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Focusing issue with vx12 and binoviewer (power-switch)

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#1 Trentend

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Posted 22 August 2019 - 03:51 PM

Having problems achieving focus with my Denkmeier standard binoviewer with power switch, panoptic 24mm and ocs a45 when used in a second hand Orion Optics VX12 (the F4 version). Have collimated the scope as best as I can using a Hotech laser per instructions on the astrobaby site (the laser doesn’t appear to need collimating itself). I have pin sharp views with the low power settings, but out of focus with the medium and high even on clear nights. By process of elimination:

 

1. The set up worked perfectly at all 3 settings in an 8inch Newtonian so I doubt it’s the binoviewer.

 

2. Appreciate the 12 inch requires more time to cool, so I’ve tried it after leaving outdoors for 3 hours but get the same result.

 

3. I have a fan on the mirror cell which I haven’t used yet but don’t think that will make a huge difference.

 

4. I have an extension tube between the power switch and ocs. Perhaps need to remove this and experiment with focus travel distances (I do though have sufficient inward and outward movement on the medium and high settings and get close to a focused view, but just not quite there)

 

5. Stars at medium and high settings appear like shuttlecocks, but I don’t think this is comma as the ocs is supposed to fix that (no paracorr required)

 

Any thoughts? Presume the higher the magnification the more pronounced the impact of poor collimation or would you expect if low power is fine then medium and high should be ok also.


Edited by Trentend, 22 August 2019 - 03:53 PM.


#2 photoracer18

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Posted 22 August 2019 - 04:52 PM

Why would you have an extension tube between them? OCS is there because Newtonian systems don't have enough back focus for a BV and you are eating a lot of that up. An F4 primary has a much shorter focus range than an F5 or slower scope. If stars appear like shuttlecocks that is coma. Are you using the A37 or A45 OCS module? The A37 is not going to work well with an F4 primary as its only rated to work up to about F5. (I was a beta tester for Denkmeier in the early years).



#3 Trentend

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Posted 22 August 2019 - 05:11 PM

It’s the A45 and the extension tube (not even sure if that’s what it’s called) is around 3cm in length. Not sure if I can post a picture here? This set up works perfectly at all 3 power levels in my 8inch f5 skywatcher.



#4 Trentend

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Posted 22 August 2019 - 05:20 PM

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#5 Eddgie

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Posted 22 August 2019 - 08:04 PM

First, I am not sure at all what you are saying.  Are you saying that you can rack through best focus to see an expanded pattern on either side of focus but that the stars are not sharp when at the best focus point, or are you saying that you cannot reach focus (not enough out-travel?)

 

If you cannot reach focus, then try slipping the OCS tube out of the focuser.  If that works, then you can try to shorten the extension tube.  Now if you shorten it too much, you won't be able to reach focus in low power so shorten it just enough to reach focus in low power and see if that gets you enough out-travel for high power.

 

If that does not work, then you are going to either have to lift the primary, put on a lower profile focuser, replace the focuser tube with a longer one, or simply continue to slip the OCS tube out for high power

 

If though, the problem is that you can pass trough focus (rings collapse then expand) and the stars are comatic when you are at the smallest focus, then my bet is that your focuser is tilted or sagging under the load.

 

If you have a Crayford like a Moonlight or other Crayford focuser, the binoviewer can put enough weight on the pinion that it allows the focuser tube to pull away from the bearings closer to you, or your focuser could be causing the tube wall to deflect.

 

To check for this, first collimate with the laser in the focuser, then check it again with the laser in the eyepiece holder of the binoviewer.  If the laser does not hit the center of the primary, then you have a focuser that is tilted, or there is sag in the focuser or the tube is deflecting away from the bearings. 

 

I found that many focusers cannot hold the weight of BV with heavy eyepieces and will deflect under the load, especially when the focuser is extended all the way out.  I experienced this problem even with my Moonlight, and eventually had to replace it with a Feathertouch, which is a far more capable design for use with a heavy load with a long moment like the Binoviewer presents.

 

So, I am not at all sure  exactly which of these cases fit your description, but you should have enough now to further diagnose the problem.  


Edited by Eddgie, 22 August 2019 - 08:05 PM.


#6 Trentend

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Posted 23 August 2019 - 04:38 AM

“Are you saying that you can rack through best focus to see an expanded pattern on either side of focus but that the stars are not sharp when at the best focus point”. 

 

Yes that’s exactly it. I’ll try with the laser in the binoviewer to see if the focuser is being tilted under the weight.

 

Also, when collimating, I’m able to get the secondary reasonably we’ll centred when viewed through a collimating cap in the focuser with all 3 clips of the primary in the right place and the secondary looking pretty circular. But when I then adjust the primary and secondary with the laser, the angle of tilt of the secondary makes it more oblong (when viewed again through the collimating cap) and one of the clips disappears. Not sure if that makes sense. I’ll try and post a few photos.

 

Thanks for all your help.



#7 Trentend

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Posted 23 August 2019 - 07:41 AM

Ok, gave this a try. The laser centred nicely in the donut on the primary with the hotec only. When I put the binoviewer in place and swapped one of the eyepieces for the hotec with the focuser fully in, the dot was about 1-1.5 inches away from the donut . When I then moved the focuser fully out there was definitely some drift, not much (around 2-3mm I’d say) but you could feel a little play, even with the various screws tightened up. So definitely need to try a new focuser. Guess what I don’t understand is how the dot can be so far out regardless of where the focuser is?



#8 Eddgie

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Posted 23 August 2019 - 08:31 AM

What happens is that the 140mm light path of the binoviewer acts as a long sight tube, and if there is any focuser mis-alignment (a possibility that I did not mention is that the focsuer itself is not collimated) it will simply be magnified.

 

 

The tube wall itself can be sagging under the weight of the focuser/binoviewer but this kind of hard to identify so you have to identify it by process of elimination. 

 

First thing I would do would be to check the tilt of the focuser itself using the laser in the focuser tube eyepiece holder. Make sure that the beam is hitting the exact center of the secondary.  You may want to mark it (center spot the secondary... This won't hurt anything).  If it is not, you may want to use the focuser collimation screws to get it to center. 

 

Now, put the BV in again, and move the laser to the focuser and look for the deflection.   Once you see it, try manually grasping the focuser and see if you can flex it on the tube and cause the laser dot to recenter. If you can lift on the focuser and get the spot to recenter, then you have some tube flex.

 

If it is not tube flex, then the next thing to do is to check that the focuser tube is not deflecting away from the bearings closest to the observer. Some focusers (Moonlight for example) have exposed bearings.  If you can see the bearings, try turning them when there is no BV and feel the resisitance, then put the BV in and try turning them again, and if they turn freely, then the weight of the BV is deflecting the pinion shaft causing the tube to sag away from the bearings.   This will quickly destroy the front bearings by the way, and before they go, they will start to drag and scar the focuser tube.  I am not saying this will happen, only that it may happen.

 

Anyway, I can point you to the likely causes of your problem, but it will take some work on your part to identify the culprit.

 

I would definitely start with focuser collimation though.   For normal viewing, this really makes almost no difference, but for binoviewing, the focuser collimation needs to be spot on.   Start there.

 

My own problem was a combination of things.  My focsuer was not exactly collimated, but even after doing this, the amount of deflection from the weight of the BV was pretty extreme.  For me, it was the focuser (Moonlight). The focuser tube was deflecting away from the bearings on the observer's side.   I tightened the bearing pad on the pinion shaft tight enough to prevent most of the deflection, but then the low speed knob started to slip.   Moonlight put a refractor preload spring into the focuser and that helped, but then the lower bearings started to fail and scarred up the focuser tube.   I replaced the focuser with a Feathertouch and that solved the deflection problem and even with the heavy weight of the BF and Hyperion zooms, focusing was super smooth and light.  I got the longest tube, and I was able to reach focus in all three powers without having to pull the OCS tube out from the focuser (though I could do that with the Moonlight, which I had ordered with the longest tube). 

 

Anyway, you know what you are looking for now, so I hope you have good success in resolving the issue. 



#9 Trentend

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Posted 23 August 2019 - 09:35 AM

Cheers I check when you say focuser collimation you mean centring the view of the secondary in the focuser?



#10 Eddgie

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Posted 23 August 2019 - 12:06 PM

I mean that the focuser may be tilted and may need to be collimated.

 

The problem with this is that it can be hard to really know using just a sight tube because you can move your eye around to compensate.

 

The best way to see if your focuser is tilted is to measure the distance from the center of the focuser to top of the tube, then remove the secondary and shine the light across the tube and measure to see if it hits exactly the same distance from the top of the tube. 

 

Here is how I do it.  I use a sheet of writing paper and tape it inside the tube over the hole in the focuser and square to the end of the tube (make cuts for the spider vanes as necessary).

 

With the focuser racked in so that the tube touches the paper inside the tube, shine your laser and mark where it hits the paper.

 

Now, move the paper to the other side of the tube and tape it square to the end of the tube.  Shine the laser on it and see if it hits the same spot.  (In most dobs, one of the spider vanes is over the focuser so you can use the opposite vane to check to see that the spot is in the correct left/right position, and the paper spot is used for up/down.)

 

If you are careful you can simply measure this using a good ruler.  This does not have to be perfect, but it should be as close as you can get it.  Most focusers have collimation three collimation screw pairs in the base that will let you tilt the focuser as necessary.

 

Now, I am not saying you have to do this.  If you find the focuser is sagging (which is most likely) then correct that and see if you get the same spot on the secondary, but if that fails then my guess is that the focuser is tilted or the tube is deflecting.  The next step to determine which is the case would be to check the focuser for tilt, and if the focuser does not have any, then that would indicate that there is tube wall deflection.  (I have a doubt that this is the problem, but it could be.  Until you check focuser tilt and focuser tube deflection you won't know.).

 

It is a pain I can assure you, but the very long light path of the bioviewer, power switch and in your case, filter switch (a total light path of about 160mm, if you have tilt, tube wall deflection, or focuser tube sag against the pinion, even a vary small amount of error in any of these places is going to throw your collimation off and it will change as the orientation and load on the scope changes.   

 

So, we have diagnosed the problem as being with one or a combination of these conditions, and now you have to make a decision as to whether you want to first eiliminate focuser tilt, or try to work around it.  My advice though is to start with focuser tilt because everything else will be affected by this.  

 

If you decide not to start with focuser tilt, then look carefully at the focuser tube for defection.  If you can't access the bearings, just look at the spacing round the end of the focuser tube with and without the bionoviewer.  There should be a small concentric gap between the outside wall of the focuser tube and the inside wall of the focuser housing.  Check it without the BV and make sure it is even.  Next, put the BV in and look at the gap and see if it changes.  In particular, the gap that is on the side of the pinion shaft is the one that usually shows the error,  The gap will get smaller with the BV. This is because the pinion is compressing the nylon or Teflon bearing that it rides in. It is a very powerful moment having 160mm of moment.  Most focusers are not going to handle this without great difficulty, and even ones that do handle it will go through bearings much faster than with even heavy single eyepieces.

 

You might also be able to probe the bearing race with a long, thin piece of paper or plastic.  With no BV, just push the thin material into the focuser housing at the place where the bearing tracks show on the focuser tube, and measure how far it goes in.  Next, put in the BF and re-insert the thin material. If it goes in further, this means that the focuser tube has pulled away from the bearing and the pinion and front bearings are suffering the entire load. 



#11 Trentend

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Posted 27 August 2019 - 03:36 AM

Thanks for this. Will give it a try. Also, thinking of checking collimation again but using a single eyepiece rather than the binoviewer just to confirm I’m able to collimate and see stars pin sharp and there’s nothing fundamentally wrong with the scope. Presume using say a x2 Barlow and 12mm eyepiece I should be able to replicate the medium and higher power binoviewer with 24mm panoptic? If this works I guess that narrows down any issues to either the focuser (sag etc) or binoviewer itself (although the latter is perfect in my 8inch dob).


Edited by Trentend, 27 August 2019 - 03:37 AM.



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