to the Orion nebula?

 

For the purpose of minimizing the light path, I was considering that the light path through the actual glass alone is exactly equal to the length of any non-diagonal side of the prism, or the length of the diagonal side multiplied by √2. For a mirror diagonal, it would be just be length of any side of the mirror times √2. I guess that any diagonal prism/mirror is slightly larger than the barrel (be it 1.25", 42mm, or 2"), but probably not by more than a couple of millimeters for the sake of light path. For a T2, you then get roughly 10mm longer light path than the 1.25" (and roughly 9mm less light path than the 2"). If being able to save 30-45mm in the nosepiece alone, then it's already worth it.

 

Interesting stuff!

[noisejammer]

I was totally confused by this, because somehow I always automatically assumed that the T2 was 2"... I mean, what else could the "2" possibly mean!? Well, it seems that "T2" is simply a threading standard with a diameter of 42mm≈1.65" (although still not sure what the "2" means), and it's used in astronomy because:

The original screw thread was M42x1 and started life with Contax and Praktica. Early Starlight Xpress ccd cameras used this because they would interface directly with Pentax screw threads that were common on SLR lenses from the early 70's (Terry, the founder told me this when we were chatting.) They switched to T2 around 2003-ish.

 

Tamron originally used a M37x0.75 thread but changed to M42x0.75 and called it the T-mount (for Tamron). Soligor started calling it the T-2, perhaps to avoid confusion with the M37x0.75.

 

Something else to watch for is the M48x0.75 which is usually found on 2" nosepieces and filters.
 

[Sarkikos]

8SE binoviewing w/ focal reducer does work, just without diagonal

 

A straight-through 8" SCT?  My neck would not like it.  

 

Mike

Edited by Sarkikos, 21 January 2024 - 11:55 AM.

[Eddgie]

There are quite a few topics on this site already regarding the combination of focal reducers/correctors with binoviewers on the Nexstar 8SE (here, here, here, here, and so on). There didn't seem to be a consensus on whether this will work, although comments lean towards claiming that it does. The main purpose of this post is to confirm that yes, indeed, it does work, but only without a diagonal.

 

Screenshot from 2023-12-18 22-05-05.png

A frigid night, terrible "seeing", trees and clouds blocking the moon... And somehow still enjoying it!

 

Screenshot from 2023-12-18 22-05-30.png

My exact setup: Celestron f/6.3 focal reducer/corrector + stock 2" to 1.25" visual back + Celestron binoviewers + Baader Hyperion Mark IV zoom eyepieces

 

To be clear, this was unsuccessful with the stock diagonal; it adds too much to the light path. Straight through, on the other hand, works just fine. Now, just because you can doesn't mean you should... There are plenty of drawbacks to this setup including but not limited to:

• Many claims that this reduces the aperture by 1-2" in diameter.
• The mount is even more off balance than when simply using binoviewers with a diagonal, because of the increased lever arm between where the diagonal would go and the center of mass of the binoviewer+eyepieces (although since these binoviewers arrived, I realized I'll need a dovetail rail extension anyways...)
• It is hard to get comfortable viewing positions without the diagonal
• Clearance between the eyepieces and the base of the mount (i.e. where the AA batteries go) is much more reduced.

Nevertheless, it was cool to get a low-power view of the moon with binoviewers! I will definitely use it here and there for when I just feel like a low-power view.

While it may work, you might want to check the aperture. My guess is that the aperture is being reduced. You would not know how much until you measured the aperture loss but my guess is that it would be about an inch. This also makes the secondary obstruction a much larger percentage of the remaining aperture. 

 

Also, the system likely has more than a half wave of spherical aberration. 

 

To measure, focus the scope at infinity with the reducer and binoviewers.  Do not change the focuser until the test is complete.

 

Bring the scope in and place it maybe a foot from and square to a wall, then shine a laser into one of the eyepieces. It will take a bit of fiddling and you may need an assistance, but you will see the entry pupil projected on the wall, and you can measure it directly. I would usually put a couple of little squares of masking tape at 180 degrees around the pattern then mark the edge of the pattern so you can more easily measure. 

 

If you are happy with the result though, then I guess that is all that matters.
 

If you do make the measurement though, please share it with us. I would like to have the data. 

[BCEagleScope]

WOW, so glad I found this (and the many other threads) specific to this topic! I just got a Binotron 27 supersystem for use in my 16" Teeter. I wanted to test it out for use in the 8se, and I just could not get it to focus using the power switch. I was able to achieve excellent focus using the neutral setting, but when I changed the powerswitch it could not resolve an image at all. I have:

Baader 2" diagonal click lock

.63 focal reducer
Binotron 27 w/ powerswitch
2 24mm panoptics

I'm going to test it out again without the focal reducer and see if I can get it to clear up.

[Eddgie]

WOW, so glad I found this (and the many other threads) specific to this topic! I just got a Binotron 27 supersystem for use in my 16" Teeter. I wanted to test it out for use in the 8se, and I just could not get it to focus using the power switch. I was able to achieve excellent focus using the neutral setting, but when I changed the powerswitch it could not resolve an image at all. I have:

Baader 2" diagonal click lock

.63 focal reducer
Binotron 27 w/ powerswitch
2 24mm panoptics

I'm going to test it out again without the focal reducer and see if I can get it to clear up.

Yeah, with the focal reducer, 2" diagonal, and binoviewer, the aperture would probably be around 6" to 6.", but this is also likely to be beyond the focus range of some SCTs. Using just the built in reducer of the Binotron power switch and you pobably will get maybe 6.6" of aperture. 

The other thing though is that at this mirror positioning, the spherical aberration is probably over 1/2 ha wave, and the obstruction does not chance in physical size, but it is now about about 45% of the aperture.

 

Denk sells a special diagonal that can mount to the rear port, and that the binoviewer can mount to the top of the diagonal, and with this, the aperture loss in low power mode is far less. For low power visual observing this is not a bad compromise because while the aperture may be slightly reduced using the low power of the power switch, when you go straight through, you are probably working at very close to full aperture, and with the high power switch you get full aperture an much lower spherical aberration.

 

This chart shows the effect of using lots of displacement of the focal plane.  This is not a C8 though, but it is a very close model (this is for a 200mm scope, but again, not exact, but good enough to show you the system behavior.) As you can see, the C8 goes into aperture reduction very easily when using the focal reducer. 

 

 

 

Once again, this model is not completely accurate to the C8, and some report not seeing any aperture loss with up to 240mm, but the light path through the Clicklock diagonal is 112mm, through the B27 is about 127mm, and through the power switch is about 20m or so (I forget the exact thickness), so at best, once you include the visual back, you are well over 240mm and even without the reducer, there is some small aperture loss, but with the reducer, it gets pretty severe.  (For every 25mm over 100mm, the spherical aberration will change by 1/23rd of a wave, so you would change the correction by 8/23rds of a wave, so well over 1/3rd wave, but in direct measurements of my C14, spherical aberration was over a half wave when using a 2" diagonal and the low power arm of the power switch. The aperture was reduced to 12.6 inches but in such a large aperture, 1.5" loss as a percentage of aperture is much less serious than 1.5" of loss in an 8" aperture. 

 

If you decide to measure the aperture using the test I did above, please share the results with us.  While charts and graphs can tell us how the system will behave on paper, field reports are more reliable. 

[rob.0919]

I've not delved too deeply into this thread or others linked, but would the Alan Gee telecompressor not be a better option ?

https://www.rotherva...or-mark-ii.html

[Happy_Camper]

I don't know how I've never heard of the telecompressor before, but it seems really cool! Throwing it on the ever-growing list of accessories I want for my scope.

 

Since a handful of people have addressed some of the drawbacks I mentioned at the bottom of the original post, I'll reiterate to anyone considering this setup that I'm not really advocating for it. It's nice to know that it works if it's your only option for low-power bino viewing (it is for me - the equipment used in this setup is pretty much the extent of my collection). I'll use it now and again, briefly, but it is far from ideal. The biggest drawback in my own opinion is how uncomfortable it is to view. I can extend the tripod legs fully to avoid sitting on the ground, but then the vibrations really get bad and even then, it's hard to avoid bending your neck in a strange way.

[Vansh]

Bring the scope in and place it maybe a foot from and square to a wall, then shine a laser into one of the eyepieces. It will take a bit of fiddling and you may need an assistance, but you will see the entry pupil projected on the wall, and you can measure it directly. I would usually put a couple of little squares of masking tape at 180 degrees around the pattern then mark the edge of the pattern so you can more easily measure. 

 

If you are happy with the result though, then I guess that is all that matters.
 

If you do make the measurement though, please share it with us. I would like to have the data. 

Can you go into more detail about how to set this up? I have a Celestron Nexstar 8i and I should be able to run this test.

 

1. Does it need to be a laser pointer or can you just use something like your smartphone's flashlight near the eyepiece? I can mount the smartphone to the eyepiece, I'd need to figure out how to mount a laser pointer so that it shines straight into the eyepiece, if the light source must go straight into the eyepiece instead of just being any light source near it.

 

2. Does it matter which eyepiece you use?

 

3. It seems like the chart is showing aperture based on backfocus. Is it talking about, if the eyepiece is positioned that far away (i.e. at the backfocus spot), and then you have to move the primary mirror to focus (at infinity), the size of the resulting aperture once the primary mirror is focused? I understand the intuition that you're measuring how much of the light from the aperture is able to reach the eyepiece by just reversing the light path, but I'm not sure how to interpret the chart.

 

4. Does the scope need to be focused at infinity (i.e. move the eyepiece to the desired backfocus distance, focus to infinity, then test against the wall) for each backfocus point being tested?

 

-----

 

With respect to what I have, I have a Celestron Nexstar 8i, a standard Celestron 1.25" visual back, a Meade reducer (Japan), a generic unbranded binoviewer from China, and a generic 1.25" diagonal from the Meade S102. I also have a Meade MA 26 mm eyepiece, which as far as I can tell has pretty much the same performance (magnification and apparent field of view) as a 25 mm Plossl (it matches a generic 25 mm Super Plossl and an Orion Sirius 25 mm Plossl; in fact I use those 3 interchangeably for binoviewing). From what I've tested, based on the distance from the counterclockwise-most end of the focus knob, against a target around 800 feet away:

 

* I can turn the focus knob clockwise a total of around 41 times before I reach the clockwise-most end.

* It takes 18 3/4 CW turns to reach focus with the 8i => 1.25 visual back => 26 mm (no diagonal)

* It takes 15 1/4 CW turns to reach focus with the 8i => 1.25 visual back => S102 diagonal => 26 mm

* It takes 13 1/4 CW turns to reach focus with the 8i => 1.25 visual back => generic binoviewer => 26 mm

* It takes 10 5/8 CW turns to reach focus with the 8i => 1.25 visual back => S102 diagonal => generic binoviewer => 26 mm

 

So the S102 diagonal "costs" 3.5 turns, the bino "costs" 5.5 turns, and the two of them together "costs" around 8 turns. Not sure if the number of turns should've added up to be the same (i.e. if I made any errors in counting the number of turns there). The binoviewer has an optical path length of 108 mm (measured using a refractor and a 6 mm Plossl) and the S102 diagonal has an optical path length of roughly 70 mm or so. Not sure the relationship between how much the focus knob turns versus how much backfocus the eyepiece is at.

 

If I stick the Meade reducer in there:

* It takes 17 CW turns to reach focus with the 8i => Meade reducer => 1.25 visual back => 26 mm (no diagonal)

* It takes 10 CW turns to reach focus with the 8i => Meade reducer => 1.25 visual back => S102 diagonal => 26 mm

* It takes 4 CW turns to reach focus with the 8i => Meade reducer => 1.25 visual back => generic binoviewer => 26 mm

* It won't reach focus with the 8i => Meade reducer => 1.25 visual back => S102 diagonal => generic binoviewer => 26 mm

 

I can test any of the above focus points easily. I don't have a standalone focuser to connect to the back of the scope to move the eyepiece back and forth though.

[BCEagleScope]

If you decide to measure the aperture using the test I did above, please share the results with us. While charts and graphs can tell us how the system will behave on paper, field reports are more reliable.

Thanks for this! I tried it without the focal reducer and visuals were much better. I tested on a light post about 500 yards away. I achieved excellent focus on low and mid power. The only difficulty I had was at high power, and even then it was close to being sharp.

At least it makes the Binoviewer usable in the c8 when I want to do so!

Edited by BCEagleScope, 27 January 2024 - 08:26 AM.

[jprideaux]

Another approach might be to use the linear BV which takes up no light path. It has its own constraints with field-stop. I’m not sure how well it would pant with the .63 Celestron Goodlett-reducer. You can also purchase a special .65 reducer to go with the linear. I don’t know if you could then “double-reduce”.