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#1 Goofi

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Posted 07 August 2014 - 02:37 AM

Is it possible to have too little weight on your mount?

 

I  know you can overload a mount, putting more weight on it than it can  handle; can you put too little and underload it?

I ask because I've always tried to keep the weight on my mount as low as possible. When I swapped out scopes recently, I more than doubled the load my mount was carrying ... and guiding performance is much, much better.

 

Looking back through my notes, I've found myself asking this question 7 times in the last 15 months. I tried googling this question, but that didn't help much. 

 

Just thinking out loud - is it possible I was too light, and the gears were not properly engaged?



#2 jgraham

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Posted 07 August 2014 - 06:38 AM

My quick response was no, there's no such thing as an under-loaded mount. However, that becomes an interesting question when you consider what a combination of a light load and long focal length does to guiding. One of my imaging rigs is very light, an SN6 on an Atlas without a guidescope. However, if I were actively guiding I'd either want an over-hung load sufficient to keep the gears enaged or a mount that was of high enough quality that the gears were always engaged. So I guess the answer is... it depends. For me, a lighter scope is generally a smaller scope and I don't use active guiding. However, if I did I would not be surprised if I saw the adverse effects of the gears not being loaded. It could be that the rule of thumb of having the scope slightly east-heavy (and possibly north/south heavy) still applies with lightweight scopes.

 

Food for thought.



#3 Jeff2011

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Posted 07 August 2014 - 08:42 AM

Although balance would be the first reason to come to mind you would also have to look at your guiding system.   Was the guide scope and camera the same between the two setups you are comparing (are you comparing apples to apples)?   Could flexure or robustness of guide scope mounting cause a difference?  Then there is seeing.  Perhaps you had a few good night of seeing with the heavier scope. 

 

My experience has been the opposite.  I got better guiding with the lighter scope, that is the AT72ED.   I mostly suspect balance issues because of the longer moment arm of the TAK.


Edited by Jeff2011, 07 August 2014 - 08:45 AM.


#4 Madratter

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Posted 07 August 2014 - 08:47 AM

I'm hesitant to say that less weight would make that kind of a difference. I think it is more likely the loads are balanced differently or something. But then at the end of the day, I do believe in the primacy of experimental results. You just need to be sure that all the other variables are controlled properly.



#5 MikeMiller

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Posted 07 August 2014 - 10:07 AM

Perhaps try putting some extra weight on the lighter scope with one of those counterbalance weights? If you get both scopes the same with weighting you can compare if it is the weight or something else that is going on.



#6 Goofi

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Posted 07 August 2014 - 10:35 AM

I dreaded putting the 110ED on my AVX - I see the guiding graphs everyone is getting and I wasn't getting them as tight. I figured the added weight would overload my mount and the stars would be horrible. But my guiding is much tighter ... I went from a dec correction every 5 seconds to averaging one every 80 seconds!

 

It's a different refractor - but still a refractor. With the AT65EDQ I found the stock counterweight too heavy, so I got a lighter one from Orion and use that. The scope weighs less than 5lbs, with everything (scope, camera, finder, guider, counterweight, etc), I'm putting just under 15lbs of weight on the mount. Swapping to the 110ED, I use the heavier counterweight, and the total weight goes up to 36 lbs. 

 

I'm setting things up the same way as before - using 2+2 stars for calibrating, ASPA for PA (I didn't bother with alignmaster last night just to make it harder). Yet my guiding graphs are flat! I changed focal length and camera settings in PhD, so it's not that. The only other change is I'm not using the OAG (I'd have to adjust the prism and I'm not going to do that unless I leave the scope on for a lot longer)... I'm using the SSAG with a fingerscope. If anything, I thought that would add to things being worse.

 

I don't know ... I'm not complaining about getting good guiding, I'm just surprised, and not understanding what's going on.



#7 Madratter

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Posted 07 August 2014 - 10:47 AM

Hmm. Given the difference in guide setups, I'm guessing that is the real difference, and the OAG was just more sensitive to deviations from "correct" than thee SSAG. That suggests to me you might be able to increase your minimum motion or other parameters with the OAG and guide as well. Was the guiding with the heavier scope actually visibly better in the star shapes?


Edited by Madratter, 07 August 2014 - 10:47 AM.


#8 rflinn68

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Posted 07 August 2014 - 11:05 AM

I'm curious to hear your answer to MR's last question here. The graph will not look as good when your guiding through the OAG as compared to the 50mm mini. The 50mm has a focal length of like 162mm and even the AT65EDQ has a focal length of 420mm. At least this has been the case with me and on a more extreme scale with the RC. My graph looked as bad as I had ever saw it but to my surprise my stars were round and looked good.



#9 Goofi

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Posted 07 August 2014 - 12:21 PM

I'm looking at some images right now (While tweaking noise in last night's image... I really should stop multi-tasking!)

 

the 110ED isn't as flat a field as the AT65EDQ, so I'm going back and looking at the center of images. 

 

 

.... and the winner is the OAG (big surprise). Eccentricity values are better with the OAG.

I compared images from last night (110ED with SSAG), a week ago (AT65 with OAG) and a month ago (AT65 with SSAG).

Both eccentricity and its standard deviation are better with the OAG .. even though the guiding curve doesn't look as good.



#10 Madratter

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Posted 07 August 2014 - 07:26 PM

Don't feel bad about that. It probably is the case that the guiding was just as good or better with the OAG. It is also possible this is simply a measure of the differences in focal length between the two scopes. Guiding the 110 would be more difficult. If you can run the OAG on the 110 that would be interesting.



#11 Goofi

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Posted 07 August 2014 - 08:32 PM

I took the 110 off this afternoon and put the AT65EDQ back on ... it's hard to cheat on my first love!  :D

 

I've got a lot of targets to work on with the smaller scope; it's good to know I can use the 110ED come spring and galaxy season.



#12 jhayes_tucson

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Posted 08 August 2014 - 05:14 PM

Keep in mind that as long as the bearings can handle it, weight isn't the thing that drives mount stability or drive capability.  It's the angular inertia of the components that drives the resonant frequency of the system.  That means that a long, light weight refractor may be harder for a mount to handle than a stubby, heavy SCT.  Keeping the mass near the center of each axis will alway result in a higher resonant frequency and that helps reduce problems with low frequency mechanical vibration.  That's why on a GEM, it's better to have a very heavy counter weight pushed high on the dec shaft rather than a lighter counter weight hanging off the very end of the dec shaft.  The fact that you were getting better performance with a heavier scope leads me to believe that you actually have it balanced better so that you reduced issues related to backlash in the drive system.  The suggestions that other have offered about the guide system are good ones as well.

 

BTW, the bearings in most even half-decent mounts can handle enormous loads.  It's the drive gears and the system resonant frequency that limits "weight" (i.e. angular inertia.) 

 

John



#13 rflinn68

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Posted 08 August 2014 - 05:34 PM

I took the 110 off this afternoon and put the AT65EDQ back on ... it's hard to cheat on my first love!  :D

 

I've got a lot of targets to work on with the smaller scope; it's good to know I can use the 110ED come spring and galaxy season.

 

Did you happen to get any color shots with it? I'd be curious to see how the ED does on RGB. $995 for a 110mm doublet doesnt sound too bad at all and your narrowband looked good to me. :D



#14 Goofi

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Posted 08 August 2014 - 08:57 PM

@John .. thanks for that feedback, definitely helpful and something to think about.

Here's a pic of my setup with the AT65EDQ ... there's little angular loading... except for the counterweight (but it's a smaller/lighter one, so it should make for a smaller moment arm).

 

get.jpg

 

 

@Robert ... No, no RGB pics. I took pics with my Canon last year with the 110 ... not good! If you were using filters, just re-focus when switching. I would NOT use this scope with a OSC though - CA was pretty bad. If you want, I can try and find some test images I did to show you. 



#15 Goofi

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Posted 08 August 2014 - 09:27 PM

Here's a sample at 100% showing CA with the 110ED using my Canon DSLR ....

 

 

Attached Files



#16 jhayes_tucson

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Posted 08 August 2014 - 10:43 PM

From what I see in your picture, you would be better off hanging more counter weights and sliding them up closer toward top end of the shaft.  By having that one weight way off on the end of the shaft, you are decreasing the resonant frequency and that make vibration more of a problem.  Get all the weight as close to the axis and you'll find that the system becomes more mechanically stable.  Just remember all that stuff you learned in physics about pendulums--it's the length that determines the period; not the mass.  (Tortional pendulums are a bit different but the same conclusion applies--it's length that is important.)

John








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