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Mark at Beaufort

Would a longer dovetail improve OTA flexing

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I recently purchased a Skywatcher 150P f/5 Newt to be used on my Skywatcher AZ5 Mount + the AZ mount extension which also sits on a metal tripod (not the aluminium tripod normally provided). When I use the Heritage 130P OTA I don't get any flexing and I note that the dovetail on the Heritage is 33.5cm  rather than the 21cm dovetail on the 150P. I accept that the Heritage is smaller and less weight than the 150P which might make a difference.

My question is would changing the dovetail to a 33.5cm improve the flexing of the 150P OTA? At the moment its great for widefield but once the mag gets high I get a great deal of shaking and the image takes a while to stop 'wobbling'.

I could purchase the new 3/8ths metal tripod to get more stability ( https://www.firstlightoptics.com/alt-azimuth/sky-watcher-38-stainless-steel-tripod.html ) but I rather check the dovetail first. 

Any thoughts?

PS - before someone asks the dovetail on the Heritage has different hole placements so won't work.

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Hi Mark

I suspect that the tripod is the issue for that scope. I am not sure of the capacity for the metal tripod you are talking about but if it's like the EQ5 one then it would make a bigger difference than a new dovetail I'd wager.

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I don't think the length of the dovetail would make a difference, Mark but I'm with Moonshane, a more substantial tripod would make more sense. A vixen dovetail on a 6£ reflector should be fine it is only on the 8"-10" Newtonians that their limitations become apparent.

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My two cents please lol, assuming you have a tripod which is suited for your OTA, a longer dovetail would only remedy flexing if the scope was secured to the dovetail along it entire length. If you have a longer dovetail and it’s supported only in the same points as a shorter dovetail then the extra length of a longer one would just be added weight.  I may be wrong but I think it makes sense.

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Thanks Shane, Steve and Sunshine. I have seen the new upgraded tripod at the recent SGL star party and it is certainly much larger and stable than my present one.

I will treat myself especially as FLO has it in stock.

 

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I seem to remember from the tal 100 days that a train of thought suggested a longer dovetail allowed more flex, the tripod will be a good investment

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It sounds more like overall shaking than OTA flexing, really, so a better mount and tripod is certainly the better bet. I upgraded the flimsy aluminium legs of an EQ2 tripod to hardwood, and that made a world of difference in the stability. Replacing teh EQ2 mount with a Mini-Giro completed the transformation into a usable system

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You mention AZ extension, I assume it’s an extension tube between your tripod and mount? 

Ive been experimenting with steel and aluminium extension tubes between my AZ4 steel legged tripod and Skytee 2 / Tak 100 setup and found the longer the extension tube the more vibration introduced at high magnification.

Top heavy! 

Thoughts?

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6 minutes ago, jock1958 said:

You mention AZ extension, I assume it’s an extension tube between your tripod and mount? 

Ive been experimenting with steel and aluminium extension tubes between my AZ4 steel legged tripod and Skytee 2 / Tak 100 setup and found the longer the extension tube the more vibration introduced at high magnification.

Top heavy! 

Thoughts?

Yes the extension tube sits between the tripod and mount. I had a SkyTee 2 mount with a Berlebach tripod. I did place an extension tube on top of the Berlebach but found it was unstable for my liking. However, the SkyTee mount + Berlebach was a brilliant set without the extension tube.

I have now ordered the new Skywatcher tripod and I am going to experiment with and without the extension tube to see what is best.

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Yeh I bought the skywatcher 16” steel tube but it was too long and heavy so managed to source a 4” OD aluminium tube and started off with it at 12” but to much vibration so cut it down into 3 pieces from 3” to 5” long.

I’m still fiddling around but I’m nearly there. With the OTA pointing towards the zenith I’ve got plenty of clearance from the tripod, stability and ease of viewing.

image.jpg

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Possibly not suited to your mount - but for imaging I recomend the use of a longer than supplied dovetail bar and also the use of a Top Bar to add extra stabilty. Beyond that I woul also suggest using the Lossmandy style bar- if your mount allows it. For my metal tube ultra fast Newts I even custom made some 1" thick, 0.5m long Lossmandy bars for ultimate support. They certainly help to lesson any flex- critical at F2.9 !

 

6" F4 Newt with a 33cm Lossmandy dovetail and a top bar fitted. Notice how widely spaced the tube rings now are (I think the the supplied dovetail was 20cm and doomed to faliure!)

38659143754_230f985d9d_b.jpg

Edited by laser_jock99

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I'm sure the tripod will make a big difference Mark. Is it possible to tell where any other vibration is coming from? I do know from larger Orion Optics newt a that there is flex in the tube walls which allows so vibration when mounted on an alt az or EQ mount. I guess your logic was that spacing further apart would help support the tube better? I still think this may be something worth investigating depending on how much of the problem is solved by the tripod.

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Guest chaz2b

Also, consider placing anti-vibration pads under your tripod, if you haven’t already.

 Adding greater strength through bigger bars is adding weight too, so you may need to add a bigger or another counterweight to compensate.

 I’ve just fitted my Capewise 8” to my paramount,which can easily take the weight, but I did fit a longer bar for extra ridgity , and, pads which do help.

 

chaz

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When I first fitted my 200p to the AZEQ6, even with the mounts larger saddle, it became evident just how much flexure is in the standard Skywatcher vixen dovetail. The mount is rock solid but I can easily induce a wobble in the bar.

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I have tried a number of different dovetails and I think there is a benefit from a longer, stiffer dovetail that securely grasps the OTA closer to the ends.  Having said that, I doubt that just changing the dovetail length from 21cm to 33cm would have had that much effect on its own.  There would be other causes such as how well does the dovetail fit in the saddle  and if there is any flexure at the tube ring mounting points. 

Graeme

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I agree with a stronger tripod but also was wondering if wider tube rings might help. The rings on my VX6 are pretty substantial as it the plate. 

 

2018-11-23_09-40-45.jpg

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