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6" F/8

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#51 stargazer193857

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Posted 24 October 2020 - 08:46 PM

The XT8 is OTA a -48” beam that weighs 20# with much of the weight concentrated in its ends. The OTA is attached to its 21# base at its balance point with two springs that pull it into a pair small diameter altitude bearings. When the OTA is disturbed by touching the focuser for example the image in the EP vibrates (shakes) in proportion to the disturbance.

Several years of experimenting leads me to believe that most of the shaking is caused by undamped ringing at the interface of the OTA and the base, which is the altitude bearings. However, I haven’t proved it to my satisfaction.

Going to a 2-speed focuser reduces the disturbance to the OTA, so reduces the shakes, but adds weight. Astro Goods (https://www.astrogoo.../bearings.shtml) sells a bolt on kit that significantly increases the size of the altitude bearings, but adds weight. When the pandemic ends, I want to take a look at these bearings and perhaps order a set to evaluate.

My original goal was to see if it’s feasible to reduce the weight of the #41 XT8 to 34#, the weight of the XT6, to combine the performance of an 8” scope with portability of a 6” scope, so adding weight is going in the wrong direction.

There’s no doubt that I can reduce the weight of the BB base from 21# to 14# by cutting enough holes in the base, which will definitely reduce the weight of the XT8 to 34#, but the precise effect on the shakes is difficult to predict, and the goal is to reduce the weight without increasing the shakes.

It seems possible that some weight can be taken out of the BB base without increasing the shakes if the primary contributors to the shakes are the altitude bearings as I suspect. For the moment, I am using the XT8 with its BB base while I decide the next step.

Ok. So we are talking about a high frequency shake that happens during focusing.

I wonder if the secondary is vibrating.

The thick tube would be too solid to vibrate.

Maybe the focuser is stressing the sheet metal and vibrating.

I don't think the tube would vibrate on the pads. I think it would be just up or just down.

Only other place is feet and az pads, but sort of doubt that. I think it would not reach the threshold to budge.

Now there is total mass vibration, but I think that heavy mass would not do that.

So my money is on the focuser angle changing, or the secondary vibrating.
The focuser attachment sheet metal can be braced or beefed up. And the secondary would do much better with a front counterweight, which has been tested and beatified to at least protect against collimation drift.



...


How you describe the OTA, sounds like it would have a big moment of inertia. Though who knows how stiff that metal really is. The problem with saving weight on metal is stiffness goes down, even if otherwise strong enough. Through some tubes have flexed under their weight, or got dented at the center rings some other how.

Edited by stargazer193857, 24 October 2020 - 08:49 PM.


#52 gwlee

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Posted 25 October 2020 - 01:57 AM

Ok. So we are talking about a high frequency shake that happens during focusing.

I wonder if the secondary is vibrating.

The thick tube would be too solid to vibrate.

Maybe the focuser is stressing the sheet metal and vibrating.

I don't think the tube would vibrate on the pads. I think it would be just up or just down.

Only other place is feet and az pads, but sort of doubt that. I think it would not reach the threshold to budge.

Now there is total mass vibration, but I think that heavy mass would not do that.

So my money is on the focuser angle changing, or the secondary vibrating.
The focuser attachment sheet metal can be braced or beefed up. And the secondary would do much better with a front counterweight, which has been tested and beatified to at least protect against collimation drift.



...


How you describe the OTA, sounds like it would have a big moment of inertia. Though who knows how stiff that metal really is. The problem with saving weight on metal is stiffness goes down, even if otherwise strong enough. Through some tubes have flexed under their weight, or got dented at the center rings some other how.

I have owned at about eight XT6/8 scopes over the years, so believe that I have fairly good feel for the nuances of the XT design. In the last 2-3 years I run quite of few different tests/experiments with several different XT scopes to try to isolate the problem, and it’s consistent from scope to scope. At the moment, I just have one XT8 with two bases that I am continuing to use for a test bed. 


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#53 Asbytec

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Posted 25 October 2020 - 06:31 AM

 Earlier this year I caught a Jovian moon overlapping its own shadow on Jupiter's face, one of the coolest things I've ever seen.  

 

Val

Me too! Not a Dob, but a 6" aperture, anyway. I was at 240x. (The sketch is embellished in terms of contrast and colors so the onlooker doesn't have to "observe" the sketch for a half hour). 

 

Jupiter 8 Jan 1700UT Io FInal.jpg


Edited by Asbytec, 25 October 2020 - 06:31 AM.

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#54 LDW47

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Posted 25 October 2020 - 08:06 AM

Ok. So we are talking about a high frequency shake that happens during focusing.

I wonder if the secondary is vibrating.

The thick tube would be too solid to vibrate.

Maybe the focuser is stressing the sheet metal and vibrating.

I don't think the tube would vibrate on the pads. I think it would be just up or just down.

Only other place is feet and az pads, but sort of doubt that. I think it would not reach the threshold to budge.

Now there is total mass vibration, but I think that heavy mass would not do that.

So my money is on the focuser angle changing, or the secondary vibrating.
The focuser attachment sheet metal can be braced or beefed up. And the secondary would do much better with a front counterweight, which has been tested and beatified to at least protect against collimation drift.



...


How you describe the OTA, sounds like it would have a big moment of inertia. Though who knows how stiff that metal really is. The problem with saving weight on metal is stiffness goes down, even if otherwise strong enough. Through some tubes have flexed under their weight, or got dented at the center rings some other how.

Using the words ‘shakes’, ‘shaking’ means a difference, a lot more to me than what you are talking might be causing ....... ! Nothing is rock solid, it would never function smoothly but the shakes ?



#55 Asbytec

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Posted 25 October 2020 - 09:22 AM

"Only other place is feet and az pads, but sort of doubt that. I think it would not reach the threshold to budge."

I do not know the construction of the XT design, but I'd add the most effective MOD on my Dob, by far, was placing the feet under the three alt pads. High frequency shakes reduced to essentially nothing. Seriously. No other MOD has made such a huge difference.

Edited by Asbytec, 25 October 2020 - 09:25 AM.

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#56 LDW47

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Posted 25 October 2020 - 10:37 AM

Place it on one of those 1” rubberized 3’ x 3’ pads that you can use for everything and anything  ! The only shake you should get then is if there is an earthquake ....... ?


Edited by LDW47, 25 October 2020 - 10:46 AM.


#57 stargazer193857

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Posted 25 October 2020 - 10:40 AM

Place it on one of those 1” rubberized 3’ x 3’ pads that you can use for everything and everything ! The only shake you should get then is if there is an earthquake ....... ?


Makes me wonder how many of these people are observing on a deck.
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#58 gwlee

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Posted 25 October 2020 - 11:56 AM

"Only other place is feet and az pads, but sort of doubt that. I think it would not reach the threshold to budge."

I do not know the construction of the XT design, but I'd add the most effective MOD on my Dob, by far, was placing the feet under the three alt pads. High frequency shakes reduced to essentially nothing. Seriously. No other MOD has made such a huge difference.

All eight of the XT series scopes that I have owned came from the factory with the feet directly beneath the pads. 


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#59 cuzimthedad

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Posted 25 October 2020 - 12:15 PM

Time to put this topic back on track. Any discussion of XT design quirks should be taken up in an individual topic, and any further discussion should focus on answering the ops original and subsequent questions. Thanks



#60 gwlee

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Posted 25 October 2020 - 12:59 PM

Makes me wonder how many of these people are observing on a deck.

Some people have no other option except observing from a wooden deck if they want to observe from home. For example, people who live in apartments, condos, and town homes do it regularly. I live on an acre in the mountains, so have choices, but must occasionally observe from a wooden deck and have no problem doing so at less than 200x when observing alone. 

 

The shakiness that comes from observing from a wooden deck is much different than the characteristic shakiness the XT Dobs exhibit on solid ground and more difficult to manage. The characteristic shakiness of a perfectly tuned and adjusted XT  dob operating on solid ground isn’t objectionable or even noticeable to many, especially to those who only observe with small Dobs. 

 

To me, it’s very noticeable because I also observe with heavily mounted small refractors that exhibit none of this shakiness, but consider the characteristic shakiness of a small Dob to be a fair trade for the generous aperture they provide. Larger, premium Dobs that I used on occasion were less shaky, so I have been experimenting and tinkering with an XT8 to see what if anything can be done to improve its performance in this respect. 
 



#61 LDW47

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Posted 25 October 2020 - 02:22 PM

Nothing has changed as we get down to brass tacks, that 6” dob is a perfect first scope for any age, for the whole family. The views will be awe inspiring, they were / are for me 10 yrs later, Andromeda and the Double Cluster and Orion Neb and the Auriga Three are all still there waiting !  PS: And the Pleiades ! 


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#62 gwlee

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Posted 25 October 2020 - 03:13 PM

Nothing has changed as we get down to brass tacks, that 6” dob is a perfect first scope for any age, for the whole family. The views will be awe inspiring, they were / are for me 10 yrs later, Andromeda and the Double Cluster and Orion Neb and the Auriga Three are all still there waiting !  PS: And the Pleiades ! 

I agree, but think a 6”f8 is much more than a “beginner scope.”   The XT6 has always been my favorite. For planets at my observing site, and probably at most sites, a 6”f8 is very good choice. 



#63 doug mc

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Posted 25 October 2020 - 09:14 PM

To remove shakes while focusing i use a tangential arm connected to the pinion shaft. You can even use a clothes peg as well.  This way you don't grab the focusing knob introducing shaking, but push forward or backwards with your finger tip. 



#64 stargazer32864

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Posted 25 October 2020 - 09:55 PM

I've never had a problem with "shaking" on my XT8". But what would be the magnifying limit on those rare nights where he has clear seeing. Henderson is smaller than my hometown of Owensboro pop. 56,000+.

 

~Robin



#65 LDW47

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Posted 25 October 2020 - 10:15 PM

To remove shakes while focusing i use a tangential arm connected to the pinion shaft. You can even use a clothes peg as well.  This way you don't grab the focusing knob introducing shaking, but push forward or backwards with your finger tip. 

Just send us a diagram or sequence of pics ! As Viny Barbarino used to say ‘ I’m so confused ‘ !



#66 Bill Weir

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Posted 26 October 2020 - 12:07 AM

After installing a 2” focuser, and rebalancing the OTA to compensate for  2” focuser and 2” EPs, what does the 6”f8 OTA weigh? 

To be honest I have no idea. I added a stack of discarded promotional fridge magnets to the lower side of the bottom end of the OTA and a small speaker magnet to the top of the bottom end. Maybe all of this added a couple of lbs but maybe not. The metal 1.25” R&P focuser was almost as heavy as the Moonlight and the Telrad I replaced the finder with added weigh The weights are more than enough to balance anything even the 31 Nagler yet not throw anything off even if only using a 5mm ortho. Only if the scope is tipped to low altitude (like with NEOWISE) and the 31 N do I need to add extra. This is achieved with a bungee and scope nuts I made out of some ocean fishing weights. This is a very rare need.

 

This scope fully assembled I find light. It’s a one hand carry by the handle to my car from the shed or out to the driveway to observe. I have altered the weight in a few ways. The particle board azimuth circles of the base I replaced with thick plywood after they warped from getting soaked in a storm. I also replaced that thin metal eyepiece tray with a large plywood wrap around tray to hold more eyepieces. Despite all of this it’s still a one hand suitcase style carry. 

 

Bill

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#67 LDW47

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Posted 26 October 2020 - 04:36 AM

To be honest I have no idea. I added a stack of discarded promotional fridge magnets to the lower side of the bottom end of the OTA and a small speaker magnet to the top of the bottom end. Maybe all of this added a couple of lbs but maybe not. The metal 1.25” R&P focuser was almost as heavy as the Moonlight and the Telrad I replaced the finder with added weigh The weights are more than enough to balance anything even the 31 Nagler yet not throw anything off even if only using a 5mm ortho. Only if the scope is tipped to low altitude (like with NEOWISE) and the 31 N do I need to add extra. This is achieved with a bungee and scope nuts I made out of some ocean fishing weights. This is a very rare need.

 

This scope fully assembled I find light. It’s a one hand carry by the handle to my car from the shed or out to the driveway to observe. I have altered the weight in a few ways. The particle board azimuth circles of the base I replaced with thick plywood after they warped from getting soaked in a storm. I also replaced that thin metal eyepiece tray with a large plywood wrap around tray to hold more eyepieces. Despite all of this it’s still a one hand suitcase style carry. 

 

Bill

You are talking about one handing your 20” dob I take it ??  LOL !




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