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Moving dovetail bar for better balance?

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#1 drneilmb

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Posted 06 June 2020 - 05:03 PM

I very recently got a used Meade Infinity 102 refractor (102mm f/6) to take the place of my Meade Adventure 80 (80mm f/5) on a photo tripod for grab and go. I definitely underestimated the increase in size that the 102 would be, but I really wanted the 2" focuser and a little more aperture (100/64 is nearly two, right?) and focal length.

 

0606201530_copy_968x777.jpg

 

The balance front-to-back on the 102 is much worse than on the ST-80. Not only is it larger and heavier to begin with, but I want to use it with a 2" diagonal and a long-FL eyepiece for wide-field scanning (35mm Bresser gives 17x and 4° TFOV) which just adds to the tail-heavy problem that it already has.

 

0606201531_copy_960x540.jpg

 

If this were one of my ATM reflectors, I would want to make it balance approximately right in between the lightest and heaviest eyepieces that I am going to use it with. I rolled the OTA on a pipe and marked the balance points on the tube (shown above).

 

Is it possible to move that blue dovetail bar back on the tube so that the balance point is better positioned for my use? Any tips on the easiest or the right way to accomplish this? Can I just take everything off the tube, drill new holes where I want them and screw it on in the new spot?

 

Is this a terrible idea or an easy tweak to improve the handling on a light mount for someone who has the will to try it?

 

Thanks in advance for any advice you can offer.

 

-Neil 



#2 B 26354

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Posted 06 June 2020 - 05:25 PM

The current dovetail's mounting screws don't attach to the OTA's thin-walled aluminum tube. They're screwed into an internal mounting plate. Meaning... you can't just drill-and-tap the OTA tube itself.

 

In your second photo, the current blue dovetail appears to be a "Vixen"-sized one. Why not just replace it with a longer one? There are lots of different lengths available from vendors like High Point Scientific or Agena.

 

https://www.highpoin...t/?q=Vixen rail

 

grin.gif


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#3 Shane1200

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Posted 06 June 2020 - 05:32 PM

I had to upgrade my Meade 102 Infinity tube with rings and a I think a 9 inch dovetail so that it would balance when using 2 inch eyepieces on a Eq mount.  You may just need to replace the blue dovetail with a longer one so that it will work on your mount.  I also replace the original focuser with  GSO crayford focuser which made the rear of the telescope heavy when using a 2 inch diagonal and 2 inch eyepiece. 



#4 cookjaiii

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Posted 06 June 2020 - 05:41 PM

You could use rings, but those would add a little more weight.



#5 drneilmb

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Posted 06 June 2020 - 05:53 PM

The current dovetail's mounting screws don't attach to the OTA's thin-walled aluminum tube. They're screwed into an internal mounting plate. Meaning... you can't just drill-and-tap the OTA tube itself.

Hmm, that makes a lot of sense that the tube isn't strong enough to attach directly to. I wonder if the mounting block inside the tube is movable?
 

In your second photo, the current blue dovetail appears to be a "Vixen"-sized one. Why not just replace it with a longer one? There are lots of different lengths available from vendors like High Point Scientific or Agena.

The photo tripod just has a 1/4 inch mounting stud that screws into one of two threaded holes in the bottom of the dovetail block, so I'm not very sure that a longer block would provide those holes further back where I want them. Do those "vixen" dovetails also support 1/4" mounting holes?



#6 t-ara-fan

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Posted 06 June 2020 - 05:58 PM

I had the same problem when adding a MoonLite focuser to my EDT80TCF. Way too heavy at the focuser end.  So I put this contraption together so I could move the focuser closer in. Didn't need to modify my scope.  Just clamp this baby on the existing dovetail. IIRC those 2 clamps are https://www.admacces...vetail-adapter/

 

Vixen under ED80 20160821_224117.jpg


Edited by t-ara-fan, 06 June 2020 - 06:01 PM.


#7 B 26354

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Posted 06 June 2020 - 06:05 PM

Do those "vixen" dovetails also support 1/4" mounting holes?

There are a zillion different ones. Most of them have combinations of 1/4-20 tapped holes and also un-tapped holes and various-length "slots" with which bolts can be used. Measure the distance between the mounting-holes on the OTA, and then try to find a bar with holes or slots that will accommodate that spacing.

 

The post from t-ara-fan shows another approach, but that will be more costly than purchasing a single longer bar.


Edited by B 26354, 06 June 2020 - 06:06 PM.

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#8 drneilmb

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Posted 06 June 2020 - 09:30 PM

I pulled the focuser off and in fact the mounting bolts for the dovetail bar appear to be threaded right into the wall of the tube.

 

0606201757a_copy_654x1162.jpg

 

Even more so, it appears that the distance between the mounting bolts is very close to the distance that I would like to move it back. So I think that I could re-use one of the holes and only need to drill and tap one new one closer to the focuser.

 

0606201803_copy_599x1112.jpg

 

And it even looks like the new hole will miss the first baffle. It might take me a while to work up the courage and clamping jigs to put this idea into practice! Or I could pay half the price of the telescope in machined aluminum just to make it more balanced.

 

Or rings, that's another possibility for sure.



#9 B 26354

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Posted 07 June 2020 - 01:47 AM

Seems like you could just use the two provided mounting-holes to easily attach one of these:

 

https://www.highpoin...ail-plate-vup11

 

https://www.amazon.c.../dp/B07GRTJPD7/

 

https://www.amazon.c.../dp/B000YPTZU2/

 

https://www.amazon.c.../dp/B01CQ0QLOK/

 

Hard to tell from your photo... are those mounting bolts just attached on the inside of the OTA with lock-nuts?


Edited by B 26354, 07 June 2020 - 01:50 AM.


#10 drneilmb

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Posted 07 June 2020 - 08:49 AM

Hard to tell from your photo... are those mounting bolts just attached on the inside of the OTA with lock-nuts?

Sorry, you're right it is hard to see in the photo. The mounting bolts are just screwed into threaded holes in the wall of the tube with no other nuts or blocks inside. The bolts will start into but don't fit the same thread pitch as the 1/4-20 tripod mount, so I'm guessing that they're M6 bolts.



#11 B 26354

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Posted 07 June 2020 - 10:02 AM

....I'm guessing that they're M6 bolts.

Most likely.

 

Pretty surprising. I just would never have thought that the tube wall on a four-inch refractor would have been thick enough on its own to hold those mounting-bolts. Seems a little scary that there are only two of them. Is the tubing steel?

 

In any case..., while some of the holes in those Vixen rails may be tapped, I'd guess that the majority of them aren't... so your M6 bolts should still be OK. I'll be interested in seeing how you end up working this out.  grin.gif




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