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use of heavy UTA on Dobs

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#1 a__l

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Posted 26 July 2021 - 08:32 PM

https://www.cloudyni...m-90-is-coming/

This thread became an incentive to open this topic.

I want to share my experience.
Many years ago, I repeated the common mistake of beginners who want to maximize their telescope with various accessories.

For example the first post on a my link with an 80 mm finder.
First photo this post, my 18" telescope. I also installed an 80mm finder. What is the finder for and what did it give me in the end?
Basically, this is the initial 2-star setting for DSC operation.
It turned out that push-to in my telescope worked well and there was no point in this finder. The star fields looked much better through the 31st nagler for 18"  how  a relatively small aperture of 80 mm. In fact, the 80 mm finder became a ballast for the telescope for the whole night, except for the start of observations.

After a few years, I found it a burden to use this finder. For balance, I had to add 10 kg weight for the mirror box. This is a significant addition, when my primary mirror weighed 18 kg and I could not make the weight of a mirror box for a classic design less than 33 kg. Those in the end I had to add +10 kg.

I ended up building a "hybrid" telescope with one plywood ring UTA and installing a Telrad on it. In the second photo he is on the left, on the right is my Zambuto 14.5" f/5.  Yes, I was able to significantly lighten the mirror box by replacing the steel mirror cell (> 10 kg) with an aluminum one (~ 3 kg). But after some exploitation, I realized that one ring does not satisfy me in terms of the stability of the telescope (compared to the earlier classical UTA).

I decided to go further. Return to the two-ring design. But already carbon fiber.
Photo by UTA number 3.

Then I had many more upgrades (replacing spider for my design, replacing a focuser, etc.). But the main thing I did. Now, my 18" telescope weighs the heaviest part, comparable to the weight of the heavy part of my classic 14.5" and has become my main telescope when traveling under dark skies. Of the 4 telescopes that I have.

The moral of all of the above is, do not try to load UTA with a variety of devices, which in the future can only make your life not comfortable. Try to make the UTA make it easy as possible by all means available to you.

Attached Thumbnails

  • финиш.jpg
  • 2_Скопа.jpg
  • Карбон_18.jpg

Edited by a__l, 27 July 2021 - 01:06 AM.

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#2 a__l

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Posted 26 July 2021 - 10:52 PM

Another trick where you can ease your UTA. Don't use SIPS.
While it may seem like it has the same weight as paracorr, it is not. SIPS requires the focuser to be removed 80mm from the plane of the focusing board. Accordingly, you will have significantly higher demands on the focusing board and UTA rings. Especially if they are wooden. Consequence, an increase in the total weight of UTA. Perhaps carbon will help you with this. It is considerably rigid than a wood (at the same thickness, about 10 times). But if you want to save some money, think about it.


Edited by a__l, 27 July 2021 - 12:21 AM.


#3 a__l

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Posted 26 July 2021 - 11:26 PM

One more nuance that allows you to lighten the weight of the UTA. Light baffle.
Usually kydex (Obsession) or other plastics are used for this. Or laminate (AstroSystems).
it has a decent weight or kydex can deform in the sun.
Use a 0.3mm thick carbon sheet. It is lightweight, durable and aesthetically pleasing. Paint the inside with black paint, otherwise this structure will show through. Disadvantage, price.



#4 a__l

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Posted 26 July 2021 - 11:47 PM

And more little things.
I have replaced all stainless screws with titanium ones. Including the bolt for the secondary holder (M10).
I recommend using a lightweight FT focuser. But it has a drawback, a weak attachment to the base (milled to make it easier). Therefore, it makes sense to consider if heavy eyepieces are to be used e.g. NV in afocal.


Edited by a__l, 26 July 2021 - 11:48 PM.


#5 sixela

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Posted 27 July 2021 - 02:28 AM

Don't use SIPS.
While it may seem like it has the same weight as paracorr, it is not.

It’s lighter. You get rid of the tunable top and replace it with the FT focuser and that more than compensates for the extra weight of the SIPS assembly (it's lighter by 38g if my memory serves me right).

[I'll grant you that I would have preferred more than two screws to attach either the SIPS or the FT focuser to the small board, though. It's convenient to be able to remove either the FT focuser or the SIPS+FT assembly when you transport the UTA and assemble it to the rest of the scope --less risk of knocking it onto things and it makes the UTA easier to handle-- but two screws aren't enough to make the focuser axis oriientation completely repeatable.]

With respect to the torque: the Paracorr lens assembly and eyepiece will sit at the exact same position. The focuser is of course further from the board but it's not that heavy compared to the tunable top that you've removed.

If your focuser board can’t cope with the SIPS plus focuser plus eyepiece, then if you go SIPSless and plan to use focuser-Paracorr-eyepiece, even though the scope might seem OK with just the focuser, adding a Paracorr and an eyepiece will strain it.

No problems using a SIPS on the UTA I designed, BTW. No problems on an 18” Canopus and a 16” Alkaid either.

 

If you want to go lighter, then you can consider the Explore Scientific HR coma corrector, ditch its tunable top, replace it with an eyepiece holder with M48 threads and parfocalise all your eyepieces, or do something similar with the Paracorr.


Edited by sixela, 27 July 2021 - 02:45 AM.

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#6 a__l

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Posted 27 July 2021 - 05:22 AM

Photo of my SIPS* and P2 with focusers.
The weight of P2 (with head) is equal to the weight of the SIPS kit with optical module for 2" applications (difference is a few grams).
The red arrows represent the plane of the board.
The blue arrows are the focuser's centers of mass.
FT focusers 1.5" travel weigh 385-511 grams (depending on models without jumbo knob). This additional weight is applied by the console at a distance of 2.5 times more for SIPS than for P2.
Further elementary mechanics for a beam with one fixed end (usually plywood).

 

*my SIPS body jammed and I cannot tear him apart, so the real distance to the focuser's center of mass is greater.

Attached Thumbnails

  • S&P (1).jpg

Edited by a__l, 27 July 2021 - 05:31 AM.


#7 a__l

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Posted 27 July 2021 - 05:50 AM

[I'll grant you that I would have preferred more than two screws to attach either the SIPS or the FT focuser to the small board, though. It's convenient to be able to remove either the FT focuser or the SIPS+FT assembly when you transport the UTA and assemble it to the rest of the scope --less risk of knocking it onto things and it makes the UTA easier to handle-- but two screws aren't enough to make the focuser axis oriientation completely repeatable.]
 

No one bothers you to drill the third hole in the FT focuser base (on top of the base, red arrow), cut the threads and attach the focuser to the third hidden screw. Which I did long ago.

Moreover, I did not buy a flat FT base for the third telescope and made it myself smile.gif

Attached Thumbnails

  • FT&S (1).jpg

Edited by a__l, 27 July 2021 - 06:19 AM.


#8 bbasiaga

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Posted 27 July 2021 - 09:33 AM

I just completed a scope with the SIPS.  Like your photo shows, the height off the focuser board of the two assemblies (P2 or SIPS) is almost identical.   I did not measure the center of gravity of the assemblies to compare them, but there is no noticeable deflection on my hardwood focuser board (I use quartersawn wood ~1/2" thick) on the new scope with the SIPS or the old scope with the P2.  

 

 

I also have large finders on a couple of older scopes.  One was a kit from Astrosystems.  It was very bottom heavy so I needed the weight up front.  I use a 60 or 80mm stellarvue finder on that one, and use it a lot since there are no DSCs on that scope.  My 22" I built to have a 60m finder.  But honestly the bearings are so large that it balances fine with or without it.  I used it a lot too, until I added DSCs.  Now I only use it to look at wide field star clusters.    

 

Adding counterweight to the back of the scope is only necessary to accommodate a finder if you add the finder AFTER you already built the scope.  Otherwise, it is possible to design for it by moving the balance point (and therefore bearing size/position) to accommodate it.  This can have some impact on scope size and transportability ,but doesn't necessarily mean the scope has to be heavier. 

 

Brian


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#9 sixela

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Posted 27 July 2021 - 11:05 AM

Photo of my SIPS* and P2 with focusers.
The weight of P2 (with head) is equal to the weight of the SIPS kit with optical module for 2" applications (difference is a few grams).

30-35, to be precise.
 

The red arrows represent the plane of the board.

Quite, but _in use_ the Paracorr will be at the same position. On your photo the focuser would be racked out more on the top assembly.
 

FT focusers 1.5" travel weigh 385-511 grams (depending on models without jumbo knob). This additional weight is applied...

...and the weight of the tunable top disappears and is replaced by extra weight at the SIPS end closer to the board.

In other words: it's a wash.

Mind you, my new scope doesn't have a SIPS either, and that's because I don't want to detune the Paracorr for all my eyepieces just because my two Nikons are a bit troublesome (the Nav HW 17 sits 2mm too far, so you have to detune everything 2mm just to get that one to focus).

Not that the other Paracorr likes the Nikons either. The 17mm Nav-HW sits 1.5mm higher still (because of the 46mm diameter baffle at the end of the tunable top) and the 10mm Nav-HW+EiC10 is still too close on position H if you use the 2" barrel (but that's fixed using a parfocalisaiton ring). 


Edited by sixela, 27 July 2021 - 11:18 AM.

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#10 yeldahtron

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Posted 27 July 2021 - 11:33 AM

I debated the convenience/torque trade-off when I mounted a 50 mm RACI finder on my Lightbridge. I concluded that mounting the finder at the top of the LTA closer the mount trunnion, as opposed to on the UTA near the focuser, was the way to go. This placement isn't the most convenient: I have to squat or take a knee to use the finder, but I can also look up the scope and use the unity finder at the same time to quickly slew to the target area.

While this placement doesn't eliminate the torque induced by the finder, especially when the scope is pointing at high altitudes, it doesn't require any counter-balancing weight. (A lot would be needed if the finder were mounted on the UTA.) I save my counter-balancing weights for my holy hand grenade, the 35 mm Pan.


Edited by yeldahtron, 27 July 2021 - 11:36 AM.


#11 MitchAlsup

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Posted 27 July 2021 - 05:08 PM

All this bickering about whether SIPS is better of not lead me to design something else, again.

 

The problem is that the Paracorr only needs very coarse control of its focusing position (on the order of ½mm)

whereas EP focusing needs focusing precision down to 0.003" (F/3) About 6× finer that Paracorr focus.

 

Secondly, this whole problem of the cantilevered weight of the Paracorr and EP causing high loads on the focuser, its base plate, and the focuser board is all because the focuser BOARD is in the wrong position !!! Think about that again 

 

Remember the telescope mechanics is designed around the OPTICS not the other way around. In fact all that heavy glass and mirror cells are to keep two almost weightless whisps of aluminum at exactly the right positions and with the proper shapes.

 

So, I did something different. After sketching the telescope optics, and placing the paracorr just outside of the incoming light columns, the focuser BOARD goes BETWEEN the exit of the Paracorr and the focuser body !!!

 

13MIPSOpticalPath.jpg

 

The cantilevering weight is almost completely balanced leading to almost zero torque on the focuser board.

 

The focuser carries less load so acts smoother.

 

The UTA is larger making it stiffer and less likely to suffer from tube currents. This also means 6-pole trusses can utilize a shroud without "shroud accommodations" !

The truss poles articulate their loads directly into the stiffest section of the UTA (and in my case directly to the focuser board so there is no path for bending moments to accrue.)

 

And to top it off, the cam of the tunable top is used as the Paracorr tuning mechanism--meaning it is easy to incorporate this into your FUTURE designs. Why do I say future? Because the size of the tube of the UTA necessarily needs to be about 6"-7" larger than the primary mirror (not the 1-2" with std design.)

 

https://www.cloudyni...ic/545307-mips/


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#12 sixela

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Posted 27 July 2021 - 05:27 PM

I like the idea but exposed like that a P2 in these parts would be quite the dew magnet.
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#13 a__l

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Posted 27 July 2021 - 05:59 PM

30-35, to be precise.
 

 

Those am I deliberately cheating by talking about a few grams of difference?
Photo for 2" use. I delete a coarse thread on the SIPS optic module (on a lathe), so the SIPS weight is slightly less than the initial one (5-10 grams is the weight of the delete thread).

Absence thread so that the SIPS module can be used with the FT upgraded by me for prime focus NV.

Attached Thumbnails

  • Вес_SIPS (1).jpg
  • Вес_p2 (1).jpg


#14 a__l

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Posted 27 July 2021 - 06:13 PM

 

Quite, but _in use_ the Paracorr will be at the same position. On your photo the focuser would be racked out more on the top assembly.
 

 

A controversial point. The SIPS module stands for dimensions. I tried to create a real situation.

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#15 a__l

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Posted 27 July 2021 - 06:20 PM

 

Quite, but _in use_ the Paracorr will be at the same position. On your photo the focuser would be racked out more on the top assembly.
 

...and the weight of the tunable top disappears and is replaced by extra weight at the SIPS end closer to the board.

In other words: it's a wash.

 

Those are you saying that only the SIPS head weighs 500 grams?
Paracorr weight with head = SIPS kit weight. And nothing disappears anywhere. So 400-500 grams between the red and blue arrows in my drawing is a pure bending moment additive. Why deny the obvious.



#16 a__l

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Posted 27 July 2021 - 06:52 PM

I just completed a scope with the SIPS.  Like your photo shows, the height off the focuser board of the two assemblies (P2 or SIPS) is almost identical.   I did not measure the center of gravity of the assemblies to compare them, but there is no noticeable deflection on my hardwood focuser board (I use quartersawn wood ~1/2" thick) on the new scope with the SIPS or the old scope with the P2.  

 

You don't seem to understand the main thing in my topic?
I am writing about how you can reduce the weight of the UTA.
Why exactly 1/2 inch? Perhaps for a device without SIPS, it could have been made smaller and easier?

 

 

I also have large finders on a couple of older scopes.  One was a kit from Astrosystems.  It was very bottom heavy so I needed the weight up front.  I use a 60 or 80mm stellarvue finder on that one, and use it a lot since there are no DSCs on that scope.  My 22" I built to have a 60m finder.  But honestly the bearings are so large that it balances fine with or without it.  I used it a lot too, until I added DSCs.  Now I only use it to look at wide field star clusters.    

 

 

All this leads to the fact that you are forced to increase the thickness of the UTA rings, make stronger UTA struts, heavier truss tubes. You end up adding weight to the bottom for balance.

Ultimately, the weight of the entire telescope.


Edited by a__l, 27 July 2021 - 07:24 PM.


#17 a__l

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Posted 27 July 2021 - 07:44 PM

For the Sixela, compare the position of the focuser knob on my telescope (white line) and Kunama (relative to the plane of the board). I think that everything is clear here in order to end the unnecessary dispute.

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  • Kunama.jpg
  • FT&S (1).jpg

Edited by a__l, 27 July 2021 - 07:47 PM.


#18 a__l

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Posted 27 July 2021 - 10:23 PM

In other words: it's a wash.

 

torque roughly

For SIPS 380x1+500x2=1380

For FT+P2 800x1+80x2=960

difference 1.4 times

If you measure the length in parrots smile.gif


Edited by a__l, 27 July 2021 - 10:30 PM.


#19 sixela

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Posted 27 July 2021 - 10:34 PM

No, but it seems nothing will convince you.

I happen to own both a normal Paracorr and a SIPS as do you, so it’s fairly easy to measure both the weight and centre of gravity of both configurations (SIPS plus focuser vs. focuser plus Paracorr with tunable top) with the Paracorr lens at the same distance from the groove where ithe combination attaches to the focuser board.

We can’t convince you and unfortunately it seems you can’t convince yourself either.

I’ll leave it to others to make up their own minds on whether your brilliant demonstration ended the debate once and for all; I’m done with it.

Edited by sixela, 27 July 2021 - 10:36 PM.

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#20 a__l

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Posted 27 July 2021 - 11:08 PM

Trying to make a mountain out of a molehill and in the process misleading the inexperienced.

 

If a Dobsonian is designed and built with all accessories taken into consideration then the balance point can be worked out prior to building.  Counterweights are not necessary if the altitude trunnions are at the proper place.  In the case of SDM scopes, Peter Read and the person ordering the scope plan the items to be included and only after that is the mirror box height and trunnion size and position decided.

 

https://www.cloudyni...ing/?p=11249015

 

Alt bearings is standard for 24 ". You not move in size here.

If m11 tells the weight of the mirror box, there would be additional information. But I am sure that with good optimization and throwing away all unnecessary, you can make the mirror box lighter in weight. At the same time, without changing the materials.
And about transporting this "fly" to a dark site, in a few years M11 will hopefully speak out. Time will judge.


Edited by a__l, 28 July 2021 - 03:30 AM.


#21 a__l

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Posted 27 July 2021 - 11:10 PM

No, but it seems nothing will convince you.

I happen to own both a normal Paracorr and a SIPS as do you, so it’s fairly easy to measure both the weight and centre of gravity of both configurations (SIPS plus focuser vs. focuser plus Paracorr with tunable top) with the Paracorr lens at the same distance from the groove where ithe combination attaches to the focuser board.

We can’t convince you and unfortunately it seems you can’t convince yourself either.

I’ll leave it to others to make up their own minds on whether your brilliant demonstration ended the debate once and for all; I’m done with it.

So what's wrong? You only have words  against my photos and calculations.
Make your photo and calculation.


Edited by a__l, 27 July 2021 - 11:24 PM.


#22 m11

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Posted 27 July 2021 - 11:27 PM

https://www.cloudyni...ing/?p=11249015

 

Alt bearings is standard for 24 ". You not move in size here.

If m11 tells the weight of the mirror box, there would be additional information. But I am sure that with good optimization and throwing away all unnecessary, you can make the mirror box lighter in weight. At the same time, without changing the materials.
And about transporting this "fly" to a dark site, in a few years M11 will hopefully speak out. Time will judge.

I won't be adding more info to my post as you seem to have an issue with my post. confused1.gif confused1.gif confused1.gif

 

I don't share the same concerns as you do, I really don't. I am more than happy with the scope and all aspects that I intend to purchase more. 

 

I know what I was getting into as it is not my first scope. I think I understand why people don't post certain things is because individuals only see the negative in things whether its real or not. I see it as my personal decision and view as do you have you own view. Whether either view contributes to the discussion is another point entirely.

 

Not adding any more to this as I don't want to fuel the fire.


Edited by m11, 28 July 2021 - 06:45 PM.

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#23 a__l

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Posted 27 July 2021 - 11:40 PM

Words again. For me, the language of numbers is more understandable. Give information on the full weight of the mirror box so that it would be clear what we are talking about.

 

I hope this information is not secret. If this is a secret, then all of  posts about SDM in this thread are meaningless. First of all, the posts is Kunama.

See the beginning of this topic for how I lightened the weight of my telescope. I am not hiding information.

All I see in your photos is unnecessary equipment, which I eventually got rid of.


Edited by a__l, 28 July 2021 - 12:19 AM.


#24 sixela

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Posted 28 July 2021 - 03:13 AM

So what's wrong? You only have words  against my photos and calculations.
Make your photo and calculation.

I just did the experiment. To compare apples to apple you need to swap out assemblies with the same eyepiece focused at the correct distance; where things end up depends on the focal plane placement of your scope and will influence both how close the FT with the SIPS sits to the focuser board and how fully racked in the FT is when using the regular Paracorr; in your examples you failed to make the two assemblies functionally equivalent.

 

I just tuned the two assemblies for my scope.

 

-Paracorr lower lens at 21mm on the scope side of the mounting flange when focused, in both cases (obviously the Paracorr is placed identically otherwise placement would be wrong for one of the assemblies.)

-focuser or tunable top set to focus a 21mm Ethos
    + For that lens placement detailed above, there is quite some focuser in-travel remaining so the "regular" Paracorr sticks out a bit.

       I need the extra reserve both because I need some in-travel to be able to swap out the SIPS for a Baader 1.7x GP/coma corrector and binoviewer.

    + The FT in the SIPS is almost fully racked in for the 21E..that's the point and also the bane of the SIPS.

-no 1.25" adapter (which reduces the difference in weight; last time I measured with 1.25" adapters installed but the Paracorr one is heavier than the FT one, so it skewed it in favour of the SIPS).

 

Measured the COG position with respect to the flange, and the weights.

 

SIPS+FT: weight 893g, COG at 51mm from the flange.

FT+Paracorr+tunable top without 1.25" adapter: weight 906g, COG at 45mm from the flange.

 

And no, you'll just have to believe me about the COG, If you want me to post photos of the assembles perilously balanced on a small wooden plank you'll have to repeat the experiment and take photos to disprove me yourself ;-). I only have two hands for this...

 

So that gives me torques in gf.cm of 45543 gf.cm vs.40770 gf.cm. The FT+Paracorr is indeed better by a factor of 1.11.

 

I'd call that 11% difference (in the context of how you need to design a UTA that is rigid enough) a wash.

 

Especially if you consider that there was actually a 21m Ethos that weighs roughly a kilo sitting at the exact same (and larger) distance of the focuser board, which reduces the ratio significantly if you want to support that in your design (after all, we don't just want it to work when the focuser is empty).
 

On my scope focal plane placement has a much larger influence on the torque than which assembly I use. I could tweak it to make it more favourable (with a focal plane closer to the UTA), but then I woud be unable to just swap out the SIPS and use the binoviewer in the FT.

 

Obviously, feel free to find 11% difference in torque with an empty focuser a monstrous difference that renders the SIPS the spawn of the devil that needs to be railed against at the end of every speech one gives in the Roman senate. We'll just have to agree to disagree on that.

By the way, the FT has a much easier time not making the 21E sag just a little when it's close to itself rather than at the end of a Paracorr, especially one in a focuser that is not fully racked in (with the 'regular' Paracorr I have to tune the FT for slightly more friction to avoid sag.)


Edited by sixela, 28 July 2021 - 04:10 AM.

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#25 a__l

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Posted 28 July 2021 - 04:18 AM

SIPS+FT: weight 893g, COG at 51mm from the flange.

 

You have a methodical error in determining the COG for torque.
You define the load as uniform along the length. This is the case for tubes that have the same weight per cm of length (for example). You need to split your device into two parts (at least) for SIPS and FT and define the COG separately for each. Or into three parts (body SIPS, Optical module, FT) and then add torque each with their own COG (from flange). Example book Kriege (page number 180) telescope balance, which is summed up from torque (truss tubes) + torque (UTA) - torque(mirror cell)-torque(mirror) - torque(box).
For P2, these are torque (FT+optics P2)+torque (paracorr head). Or a separate torque (FT)+ torque (optics P2)+ torque (paracorr head). Then you get about 40 percent differences, which I calculated earlier.


Edited by a__l, 28 July 2021 - 05:04 AM.



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