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Setting up a Dob for Binoviewers

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#26 bcarter1234

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Posted 31 January 2020 - 02:56 PM

According to UPS I now have 1.25" 2.5X PowerMate and T2 adapter. I also of course have rain and clouds through tomorrow afternoon.;-)

Take care,
Brent

#27 bcarter1234

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Posted 31 January 2020 - 05:52 PM

Installed. Thanks to our sponsor for such a quick turnaround on a standard order. I called yesterday about 2pm EST and they were here 24 hours later.

PowerMate
 
I think I'll turn another 2" adapter to be able to push big flange that says "Tele Vue" 1-1/4" Powermate" right up against the focuser drawtube. Seems a shame Tele Vue didn't just make the body with a T2 thread. That would save another 10mm.
 
Tomorrow night shows some clearing so I'll try it then.
 
Once I see where the unit sits I may make an alternate "focuser" plate for my UTA. It will just have a 2" bore with a place to put two or three nylon setscrews. That will give me a very stiff, very low profile holder. I'll rough focus manually and then use the helical focusing on the eyepiece holders to fine tune. Might as well use them since they are there. 
 
Take care,
Brent

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#28 Eddgie

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Posted 31 January 2020 - 07:35 PM

Looking forward to see how it works out. I know in my Lunt 80, this configuration required about 5mm of inward focuser movement, but I was only using a BV with 96mm light path, though with the telecentrics, it should not make much difference. 

 

I used a 3D printed 2" plug.  Worked great.



#29 bcarter1234

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Posted 31 January 2020 - 09:31 PM

Eddgie,

 

Thanks, me too.

 

According to Denis the light path on the Leicas is 146mm if I recall correctly and that does not include the adapter on the quick changer. Once things settle down I'll actually measure the light paths of the various components and combinations.

 

It's all clouds here tonight so I went ahead and turned the 2" plug. As above here is the PowerMate installed.

PowerMate
 
The new "no profile" adapter fits over the 1.25" and 1.5"  sections so the larger flange will fit right up to the drawtube. If I find I need more I can take that lovely TV adapter and turn the flange down to 2". That would recover another 10mm of backfocus.
PowerMate adapter
 
The one for the Baader T2 adapter is next to it. They both look very similar on the inside.
 
The large chunk of PVC next to them will become my low profile non moving drawtube. It will allow the BV and any adapters to get as close to the secondary as possible. I hope to be able to get the Burgess OCA to come to focus with no extensions between it and the Baader adapter. If that works I can use empty filter holders as extensions to provide a range of magnifications and work with one set of eyepieces if so desired. 
 
If there is back focus to spare the UTA will get raised to provide a larger fully illuminated zone. Currently it is about 6mm and I'd like to raise it to 13.
 
Take care,
Brent

 


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#30 bcarter1234

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Posted 02 February 2020 - 02:07 PM

Good news first. They come to focus easily with a whopping 20mm of backfocus to spare. Based on the view of the Moon and two timed passes of a star I'd say it worked between 2.2X and 2.3X. So no full disc views available with this combo but close.

 

Now for the bad news. At least with the 30mm Parks GS-5 there was pretty significant vignetting of the off axis rays. Back to the ray trace. Is there an easy way to calculate and then draw the lightpath as it enters PowerMate?

 

It could be my imagination or lack of experience but it seemed to me that individual stars were larger relative to the entire field of view than I'm used to seeing as if I was losing some resolution. Is that even possible? I didn't notice this when using a Barlow previously at nearly twice the magnification.

 

It also seemed I could only get a good image right at the center, once a star moved off center the image deteriorated quickly. Seeing or something else? I put a single eyepiece in later (not advised as you really notice the difference in mage brightness when you switch) and while seeing was only average at best the image wasn't subject to the rapid deterioration.

 

With the benefit of daylight I'll also try Eddgie's excellent suggestion from another thread and see how much of the primary I can see through the binoviewer with it set at infinity focus.

 

Will I need to switch the 1.25" PM for the 2" version? 

 

The saga continues.

 

Thanks for reading.

 

Take care,

Brent



#31 bcarter1234

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Posted 05 February 2020 - 10:00 PM

The adventure continues. The 2" 2X PowerMate and T2 adapter arrived. I put them in the Dob to take a quick look at the Moon high in the in the twilight sky. The view was, well I'll have to let you know after finish the PVC low profile non moving drawtube, mount it to a new focuser plate and replace the existing assembly. Focus could not be achieved with 20mm of backfocus. Does anyone use a 2" 2X PowerMate with a Dob for this purpose? If so could you let me know how much additional backfocus was needed?

 

The new assembly will provide another 42mm of backfocus, 45mm if I go with a slightly thinner mounting plate. My hope is that I'll need somewhat less, in fact something like 30mm less. If that turns out to be the case I can make new 30mm longer truss poles to provide a larger fully illuminated field.

 

At this point I'm easily in deep enough to have purchased the primaries for a 12.5" binoscope. At least I'll have a lot of surplus glass to sell once I find the solution. ;-)

 

Thanks for reading.

 

Take care,

Brent 



#32 25585

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Posted 07 February 2020 - 10:23 AM

I have come to feel that the Televue 2X Binoviewer Amplifier is an excellent option but the Baader 1.7 Newtonian unit is also a very attractive choice, and some people recommend the Seibert OCA which is supposed to give 1.3x or something. 

 

So, lots of options if you want a bit lower power. 

 

But yeah, it is the undersized primary here that would have the big impact but once you get down to the f/4.5 speed, there really is not a 1.25" binoviewer that will allow full aperture performance.  Here though, it would be the undersize primary doing the most harm. Just the geometry of Newtonians.  You can't beat it with bigger apertures really because bigger apertures only work if the prisms are bigger, and the bigger the prism, the longer the light path, which would require even more movement of the secondary to the primary, so that kind of nulls out. You would get a bigger field, but at the cost of more aperture loss. 

h

BV Is very constrained by geometry. 

Would using identical Barlows under each eyepiece, have the same or a different effect?



#33 junomike

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Posted 07 February 2020 - 06:07 PM

Would using identical Barlows under each eyepiece, have the same or a different effect?

Using a Barlow before the Prism (under eyepiece) increases the mis-collimation of the Prisms by the factor of the Barlow (2X, Etc).

IME It's always best to use the OCS where it was designed to go.



#34 bcarter1234

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Posted 08 February 2020 - 09:19 AM

The PVC low profile holder and new focuser plate were installed yesterday. As usual I missed something by some small amount and had to raise the secondary on its stalk by about 5mm. On the plus side that means an extra millimeter of fully illuminated zone. ;-)

 

Stars came to focus with 27mm of backfocus to spare. Measuring this against the GSO 2 speed linear bearing focuser shows a shortfall of 16mm. To restore the convenience of the focuser vs the manually adjusted PVC holder a new focuser plate will me made that will allow the baseplate of the GSO focuser to be mounted flush with the inside face of the focuser plate. It will provide the benefit of a low profile focuser for about an hour's work without springing for a new Feather Touch for now. Of course the low profile focuser's drawtube insertion into the light cone will be there too but everything is a compromise right?

 

Images looked promising considering nothing was allowed sufficient time to cool and seeing was not amazing. Assuming the 30mm Parks to be 52 degree according to the specs the 2" Powermate was operating at about 2.2X. A given star made the passage from edge to edge in 116 seconds. That works out to a 0.48 degree field of view and 108X verses the eyepiece only magnification of 49X. As the intent of this is to complement a binocular telescope does an excellent job from 23X to 117X that low end number is a good fit.

 

A pair of 18mm UFF eyepieces also showed a decent image for the conditions. No star transit time was measured but the magnification would have been around 180X. No change in merge even with my bizarre alignment. I have to say the Moon was impressive at both magnifications. At 108X it just exceeded the field stop in every direction.  

 

Once the focuser is reinstalled checking collimation while observing and visually observing the amount of cut off as the image leaves the center will be possible. It would be nice to have even a decent approximation of the ray trace of a Powermate. I suspect I'll see more image cut off as an out of focus star nears the edge than I do when using a standard Barlow operating at closer to 3X. Does that come from the extended light path of the binoviewers? If so moving the secondary up to get another 3-4mm of fully illuminated zone may not yield much visible benefit.

 

Recognizing I know nothing about optics could a Powermate like device be made that reduced the image magnification and extended the optical path? Something like that would be a boon to binoviewers and binocular telescopes.

 

Thanks for reading.

 

Take care,

Brent



#35 bcarter1234

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Posted 09 February 2020 - 03:10 PM

The focuser board has been modified to allow mounting the focuser on its inside face. The result is a poorman's low profile focuser with an additional 20mm of backfocus. 

 

The three photos to follow which could end up in any orientation. They are all correct on the phone. ;-)

 

Here is the non moving low profile drawtube used temporarily to determine the focus location of the Leicas and 2" PowerMate.

Low Profile Draw Tube
 
It worked fine but the having to rough focus by hand and fine tune with the helicals on the Leicas, it was never intended to be used for more than testing. For viewing it would work fine. For checking an out of focus image it was pretty clunky.
 
This shows the modified position of the GSO focuser mounted on the inside of original board.
Focuser board
 
Here it is installed on the telescope.
Focuser installed

 

Focus can now be reached with about 3mm of backfocus to spare. Should more prove necessary it could be done by modifying the front face of a new focuser board to allow the knob assembly to be recessed slightly into the face.

 

Thanks for reading.

 

Take care,

Brent   


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#36 bcarter1234

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Posted 10 February 2020 - 09:32 AM

Last night was an opportunity to run it through its paces and it worked with no issues. There is sufficient focuser travel to get to a nice disc on both sides of focus so collimation can be checked while observing. M42 and Sigma Orionis looked great with the 30mm and 18mm pairs providing 108X and 180X respectively. 

 

With considerable weight of metal and glass hanging off the UTA the F/4.5 system does reward resetting the collimation for significant changes in altitude. Next up is a set of new trusses and truss connections to minimize this tendency. The original wooden trusses look great and did fine with just an eyepiece but the 2" PowerMate, binoviewer and dual eyepieces add a substantial eccentric load to the structure. 

 

I'm sold on the benefits of tiny adjustments to collimation while observing. Most of the time a 10 to 20 degree of turn of one knob is all that is required. On average nights the star image in the center changes from looking like the end view of a marshmallow to a used foosball. When seeing is good the foosball is replaced by a ball bearing. Off center coma is greatly reduced, particularly to one side of the field.  

 

My favorite accidental find of last night was a new to me red blue pair that was located at the time about 10 degrees east and 4 degrees south of Sirius. I thought "Wait that looks like..." Sure enough when I looked up my "discovery" it turns out to be H 3945 "Winter Alberio". If you like the original this pair will not disappoint.

 

As I was trying to make a mental note to remember the location of this pair it occurred to me to call it "Sirius Radio". 10 degrees east, 4 south... 10/4, Sirius...radio as in C/B, yeah you need to be of a certain age.

 

Thanks for reading. 

 

Take care,

Brent


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#37 Bob4BVM

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Posted 11 February 2020 - 10:31 PM

Last night was an opportunity to run it through its paces and it worked with no issues. There is sufficient focuser travel to get to a nice disc on both sides of focus so collimation can be checked while observing. M42 and Sigma Orionis looked great with the 30mm and 18mm pairs providing 108X and 180X respectively. 

 

With considerable weight of metal and glass hanging off the UTA the F/4.5 system does reward resetting the collimation for significant changes in altitude. Next up is a set of new trusses and truss connections to minimize this tendency. The original wooden trusses look great and did fine with just an eyepiece but the 2" PowerMate, binoviewer and dual eyepieces add a substantial eccentric load to the structure. 

 

I'm sold on the benefits of tiny adjustments to collimation while observing. Most of the time a 10 to 20 degree of turn of one knob is all that is required. On average nights the star image in the center changes from looking like the end view of a marshmallow to a used foosball. When seeing is good the foosball is replaced by a ball bearing. Off center coma is greatly reduced, particularly to one side of the field.  

 

My favorite accidental find of last night was a new to me red blue pair that was located at the time about 10 degrees east and 4 degrees south of Sirius. I thought "Wait that looks like..." Sure enough when I looked up my "discovery" it turns out to be H 3945 "Winter Alberio". If you like the original this pair will not disappoint.

 

As I was trying to make a mental note to remember the location of this pair it occurred to me to call it "Sirius Radio". 10 degrees east, 4 south... 10/4, Sirius...radio as in C/B, yeah you need to be of a certain age.

 

Thanks for reading. 

 

Take care,

Brent

Hey Brent,

Nice job on  3945 ! :)  I love "discovering" stuff by just scanning around like that. And binoc-vision makes it so easy & fun. That's one reason I enjoy kicking back in my motorized bino-chair for an evening tour of the Heavens. So many treasures to discover or re-discover !

 

Nice job on getting the BVer running on the dob. So sweet to have all that aperture and 2-eyed viewing rolled into one.

 

I agree with you on the bennies of OTF collimation tweaks.  Do you have knob extensions or do you have to go behind the mirror ?  Since building the cells for my B-scope and testing them in my Old Dob structure, I too am sold on Collim Tweaks. My B-Scope cells have collim motors so I can do it with a couple pushbuttons right at the eyepiece, and I do it whenever I think I can get a sharper star image...  So you might consider motors to drive your bolts. Or if you are replacing the trusses you could go with 6 vs 8 and do the hexapod collimation route per Oberon's "Merope".  That was what I had in the works for the rebuild of my Old Dob, before I acquired a second 17.5" optics set and got diverted on the hairbrained binoscope project. Hexapod collim is the cats meow and like motors, it also can be done while observing in the EP.

 

CS

Bob




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