My binoviewers with 8" sct, why won't they focus?

[Emdeejay]

I just bought a pair of William optics binoviewers and a 8" nextsar evolution, love the telescope, and I want to love the binoviewers as well but I just can't reach focus. I've read articles and I tried the Barlow lens solution, it didn't work. The bino's worked in my 4"inch refractor so what I don't understand is why won't they work in an 8"sct that has much higher quality optics. I really wanna get those working, only using single eyepieces gets tedious after an hour or so with all the strain on the one eye. So any advice would be very much appreciated. Do I just need a higher magnification Barlow lens maybe. Lemme know your thoughts thanks.

[Louis D]

1. Are you using a 1.25" diagonal?  This will help minimize the amount the mirror will need to be moved forward.
2. Have you tried both with and without the Barlow?  It should work either way.  I'm sorry to read it's not working with the Barlow, either.
3. Have you tried achieving focus in the daytime on a distant object?  This can help sort out what's going on.
4. Does the image appear to be converging to a focus, and the focuser just runs out of travel before reaching focus?
5. Do you need to use a Barlow with the 4" refractor to reach focus?
6. Have you tried racking the focuser all the way from one end to the other of its range of travel while looking through an eyepiece to ensure the mirror appears to be moving the entire distance?
7. Have you tried adding a 4" extension tube in place of the binoviewer to see if you can reach focus.  The binoviewer adds about 4" to the optical path length.

I've had no trouble reaching focus with my 127 Mak without a Barlow, and I've read numerous places that 8" SCTs should also have no trouble reaching focus natively.  Keep trying to narrow down the issue.

[globular]

You say you have "just" got your scope? So maybe you don't know about the evolution's mirror locks?

These knobs, when engaged (i.e. fully screwed in), clamp the mirror - useful for avoiding mirror flop.

When you're focusing though, they should NOT be engaged (i.e. unscrew them a good number of full turns) so that the mirror is able to move freely and come to focus.

[Apologies if you already know this.... just thought it was worth mentioning.... it caught me out the first time out with my evolution.] 

[Shimrod]

5 hours ago, globular said:

You say you have "just" got your scope? So maybe you don't know about the evolution's mirror locks?

These knobs, when engaged (i.e. fully screwed in), clamp the mirror - useful for avoiding mirror flop.

When you're focusing though, they should NOT be engaged (i.e. unscrew them a good number of full turns) so that the mirror is able to move freely and come to focus.

[Apologies if you already know this.... just thought it was worth mentioning.... it caught me out the first time out with my evolution.] 

It's worth pointing out you only get the mirror locks with the 'HD' optics and not with the standard SCT assembly.

[Stu]

Binoviewers should work with no problem with an SCT as it has a very wide focus range due to the moving primary. Have you gone through the whole range of focus? What’s the setup in terms of diagonal etc?

[michael8554]

7 hours ago, Emdeejay said:

I don't understand is why won't they work in an 8"sct that has much higher quality optics.

Nothing to do with the quality of the optics, only the design of the optics would impact (Newtonian, SCT, Refractor).

To expand on what globular said, SCT's have a huge focus range, sometimes requiring many many turns of the focus knob to get coarse focus.

Despite that, a diagonal + binoviewer + eyepieces may be too long for even a SCT, a Barlow will make it even "longer".

Hopefully someone with this setup will post.

Michael

[Cosmic Geoff]

I have used a binoviewer with both my 127mm Mak and mv C8 SE. The binoviewers add about 100mm to the optical path, so you have to wind the focus knob a lot to compensate for this, but these scopes will handle this. The lengthened optical path will increase the magnification.  You do not need a Barlow lens, unless you want more magnification than the pair of binoviewer eyepieces can deliver. 

If you get the scope in focus with an eyepiece (any eyepiece) as normal, and then pull out the eyepiece a few mm and refocus, this will show you which direction you have to wind the focus knob to get the binoviewer in focus.

[Emdeejay]

Yes I have to use the Barlow on the 4 inch refractor. I feel like when I added the Barlow it almost got more put of focus, I tried it with the 2x Barlow, tried it with the low power Barlow extension piece it came with, tried it without the extension piece. I'm gonna try it during the day today and see what's going on, cause I love the binoviewers and I was so excited to try them on the sct cause the images are just so crisp I couldn't wait for the more "3d" effect they provide. I only tried them looking at stars, maybe I should try it on something bigger like the moon or planets. You're suggestion of an extension tube might help, I was thinking maybe it needed a tube with a few inches since everyone says how the binos create a 100mm optical path etc, but I didn't know if you can just get an extension tube that isn't a Barlow. Also yes I'm using a 1.25 inch diagonal and also it's not the HD so I don't have those knobs... Ugh, I hope I can figure this out, I really wanna be able to use those. 

[Cosmic Geoff]

56 minutes ago, Emdeejay said:

You're suggestion of an extension tube might help, I was thinking maybe it needed a tube with a few inches since everyone says how the binos create a 100mm optical path etc, but I didn't know if you can just get an extension tube that isn't a Barlow. Also yes I'm using a 1.25 inch diagonal and also it's not the HD so I don't have those knobs... Ugh, I hope I can figure this out, I really wanna be able to use those. 

You do NOT need an extension tube to make the binoviewer work with a SCT.  I should know.🙂 Read my previous post.🙂

For a refractor you need LESS tube length, and the focuser may or may not accomodate this, depending on the particular hardware involved.

[Emdeejay]

 

8 hours ago, Cosmic Geoff said:

You do NOT need an extension tube to make the binoviewer work with a SCT.  I should know.🙂 Read my previous post.🙂

For a refractor you need LESS tube length, and the focuser may or may not accomodate this, depending on the particular hardware involved.

 

They work with the refractor alright, the SCT is what I'm having trouble with. 

[Louis D]

23 hours ago, Emdeejay said:

You're suggestion of an extension tube might help, I was thinking maybe it needed a tube with a few inches since everyone says how the binos create a 100mm optical path etc, but I didn't know if you can just get an extension tube that isn't a Barlow.

I suggested the extension tube to REPLACE the binoviewer's optical path length while trying to figure out why you can't reach focus.  Often, you can simply unscrew the optical section from a Barlow and use the tube alone as an extension tube to get a couple of inches of extension for focus investigations.

Do NOT use the Barlow with the SCT.  You should be able to reach focus without it after turning the focus knob many times in the proper direction.  Sure, the focal length will increase some, but not nearly as much as adding the Barlow will increase it.

[Starwatcher2001]

Hi Emdeejay,

As Stu mentioned, have you actually moved the focuser all the way from one end of the scale to the other?  On an SCT it's a ridiculously long distance and far further than you might imagine. On my 925 SCT it more than a minute to get from one end to the other and makes my hand ache. The first time I did it I thought my scope was broken as it just kept turning and turning. Keep winding until it stiffens up, it won't fall off the end. Thankfully the actual range that's needed in practice isn't that big, except when I put my focal reducer in or out, in which case I've got to wind the hell out of it again*.

You might try a little experiment. It's a bit of a faff, but worthwhile and quite educational.

1. Set the scope up during daylight, fairly horizontal and looking at something distant.

2. Set the scope height so that you can sit down to look through the eyepiece.

3. Aim and focus on your object using a long focal length eyepiece.

4. Wind the focuser all the way in until it gets stiff (I believe that's clockwise)  (from Rod Mollise - Author of Choosing and Using a Schmidt Cassegrain Telescope)

5. Remove diagonals, extensions etc.

6. Temporarily attach a metal ruler, straight piece of wood or similar to the bottom of the telescope's visual back (that's the threaded adapter on the back of the telescope tube). Use a strong rubber band, duct tape or whatever.

7. Hold your eyepiece on the ruler/wood, using it as a slide and view through it and the scope to the object.

8. Slide the eyepiece backwards until you finally reach focus. It's a bit tricky to hold everything in place, but easier sitting down.

9. With a bit of care you should reach focus at some point.  Mark the position of the front lens on the ruler, and measure it off (distance A). If you can't reach focus, restart from step 4, but wind the focuser the other way before you start.

10. Repeat steps 7,8,9 with your binocular. Mark the distance to the front lens of the bin, and measure the distance (distance B). If you reach focus, that will prove to yourself that it's possible with that scope/bin combination.

Distance B is how far you need to achieve in total once you put the diagonal and any extension tube back in.  The difference between A and B will tell you how different it is between your eyepiece and bin, and whether this is more, or less, back focus.  I think most of us are convinced that you should be able get that difference using the focus knob alone and shouldn't need any extensions/barlows etc.

Good luck, you will crack this.

 

(* As the change in focus was... erm... winding me up, I bought a motorised focuser. It still takes a fairly long time to move from one end to the other.)

[Emdeejay]

Starwatcher thanks for all the detailed info, I will definitely try that, and yes I was wondering if the focus know was broken haha or maybe they just work different and don't come to a full stop. I'm gonna try that experiment for sure that should definitely clear things up. I just got a focal reducer too so good to know, I probably would have started to panic if that wouldn't have worked either haha. Thanks to all for the info 'll try it out.

[Starwatcher2001]

Emdeejay, if you try your focal reducer during daylight with a long f/l eyepiece, don't worry if the middle of the field of view looks dimmer. Mine does that with a 40mm e/p. I don't see that at night and the wider views are very nice. Cheers, Mark.

Edited June 29, 2021 by Starwatcher2001

[JTEC]

As others have said, should work without a problem.  I routinely used my Baaders with Baader 1.25 prism on C11 with 2x Powermate ahead of the bino. No focus issues at all. And absolutely worth it. The views of many objects - the Moon, for example, and brighter DSOs were so engaging and immersive that, for these targets, I never bothered with mono viewing again. Every reason to think that you’ll soon sort this out - hope so.

[Emdeejay]

Yea I tried the binoviewerast night and they focused but not all the say as crisp as I would have liked, it seemed the 20mm episode were too powerful or something which should not be the case. I could see good enough but if the focus would have been just a little more crisp it would have been perfect. Then there was a double image of Saturn but I got it to dial in to just one image by turning the eyepieces, apparently they have focus knobs, I was not aware of that, but even after all that there was a bright spit in the center field of view, it didn't prevent me from seeing anything but it was just like an illuminated circle in the middle. And as far as DSO objects go like Hercules globular cluster or verious other M objects I can't see see anything like that very good with these. I'm very frustrated with them. I feel like for 300 dollars they should work better than this, much better. Also I tried the focal reducer and I don't think I'm doing it right, it was this tiny little image that made everything harder to see, its supposed to help with DSO's right? I have never used a focal reducer before, so maybe I'm doing it wrong I dunno, but it looked like a small image and super zoomed out the Hercules cluster was so small I could barely even see a white fuzz blob. I took it out so I was back at f/10 and I could resolve it very well and put in a 20mm ep and it looked even better... so I dunno... in not having good luck with my new stuff 😕 SMH, the telescope is awesome my single ep's work great, the binoviewer and focal reducer leave me feeling completely hopeless and ready to pull my hair out. Lol.

Edited June 29, 2021 by Emdeejay

[Louis D]

The focal reducer is doing its job, reducing the focal length and thus the magnification.  This is why everything looked smaller (super zoomed out).  You really only need it for observing objects too large to fit in your lowest power 1.25" eyepiece.

The double image is most likely caused by one or both eyepieces sitting tipped in the eyepiece holders.  This is usually caused by the eyepiece undercut.  Try holding the eyepiece firmly down in the eyepiece holder while tightening the locking collet.  If it still tips, you may need to fill the undercut with narrow tape (auto pin-striping tape works well for this purpose).

The eyepiece "focus knobs" are actually diopter adjusters to allow for different focus positions for each eye if you're not wearing eyeglasses.  If you have the same diopter power in each eye or are wearing eyeglasses, both should be all the way down.  The reason they merged the images for you was because by spinning the eyepiece, you were able to rotate one tipped eyepiece's optical axis to match the other eyepiece's optical axis.

I have no idea what could be causing the bright spot in the center of the FOV.  Perhaps you're actually seeing massive edge vignetting.  It might also be stray light from somewhere in the optical train.

Edited June 29, 2021 by Louis D

[Emdeejay]

Ahh okay, that explains why moving the knobs fixed the double image. The bino's came with no instructions at all so I'm just flying blind here. Also there was no bright spot when I viewed stars only when it was on Saturn, it was like the brighter larger image caused it, I dunno I'm gonna try em during the day and again at night. I wanna get some zoom ep's for it so I can lower the magnification and see if that helps, it looks like if I lowered the magnification a little they would focus real well..... also, so you're saying the focal reducer isn't really necessary if I'm not having trouble seeing things, it's only if I wanna fit something large into view? It won't improve the image of any DSO's? Everything says lower f ratio is best for DSO's so I thought I'd try it but I noticed no difference really. Also it's just a cheaper .5x svbony f/r it just screws on the back of the lens, I could t afford the celestron 6.3 f/r, I don't know if that would make a difference....

[Louis D]

5 hours ago, Emdeejay said:

Also it's just a cheaper .5x svbony f/r it just screws on the back of the lens

I use one in conjunction with a 2x Barlow nosepiece and 45mm of extension between them to create a home brew OCS/GPC to reach focus at about 1x power in my Dob.  However, the 0.5x adds loads of field curvature, so only the central region is sharp.  They are mostly intended for use with cameras with small imaging chips to fit large objects onto them or to reduce exposure time.  They don't see the blurry edges due to their small coverage area.  The Celestron 0.63x R/C not only reduces the focal length, it also flattens the field.  However, it also adds a bit of spherical aberration at high powers that is not visible at low powers.  Thus, you shouldn't use one at high powers to get the best views.