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Is it time to kill the "Rat Box" myth


Vox45
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I know I am going to take a lot of heat for this as I am going against common wisdom. A lot of people are going with the "Rat Box" design for their pier. I'd like to see an open discussion on this as is seems that it is a very bad idea!

Now, it may be that for most of the amateur out there it does not have a significant impact on their imaging sessions, other issues will arise way before this one does but still, I think this should be clarified for the benefit of anyone who is serious about building its own obervatory.

What do you think ?

 

 

Edited by Vox45
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Nothing wrong with a rat box IMHO I've got one and can do 30min subs continuously, you don't need the screws to be that long and 4 is better than 3, I think the reason for 3 is on the milking stool principal of being easier to level.

I really need my 10Micron mount to start off level in all directions to get good modelling as recommended by our sorely missed friend Per.

The main fault with all the proprietary piers is the size of the base that screws down to the ground, if the thing is over a meter high with a heavy mount and big scope slewing around I think there is more likely to be movement at this point than anywhere else with a base hardly any bigger than the top.

All the piers I have built have bases with bolts 600mm apart.

What causes these vibrations would you be banging your pier with a hammer while imaging ?

Dave

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I guess what gave me pause is when he said that it is very difficult to tell apart bad seeing from vibration.

Maybe for those who have high-end mounts vibration is not a big issue, but lower-end mounts can induce vibration through motors and gears as I understand from the video. So someone might say, my pier is rock solid but I don't have good seeing at my location when in fact it's the vibration from the mount that cause a big chunck of the "bad seeing" 

Should we really dismiss this or should we challenge the common wisdom of this design ?

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Considering I actually lift the mount and it's plate off after every session (it sits in the edge of a shrubbery, so I have to bring it in for weather protection)  I thought my problems were due to my pier compromise, but most of it was poor PA, even though I thought it was good, general backlash in the mount and PE.

Having said that, I don't always get 30 min subs, but I'd take any bet that says it's my peir to blame.

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46 minutes ago, Davey-T said:

I really need my 10Micron mount to start off level in all directions to get good modelling as recommended by our sorely missed friend Per.

 

I would not want to challenge Per as he was well respected here, but, and this may be the start of a different thread, isn't it a fact the the mount does not have to be leveled as long as the PA is good ? I see a lot of people going in extrem lenght to level their mount with great precision (hence the rat box) ... but I've been told that a rough level is sufficient (so the whole setup does not tip over) as long as PA is precise. Have I misunderstood ?

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When you think that modern-day sports car can accelerate 0 to 60 in 3secs and the wheel usually hold on by 4 small wheel studs these cars are Thousand break horse, I don't think that they have any degrees instructional engineering or xyz in working out with the main strength come from.

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Per and I talked about this quite often and he did say that the 10 Micron software favoured a level mount for unguided imaging via 'Model Maker.' Since he wrote the software I felt no inclination to argue! 

However, the normal autoguided GEM does not need the pier top to be level, or even slightly level, to be perfectly polar aligned. Unless you need the raising effect of the bolts to get your hand under the mount head to bolt it in place there is no need whatever to make your pier top adjustable for levelling. The rat box, in this case, is both pointless and potentially vibratory. You cannot level a Takahashi tripod (the legs are of a fixed length) but you can polar align their mounts in less than a minute.

When you level the popular makes of mount you are 'levelling' only one thing, the orientation of the polar scope reticle. Given their innate inaccuracy I find it hard to believe that a small tilt is going to be very life changing. When you drift align the interaction between Alt and Az adjustments is reduced on a level mount but on a fixed pier this is another non issue.

So I would not compromise my pier top by perching it on studding if I could fix the mount head down from on top, as I can with Mesu, 10M and Avalon but not Skywatcher or Takahashi.

What I don't like about the pier in the video is its small footprint.

Olly

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2 hours ago, Vox45 said:

I would not want to challenge Per as he was well respected here, but, and this may be the start of a different thread, isn't it a fact the the mount does not have to be leveled as long as the PA is good ? I see a lot of people going in extrem lenght to level their mount with great precision (hence the rat box) ... but I've been told that a rough level is sufficient (so the whole setup does not tip over) as long as PA is precise. Have I misunderstood ?

Not for the 10Micron it needs a reference point to model the sky using Pinpoint and that needs it to be level to start with :)

Dave

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There's no substance to the argument/explanation of the design parameters, merely an opinion based on [some parts of] mechanical theory. To convince me (beyond the intuitive 'bloody obvious' level) he would need to produce actual data to back it up - i.e., vibration data generated which establishes the optimal dampening design with various loads. Call me a cynic if you like but this is just a sales pitch.

ChrisH

Edited by ChrisLX200
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This is mine:

DSC00655_zpsokpre1xr.jpg

Whilst I did machine an adjustable top plate which could be levelled I found making such adjustments were not required, still the facility is there. The plates are 25-30mm thick and do not flex/rotate at all. The main pier (the green bit) is 8-3/4" O.D. (nominal 8" steel steam pipe having a 3/8" thick wall), and there is actually more of this pipe below ground than above. The concrete support is 5ft 6" diameter and 6ft deep - that's a heavy base. As far as vibration is concerned the most significant source is traffic on the minor road at the front of the house, cars and especially buses that bounce over the 'speed humps' cause micro-earthquakes which I can actually feel inside the house, and no doubt such vibration reaches the pier too. Thankfully there is very little traffic at night but that by far is the largest contributer of unwanted vibration. I cannot see any way that normal functioning of the mount will induce vibration. However, an Owl sat on the end of my OTA is another problem altogether :)

ChrisH

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4 hours ago, ChrisLX200 said:

There's no substance to the argument/explanation of the design parameters, merely an opinion based on [some parts of] mechanical theory. To convince me (beyond the intuitive 'bloody obvious' level) he would need to produce actual data to back it up - i.e., vibration data generated which establishes the optimal dampening design with various loads. Call me a cynic if you like but this is just a sales pitch.

ChrisH

Agreed.  Just because there's a potential for something to behave in a given way doesn't mean that it will behave that way in a "real-world" situation.  It might well vibrate if you smack it with a lump hammer, but how many astronomers are likely to be in that situation?

If someone were to point to a "rats nest" pier using a few short lengths of decent size rod for support and tell me it wouldn't be solid enough without getting into real numbers and some proper engineering then I'd probably invite them to keep smacking their head on it until we could measure some actual vibration.

James

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Although I see it in the flesh every day, it brought a lump to my throat to see Per's rig unexpectedly on here. Gosh.

More cheerfully... the 'ultra low profile ratbox' piers, four of them, were beautifully fabricated by Jonas Grinde and no amount of video snake oil will stick to them. If you don't need to get a hand under them the threaded bars might as well be as short as possible.

They are also bolted to a 6 tonne base laid by two old men with a combined age of 130.  The concrete truck was delayed by a pump failure at the factory, a closed road and then a lane blocked by an upside down car, rolled in a moment of gallic over-excitement by its owner. Not only had the fit young workforce had to go off to work by the time it arrived but the concrete was also 'going off.' The fun we had!

Olly

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I am inclined to agree with ChrisH that some data would be a good idea. However, the example shown is poor design with the threaded adjustment "bolts" being far too long. To me it makes no sense to top an otherwise rigid pier with such a weak spot. However, I do think the move to relatively light mounts and short small APO refactors has allowed astro-photographers to get very good results with such designs.

I have always used large Newtonian telescopes and the wind-loading induces a significant variable torque that could potentially induce oscillations. However, as ChrisH said measurement is key.

Regards Andrew

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39 minutes ago, DannyLee said:

Chris H

 

how do you cope with all that spaghetti? All I have is power lead, usb and hand controller and I get totally into a rat's nest with cable around everything. God knows how you manage with that lot.

Dan

That's an old photo which doesn't show it setup for an imaging session, there's actually only one cable loom going up to the scope which is attached at two points - one at the dovetail clamp and the other on the rear of the RA housing. The scope can meridian flip all it likes (and it needs to for an unattended modelling run). There are no snags, no dragging on the mount, no weight pulling on the focuser. I don't know if I have an up to date image but this one is close (I've since re-painted the little rusty shelf :) and that box shown sitting on it in the photo is the Ethernet interface for the Moravian G4-16000 camera, and that is currently attached to the Mount control box as one unit).

DSC00716_zpsdbgieao5.jpg

DSC00719_zpsnq04z2k7.jpg

 

ChrisH

 

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Thank you all for your expert opinions on this matter. I think it is sane to have these kind of conversations and to challenge common wisdom once in a while :)

From what I gather here, when the time comes for me to build a permanent pier, I will go with a mount that does not have to be screwed from the bottom and leave the bolt/plate out of it as it is unnecessary.

There is another solution that I've seen in another thread where the concrete pier had been poured with a hole in the center to be able to screw the mount from underneath, I guess that this would be better than the rat box for those who don't have the luxury of changing their mount.

Edited by Vox45
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