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Newbie Here! Type 4 Barndoor Mount Geometry?

MikeK2

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MikeK2

MikeK2

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Hello,

I'm planning to go to a place that has very dark skies in a few months and thought it would be fun to try some pictures of the night sky. I came across several examples of barndoor mounts online and liked the simplicity and economy.

Since I have access to a 3D modelling program (Solidworks) I used it to design a type 4 mount  that I could manipulate on screen and check for tracking errors. I tried several hinge locations and was dissapointed in the errors that I got. The best that I managed was about 25 arc seconds of error over two hours. Pretty poor compared to the 1 or 2 arc second results mentioned several places online.

Are there any kind souls out there that are familiar with these mounts that would be willing to give me some advice? I can provide details of the design if that would help.

Mike

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MikeK2

MikeK2

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Kalasinman- Good point about 25" probably being good enough in most cases. But the descrepancy between results claimed online and my own bug me. I have access to a well equipped machine shop that will allow me to put everything where it is supposed to be within the proverbial gnat's butt. But it seems kind of pointless if 25" is the best I can hope for.

Happy-kat- I have heard of a cam that can be used to correct errors in a single arm mount. Is that what you are talking about? That should theoretically give perfect compensation, but the ones that I have seen online look pretty shaky mechanically. Can you tell me where to find the one you mentioned?

Mike

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MikeK2

MikeK2

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That is the cam I was thinking of. It seems like it could theoretically compensate, but I think that in actual use the tip of the threaded rod would wobble as it rotated against the cam. Has that been mentioned?

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happy-kat

happy-kat

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Not that I have seen.

I have not yet sorted making one for my mount but I will if when the motorising project is finished I identify a need.

Just so simple to try really.

I have a captive nut at the end of my carriage bolt. It currently slides with ease so I guess it still might.

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Ptarmigan

Ptarmigan

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That is the cam I was thinking of. It seems like it could theoretically compensate, but I think that in actual use the tip of the threaded rod would wobble as it rotated against the cam. Has that been mentioned?

In this thread :

http://stargazerslounge.com/topic/235493-difference-between-type-3-and-4-barn-door/#entry2550749

Fred_76 was asked a similar question (juddering at the contact with the cam) in post #5 by Stainlesssteve,  Fred said "no" etc in post #6

Is that what you had in mind ?

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MikeK2

MikeK2

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Yes, that is exactly what I was concerned about. No desrespect meant toward Fred, but I find that hard to believe. I keep thinking about how drill bits in a drill press act if you don't use a center drill or punch before using them. Maybe an experiment is in order!

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kalasinman

kalasinman

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Wait, you did day 25" over 2 hours? Wow, that seems actually quite good if you divy that up over 30 second to 2-3 minute exposures.If you were going 2 minutes, that works out to .41" per exposure. In that example if you DSLR pixel scale was 2.5"/pixel you'd be into sub pixel tracking, which is astounding.

Depending on seeing, 10-20 pixel star widths are far from rare. Errors of 3-5 pixels, even at a huge crop, would be hardly noticeable, right? So just guestimating, you'd just get into eggs at full crop with a 20 minute exposure. I may have my rough estimates a little rough here, but you get my point? Or am I missing something?

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glowjet

glowjet

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I was quite taken with the Dave Trott tracker a while back, during my research I came across a design using the type 4 systems, a very solidly built unit from stainless, alloy, brass and roller bearings, using a programmed stepper motor as a drive. From what I recall the results came very close to those given in the article on the DT design type 4.

Your 25" of error over two hours sounds extremely good, as already pointed out. If I remember, in the design models, error seem to increase at various points along the time line, as it progresses, however, using such a unit for the first period of exposure, tracking seems to produce very good results.

No matter which design you choose, it will have a safe tracking period, using this to make multiple exposures and stacking the results, must give some rewarding images, in some cases at little cost to the constructor.

I have a type 4 design, still on the drawing board, to be constructed using hardwood, alloy, stainless steel, roller bearings and a powerful 1RPM 12V geared drive motor, which I imported from the States, to provide a constant speed, this will have a modulated 12V supply. The whole unit is intended to be electromechanically operated, tracking adjustment will take place by altering the hinge spacing, using a drive screw mechanism, which can alter the distance by increments of 0.001"  One of these days I will get it built :)

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MikeK2

MikeK2

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happy-kat- Interesting links. Now I want to build a 3D printer!

The mount in the 2nd link looks pretty serious. And heavy.

glowjet- Your comments are encouraging. Maybe I'm worrying about this too much. I think I'll try a few more pivot distances and if I don't improve things I'll call it good enough!

If I can figure out a way to output an image from Solidworks I'll post it so people can see what I'm doing and perhaps offer suggestions.

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MikeK2

MikeK2

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The camera is not shown. It will mount to the hole in the mustard colored plate that is driven by the brown plate. The green bit on the end of the base represents an inclinometer I scrounged from an old level. This will be used to set the wedge for the latitude I am at. At the opposite end is a red piece with a sort of diamond cross section that represents a scope rail for polar alignment. I have a 4X rifle scope in mind for that.

The plates are 3/8" (9.5mm) thick aluminum (aluminium for you Brits out there!) and the hardware will be stainless steel and brass. I'm avoiding magnetic materials so I can take advantage of the wonderful compass that is in the thodolite base that this will be mounted to.

If anyone has any suggestions or sees any pitfalls, I would sure appreciate hearing them!

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MikeK2

MikeK2

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I just tried a few changes to my 3D cad model and the goal of 1 arc second tracking accuracy appears to be as remote as the goal of perpetual motion. I could be missing something here, and I hope that I am, but I am starting to think that these claims of one second accuracy are just dust in the wind. Too bad. It all seemed so elegant for astrophotography! Not that 25" error is bad at all, but all of the claims of 1" or less accuracy to me at this point are just B.S. I mean what the he**, why doesn't anyone just be honest?

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