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Planetery observing with Sky-Watcher 10" Flextube

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#1 Eddgie

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Posted 19 October 2020 - 10:29 AM

Just a note here.  This telescope is a very compromised design and it has a lot of behavioral quirks.

This scope looks small for a 10" and one of there reasons for that is that it is f/4.7, vs f/4.9 or f/5 for many other scopes, but also it uses what would be considered an unconventional tube sizing. Rather than have maybe a more traditional 12" tube, the tube here is only slightly larger than 11".

 

Now there is a consequence here, and it is one that planetary observer's and people that use binoviewers to do planetary observing should be aware of if the goal is to get the best possible planetary performance.

 

Because the scope uses a small tube diameter, this means that the focuser tube can very easily project into the light path.  In fact, this projection can be quite deep. 

 

This picture shows the extent of intrusion when the focuser is racked all the way in and this configuration should be avoided at all costs for planetary observing because as can be seen, the area of the obstruction is quite large and probably combined with the secondary mirror, brings the total obstruction to probably 35% or 40%.  

 

XQE_01261.JPG

 

Even when racked only half way out, the intrusion is still considerable. This focuser has a travel range of 1.5" and at 3/4th inch, there is still considerable intrusion bringing the obstruction to probably 30% 

 

XQE_01301.JPG

 

To be completely free of intrusion, the focuser has to be raked out 1.25".  

 

Now the quirk here is that many eyepieces won't reach focus without the extensions but the extensions are so long that the will force the focuser to be racked in considerably.

 

For this reason, when doing planetary observing, my recommendation would be to avoid extensions and instead try to use a configuration that requires the most outward travel of the focuser tube.  This can be done buy using 2" extensions on the eyepiece of on the 2" to 1.25" adapter (if using 1.26" eyepieces). Rack the focuser all the way out, then draw or slip focus the extended eyepiece as necessary to get the correct rough focus, and then you have .25" of fine focus to play with.

 

Now this only matters if it matters to you.  I have been doing planetary observing for going on 40 years now and my experience is that while no one small thing by itself will usually be an issue, small things do add up, but in this case, if you are using an extension that requires considerable inward focuser travel to reach focus when planetary viewing, the obstruction becomes abnormally high and does lower contrast.  I can see the extra diffraction at the eyepiece.

 

When properly set up (Get rid of three of the mirror retention plates and reduce the size of the remaining ones) this telescope gives quite excellent planetary performance, but using it with the extensions as shipped by Sky-Watcher and having excessive inward focus travel (anything more than half an inch) and you are in SCT land for contrast, and worse if you are running with the focuser nearly all the way in.

 

It is not the best design.  This scope should have one of the low profile Hybrid focusers as an option.  The standard focuser is ill suited to the design

 

And apologies for the poor images.  I did not want to invest more time than essential here to make the point, and while the quality is poor, they are clearly sufficient to show the issue.


Edited by Eddgie, 19 October 2020 - 10:42 AM.

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#2 Eddgie

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Posted 19 October 2020 - 10:39 AM

As can be guessed, I continue to be displeased with the design of this telescope.  While I have mine sorted out reasonably well by now, it is not a scope I would recommend unless someone specifically needed the Flextube compact platform or function (Focal reduction or Binoviewing) but even here, the performance will often be impacted unless care is taken to minimize issues like this.  

 

When doing planetary, Binoviewer users in particular need to be wary of the focuser behavior here.  Many BV users try to keep the power to a bare minimum and this will usually mean the focuser is racked all the way in, and in this particular scope, that is a lot of added obstruction. The first picture above tells the brutal truth.  The only other scope I have seen that had this level of intrusion was the Sky Watcher 130PDS.  



#3 lee14

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Posted 19 October 2020 - 11:05 AM

While I agree that the extra blockage from the focuser tube is an annoyance, and certainly contributes to the total area obstructed, measurements of the first image yield the total area obstructed to be 17.2 %.

 

Lee



#4 ShaulaB

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Posted 19 October 2020 - 11:06 AM

From what I have read in several posts on CN, binoviewers are not recommended for reflectors.

 

Every telescope is a compromise. With this model, the relatively low cost and the convenience of the unique design provide a downside or two, or four, as you have seen.



#5 Ihtegla Sar

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Posted 19 October 2020 - 11:15 AM

Question: Other than lowering the total light gathering power, what effect does this type of obstruction have on the image? Is an asymmetric obsctuction on the periphery worse than having a larger center obstruction?
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#6 Barlowbill

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Posted 19 October 2020 - 11:16 AM

I do not mean to be argumentative but I don't understand your percentages of obstruction.  In my eyes, the first picture shows far less than 35%-40% obstruction and the second picture is much less than 30%.  I appreciate your point, just wondering why I see much less area of obstruction.  Thank you


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#7 Eddgie

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Posted 19 October 2020 - 12:05 PM

From what I have read in several posts on CN, binoviewers are not recommended for reflectors.

 

Where in the world have you read that?  I have been using binoviewers with reflectors for 8 years and they work fine. 



#8 Eddgie

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Posted 19 October 2020 - 12:08 PM

While I agree that the extra blockage from the focuser tube is an annoyance, and certainly contributes to the total area obstructed, measurements of the first image yield the total area obstructed to be 17.2 %.

 

Lee

What you are looking at is not the light gathering here, it is the diffraction effects that lower contrast. Having this much added obstruction is like having a secondary mirror that would be 35% or larger in diameter. It has a very large affect on contrast transfer. 



#9 Eddgie

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Posted 19 October 2020 - 12:11 PM

I do not mean to be argumentative but I don't understand your percentages of obstruction.  In my eyes, the first picture shows far less than 35%-40% obstruction and the second picture is much less than 30%.  I appreciate your point, just wondering why I see much less area of obstruction.  Thank you

I am talking about the total level of diffraction when you add the diffraction of the focuser intrusion to the diffraction of the secondary obstruction.  This would be like having a secondary mirror that was 35% or more in size.   At 25%, the native secondary is well sized for planetary work, but when you add in the focuser diffraction it is considerably more than this.

 

If it does not bother the observer to know that they are loosing contrast that is fine by me, but for people that want the scope to perform at its fully potential, they should make sure that the focuser is racked out as far as possible when viewing planets. 


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#10 lee14

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Posted 19 October 2020 - 12:27 PM

What you are looking at is not the light gathering here, it is the diffraction effects that lower contrast. Having this much added obstruction is like having a secondary mirror that would be 35% or larger in diameter. It has a very large affect on contrast transfer. 

You stated the area percentage of obstruction as being between 35 and 40%. That is simply not the case. As stated, in the image you supplied, the measured percentage is close to 17.2%. If your objection concerns the degree of diffraction effects, that is a different phenomenon. 

 

Lee



#11 Eddgie

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Posted 19 October 2020 - 12:45 PM

You stated the area percentage of obstruction as being between 35 and 40%. That is simply not the case. As stated, in the image you supplied, the measured percentage is close to 17.2%. If your objection concerns the degree of diffraction effects, that is a different phenomenon. 

 

Lee

Yes.  I would have thought that in the context of my post in which I was saying that it would be advisable to use the focuser fully extended for planets, it would be implied that the concern was the the contrast lost to diffraction.

 

A C8 has only a 10% obstruction by area, but the secondary baffle is 33% of the diameter.  Having such a large intrusion in the light path as is caused by the focuser tube is like having a larger secondary mirror which lowers contrast.

 

Forgive me, I thought that in the context that it was posted, it would be obvious that I was concerned about diffraction effects, but clearly I was mistaken to think that everyone would realize this.  So, totally about diffraction and nothing to do with light collection though in the end, these are related. The more area of the primary you cover, the more diffraction, and the more contrast loss.

 

Anyway, my apologies for note being specific and stating that it was the contrast loss that was the concern. 


Edited by Eddgie, 19 October 2020 - 12:54 PM.

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#12 tphili1959

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Posted 19 October 2020 - 01:10 PM

Not sure if it applies to my skywatcher 12 inch collapsible ..when I look at jupiter,it's so bright I sometime s use a polarizing filter to dim it down some..but I think I'll check my focuser anyways

#13 Ihtegla Sar

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Posted 19 October 2020 - 01:11 PM

Now that we got that sorted, any thoughts on whether an asymmetric obstruction on the periphery is worse than having a larger center obstruction?

#14 Eddgie

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Posted 19 October 2020 - 01:23 PM

I do not mean to be argumentative but I don't understand your percentages of obstruction.  In my eyes, the first picture shows far less than 35%-40% obstruction and the second picture is much less than 30%.  I appreciate your point, just wondering why I see much less area of obstruction.  Thank you

The total area (the obstruction of the secondary and the obstruction of the focuser tube shadow) are combined together and when combined, it would be similar to adding this area of the focuser tube shadow to the area of the secondary mirror. 

 

It does not matter where an obstruction is. It can be mirror clips, secondary spider, dust, or in this case, the excessive intrusion of the focuser tube.

 

You can estimate the total obstruction of the system by adding these areas together (this is not at all exact) and this will give a rough approximation of the amount of contrast loss that would occur for a secondary with a similar size area. 

 

For example, while we often omit it when talking about reflectors, a standard thin vane secondary spider adds about 2% contrast loss to the system.  We don't usually include it, but that is just being sloppy. 

 

In this case, if you added the area of the focuser shadow to the area of the secondary shadow, the result would be that the contrast would be reduced about the same amount as if the secondary had been made large enough to encircle the extra area.  The secondary shadow does not really grow in size but the overall effect is that the contrast were lowered in a similar way as if the secondary were made bigger.   Here, the obstructed area is quite large.  The focuser tube is 66mm wide and projects into the light cone by 30mm".  That means that approximately 2.5 square inches of area is being added to the diffraction of the aperture.

 

The secondary of the Sky-Watcher is about 4.8 square inches, so adding 2.5 square inches would bring the total area to 7.3 square inches, so a considerable increase in the area of diffraction artifact.

 

In reality, it is not modeled quite this way. In reality, the two diffraction surfaces would be modeled individually and the contrast loss of one would be added to the contrast loss off the other. This method though gives a decent approximation of the amount of diffraction that would be induced by the focuser tube if left to fully extend into the light path.  

 

It is just like  when we say that as a rough guide, you can subtract the diameter of a secondary mirror from the primary to give the amount of unobstructed aperture you would need to match the contrast.  That model is just a rough model and should be taken for exactly what it is which is an approximation, which is the case here. In this case, it would be the same as going from a 25% obstruction to an obstruction similar to a 33% obstruction.  That is a meaningful increase in contrast loss. 

 

One can accept that if they don't mind, but when it one can avoid it, why not do so? To avoid it, is all one would need to do is to use a configuration that prevents the focuser tube from extending deeply into the light column. That can be free to do.. If you know you can do it and improve the contrast on planets, why wouldn't you do it?


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#15 Eddgie

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Posted 19 October 2020 - 01:32 PM

Now that we got that sorted, any thoughts on whether an asymmetric obstruction on the periphery is worse than having a larger center obstruction?

Using the above remarks, I was using more or less rough aproximations.

To be accurate, you would have to calculate the contrast loss for each obstruction individually.  Once known, the curves are added together. 

For example, suppose the contrast transfer loss were 50% at half of the spatial frequency of the telescope when using the native obstruction.  Now, suppose you model the second obstruction and find that it causes a 25% contrast loss at at the same frequency.  You would now subtract 25% of the remining contrast from the the first obstruction (in these case 50%) so that the total contrast loss at this frequency would be (50% - 25% =) 37.5%.  This is the way one calculates transfer for optical errors. You start with your secondary, then the percentage of loss caused by any kind of error is applied to the existing contrast at different points in the spectral range of the telescope. 

 

The method I used in my post was not going to be exact. It was just a rough approximation.  The idea though, is that in a 10" scope, adding another 2" or 3" of diffraction area is going to have the same effect as making the secondary larger. In this case, it would be similar to going from a 25% obstruction to about a 33% obstruction.



#16 lee14

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Posted 19 October 2020 - 01:41 PM

There seems to be a false equivalency here regarding area and diffraction effects and the effect on contrast. Loss of contrast is a function of the area of the obstructions in the light path. Diffraction is a function of the edges in the light path. Diffraction is a result of the disruption of the wavelets at an edge, producing interference patterns. This is how the negligible surface area of spider vanes produce only the most minor decrease in contrast, but significant patterns of 'spikes'.

 

Lee



#17 Eddgie

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Posted 19 October 2020 - 01:47 PM

Not sure if it applies to my skywatcher 12 inch collapsible ..when I look at jupiter,it's so bright I sometime s use a polarizing filter to dim it down some..but I think I'll check my focuser anyways

The larger the scope, the less of an issue this is going to be because by percentage, the area gets smaller. 

 

In very small telescopes like the Sky-Watcher 130 PDS, it is far more severe because the area of the mirror is already small, and the same size focuser extending the same distance into the light cone covers a bigger area than the secondary mirror.  I owned one of these and abandoned it for my own use when it became apparent that the amount of obstruction was quite serious.  The 130PDS  is a telescope that should not be used in a configuration that results in an almost fully racked in focuser (I was trying to use a reducer which required the focuser to be racked in all the way.. Not a good configuration for the 130PDS). 

 

Still, it is worth checking and if noticed, it can be free to avoid any contrast loss when viewing planets (which is about the only time it will matter).  If you see that you have excessive intrusion when using your extension for planets, it might be possible to reduce it. If you use an extension, see if you can reach focus without it.  If you are using an extenstion and can find a way to configure for less focuser intrusion, it is the same as making your secondary mirror smaller.  It becomes less worth doing as the scope gets larger though.


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#18 doug mc

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Posted 19 October 2020 - 07:18 PM

The crayford focusor on these Skywatcher dobs will without doubt intrude in the light path. The focusor tube even when fully extended out is still within the optical tube. Any inward focusing allows the focusor tube to intrude the light path. I purchased a used Skywatcher two inch crayford focusor to fit on my six inch f/8. I had modified it by moving back the bottom suport rollers by 12mm and shortening the tube by the same amount.   I am hoping to buy a GSO 10 inch dob in the future, having owned the 8inch model a few years back. Did not have that problem with that scope. As Eddgie stated, a low profile focusor raised a little from the tube would fix the problem.



#19 stargazer193857

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Posted 19 October 2020 - 08:01 PM

Don't coma correctors protrude into the light path?

Do binoviewers and coma correctors get used together?

#20 Eddgie

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Posted 19 October 2020 - 10:24 PM

The crayford focusor on these Skywatcher dobs will without doubt intrude in the light path. The focusor tube even when fully extended out is still within the optical tube. Any inward focusing allows the focusor tube to intrude the light path. I purchased a used Skywatcher two inch crayford focusor to fit on my six inch f/8. I had modified it by moving back the bottom suport rollers by 12mm and shortening the tube by the same amount.   I am hoping to buy a GSO 10 inch dob in the future, having owned the 8inch model a few years back. Did not have that problem with that scope. As Eddgie stated, a low profile focusor raised a little from the tube would fix the problem.

I considered cutting the focuser tube but in the end, I decided to just go ahead and get the Orion Hybrid low profile focuser.  This is maybe 10mm shorter than the stock focuser but even when fully racked in, the focuser tube barely extends below the bottom plate of the focuser.  So between the shorter profile and the shorter focuser tube, I should have zero extension into the light cone. 

 

I was thinking about getting a different focuser anyway. The single speed is OK with lighter loads but I recently got back into binoviewing and it was not working so well with that at high power. If the only thing I used it for was with the image intensifier, the factory focuser would have been fine.

 

I am willing to make this investment because I need to get this scope right. It has to be both my planetary scope and my night vision scope. It is the biggest scope I see myself owning now and I want to get it as right as possible for all of the applications I want to use it in.



#21 Eddgie

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Posted 20 October 2020 - 08:33 AM

Don't coma correctors protrude into the light path?

Do binoviewers and coma correctors get used together?

Easy answer first.  There is only one coma corrector that works with binoviewers and that is the Baader 1.7x Newtonian Glass Path Corrector.  Because the light path of most binoviewers is 100mm or more, and because coma correctors have to be spaced a certain distance from the focal plane (note the tunable top on the Paracorr), the coma corrector would have to be designed specifically for the application and again, Baader is the only one that I know of that produces such a corrector.

 

Now there is a way you can use a Paracorr, but it needs to be used in conjunction with the Televue 2x Powermate.  There may be other ways to do it, but typically it is not done and the simple reason is that since you are limited to 1.25" eyepieces so can't see that far from the edge of the mirror, and because most Newtonians can't reach focus without some kind of amplifying optic, coma is typically not a big issue.  When I use the Sky Watcher at the binoviewer truss setting though, since no Barlow is used, I can see a bit of coma, but at f/5.1 and with only a 1.25" eyepiece, it is not all that bad.

 

And yes, they coma corrector could stick into the light path.  In a scope like the Sky Watcher 10" with its undersize tube.  Based on my experience with things like the Baader, it would be pretty easy to get to a configuration where the end of the GPC is intruding into the light path.   

Again, the larger the scope (for a given size intrusion) the smaller the problem, but for a scope like the 10" which is going to be used for high resolution observing, all of these issues do mount up.  For whatever reason this scope was shipped with six mirror clips, when three would be sufficient, and a focuser that can protrude deeply into the light path.  For someone that wanted the best possible performance from their telescope, it would be best to eliminate as much diffraction as possible. I have removed three of the mirror clips and I re-shaped the remaining three to greatly reduce their protrusion, and I am using a configuration that ensures that the focuser tube is almost fully extended.

 

Does all of this make a difference? It think so.  I had one of the best views of Mars a few nights ago that I have had in a very long time. 



#22 stargazer193857

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Posted 20 October 2020 - 11:25 AM

Easy answer first. There is only one coma corrector that works with binoviewers and that is the Baader 1.7x Newtonian Glass Path Corrector. Because the light path of most binoviewers is 100mm or more, and because coma correctors have to be spaced a certain distance from the focal plane (note the tunable top on the Paracorr), the coma corrector would have to be designed specifically for the application and again, Baader is the only one that I know of that produces such a corrector.

Now there is a way you can use a Paracorr, but it needs to be used in conjunction with the Televue 2x Powermate. There may be other ways to do it, but typically it is not done and the simple reason is that since you are limited to 1.25" eyepieces so can't see that far from the edge of the mirror, and because most Newtonians can't reach focus without some kind of amplifying optic, coma is typically not a big issue. When I use the Sky Watcher at the binoviewer truss setting though, since no Barlow is used, I can see a bit of coma, but at f/5.1 and with only a 1.25" eyepiece, it is not all that bad.

And yes, they coma corrector could stick into the light path. In a scope like the Sky Watcher 10" with its undersize tube. Based on my experience with things like the Baader, it would be pretty easy to get to a configuration where the end of the GPC is intruding into the light path.
Again, the larger the scope (for a given size intrusion) the smaller the problem, but for a scope like the 10" which is going to be used for high resolution observing, all of these issues do mount up. For whatever reason this scope was shipped with six mirror clips, when three would be sufficient, and a focuser that can protrude deeply into the light path. For someone that wanted the best possible performance from their telescope, it would be best to eliminate as much diffraction as possible. I have removed three of the mirror clips and I re-shaped the remaining three to greatly reduce their protrusion, and I am using a configuration that ensures that the focuser tube is almost fully extended.

Does all of this make a difference? It think so. I had one of the best views of Mars a few nights ago that I have had in a very long time.


Thank you for that.

Is it also true that a low profile focuser lacks enough range to reach focus with several eyepieces and barlows with a coma corrector while also reaching focus without one?

A tall focuser like yours likely can, but intrudes into the light path in the lower position for CC.

Astrosystems mounts a 0.75" wooden block on the UTA so a low profile focuser will reach focus without a CC . Remove the block, and it reaches focus with the CC. No profusion.

Buy if a CC protrudes, then the bottom of a focuser is not much worse.


Sounds like the answer for binoviewers is truss poles whose length is designed for them. Or a removable spacer below the UTA.



Back to the high profile focuser. It's only value is the increased travel, but it gets the travel by protruding into the light path. That is ok for locational cc use, but seems suboptimal otherwise.

A high profile focuser costs less, and getting the eyepiece further from the light path may increase low power contrast.

#23 Notoriousnick

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Posted 21 October 2020 - 02:37 AM

Right or wrong, your post inspired me to take a hacksaw to my focusing tube.

 

I cut off 22mm. Of course, this means that when part way out, the end of the focusing tube drops off the supporting roller bearings.

 

So I cut out some supporting plastic guide-rails, which I promptly glued to each side of the curved metal piece that supports the roller bearings. These rails are at the height that the bearings were.

 

However, there is more friction this way and so now my focuser is of the old-fashioned type - to reach focus I have to pull-push the tube by hand, then lock it in place. The scope still collimates OK and the focuser tube has no slop. Nor does it stick into the light path anymore banjodance.gif.

 

So not the most elegant solution, but it should work (famous last words ...)  I'll know for sure when I test it out tonight bigshock.gif.

 

Edit: Just tested it. Everything looking sharp (see what I did there?) Note that earlier I had also removed *all* the mirror clips and have had no near-death experiences using the scope like this. So, with clips gone and focus tube shortened, the night sky is looking good!


Edited by Notoriousnick, 21 October 2020 - 06:17 AM.


#24 Eddgie

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Posted 21 October 2020 - 08:16 AM

 

 

I cut off 22mm. Of course, this means that when part way out, the end of the focusing tube drops off the supporting roller bearings.

 

 

Well, that is a radical solution. That would greatly limit your focuser travel.   Hopefully it still has enough for your needs.



#25 Baatar

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Posted 21 October 2020 - 09:00 AM

I just adjust the truss position (without using the 2" eyepiece extension holder) in such a way that allows me to reach focus where the focuser drawtube is almost fully racked out (thus less intrusion).  This also minimizes aperture loss.

 

But this configuration only works with Baader MB2 binoviewer with x1.25 GPC.

 

.




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