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Would this work??!

badgerchap

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badgerchap

badgerchap

Posted
Posted

OK so here is a (very) rough sketch of my Dob.

basic dob.bmp

If I were to add a wedge at 38.2 degrees ( I live at 51.8 N), align the axis with polaris....

modded dob.bmp

...and then add bearings and a motor to track at the sidereal rate, would I be able to take Astrophotos?

If not, why not? What problems am I likely to encounter? Is there anything I can do?

Basically, I don't want to buy a shed load of astrophoto gear unless I really have to. I can afford it, but the missus would rather spend the money on other things and you know what they're like lol.

Also, if this rudimentary design would work, then it would be a nice little project!

Sorry about the standard of the drawings, very quick!

Thanks for any help,

Guy

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boboco

boboco

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I think you also need to have the platform change angle (tilt) on the "horizontal" axis as it rotates about its "vertical" axis. Something like this http://www.equatorialplatforms.com/Alumplt_anim2_smallb.gif

If you look at this site Equatorial Platforms: Compact Model in the second picture you can see that there are small curved edges to the base board which will tilt it as it rotates. They are also quite clear in this Equatorial Platforms: The Awesome Aluminum Platform .

They probably are not especially difficult to make (okay impossible for me ;-) ) , but you would need to know exactly what the curve should be.

Am very eager to hear if it works.

Cheers,

Karl.

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badgerchap

badgerchap

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OK, well I've had a look at all of these designs and there's only 1 thing that irks me so far, and that is the chance of the telescope falling off the mount! I believe I can solve this, howerver, and can otherwise find no reason why this simple solution should not work. To envisage it, I have imagined the scope standing vertically on it's base, and in turn atop the wedge, so the scope is pointing directly at polaris. At this orientation, even with a full rotation of the base, Polaris should remain in the same place through the viewfinder. If this is correct, then I surmise that, after tilting the scope to point at a star on the celestial equator, and the base being revolved at the sidereal rate, the star should remain within the viewfinder.

Agree with my thinking?

If so, then, surely, this is an effective tracking system. Obviously, there will be issues with stability that need to be overcome, but I cant see any major issues there that can be resolved?

Basically, I think it just involves a bit of trigonometry and some considerable prescision!

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Merlin66

Merlin66

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Posted

The bottom bearing bolt and teflon pads are designed to take a vertical load. When you tilt the base the whole weight of the scope want's to pull the rotating part away from the base, and the centre bolt starts to take a great side load. These parts would need to be redesigned to function in a "polar" mode.

Likewise the dec bearings would need to be reviewed to work at an angle ( with side loading) - normally the forces are vertically straight down.

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badgerchap

badgerchap

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I was thinking a skateboard bearing would do nicely for the central axis, seeing as these are designed to take load from a variety of angles. I was also going to add some small castors to the bottom of the rocker between it and the base. As for the dec bearing< I was going to add some steel hoops to hold the telescope in place.

Erm...Phsychobilly, I thought I had got my geometry right, but the ... at the end of your comment makes me think I may have gone awry! My thinking was thus:

With a 0 degree wedge, my scope points to the zenith.

With a 90 degree wedge, it points to the horizon.

An observer at the equator (0 degrees) would need a 90 degree wedge.

An observer at the pole (90 degrees) would need a 0 degree wedge, so wedge needed = Latitude - 90.

At my latitude, this makes 52.26 - 90 = -37.74 (By the way I looked up my lat and was nearly a degree out - whoops!)

I reckon I had this right, but if not PLEASE show me where I'm going wrong!

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Psychobilly

Psychobilly

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if you were at the North Pole which way would the scope be pointing to point at the NCP and how would the base be sitting...

If you were at the equator which way would it be pointing....and the base again...

I got it wrong the first time i made a prototype wedge in wood...

Peter...

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badgerchap

badgerchap

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You know what, I swore you were wrong, so I went downstairs, found out the exact altitude of polaris, drew some triangles (with mutterings of "I'll show that PsychoBilly....") matched ther triangles up and....

[removed word]....

Hats off mate, thank you very much. You nicely stopped me from making a bit of a tit of myself haha!

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runoffshed

runoffshed

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I think you have it correct, Badgerchap. In your diagram, if you continue your line to the celestial pole back through the base board of the dob to the ground you get your latitude angle - 52.26 The angle where the line passes through the baseboard is 90 and the angle of the baseboard to the ground (and your wedge angle) is 90 - 52.26 = 37.74.

However, this may not be the same as the sector angle calculations for a platform as there are at least 2 main methods for obtaining these - supports at right angle to board and supports angled to board.

It looks like your tilted dob design is basically the same as a fork mount and I think you have the angle of the board to the ground correct.

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runoffshed

runoffshed

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Yes, that's correct. There is loads of stuff on the web about platform building and a very good chapter in the Springer books series "Telescope Making" should you decide to go down this route. Although it may be simple to 'wedge up' the dob you may encounter balance problems not met when it's level on the ground. The box of a dob isn't, and doesn't have to be, balanced. ie because of the front board section, the centre of gravity of the box isn't immediately under the altitude rings ....it will be more toward the front of the box. This doesn't matter when it sits flat on the ground but it will when it's tilted. You may have to employ a brake or additional weight to the back of the box.

Very interested to know though, how you get on with this as it's such a simple design. Good luck with it

John

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runoffshed

runoffshed

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Should have added that the balance problem isn't really an issue when a platform is employed as the platform usually only allows for a run time of 1 hour or so therefore looking at 7.5 degrees or so either side of level. However, with your 'wedged up' dob, the c of g is immediately flung over the edge of the wedge/support and, with a tall box may even simply tip over. There is still then the problem of balance. I wonder what latitude it might work well at, though? 70 degrees + maybe?

John

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