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Field rotation solution ??

steelfixer

By steelfixer
in The Astro Lounge

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steelfixer

steelfixer

Posted
Posted

Hi all

I am going to building my own mount to carry a very large newt.

Now for imaging the best mount has got to be an EQ type to stop field rotation.

The advantage of the Alt Azi mount is there is no meridian flip problems.

Here is the question.

Would it be possible to build a ALT AZI type mount with a built in facilaty to rotate the tube to overcome the field rotation.

Opions invited.

thanks

Graham

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steelfixer

steelfixer

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Moonshane - I am trying to get away from the EQ because the location of my obs means most of my imaging is done near to the Meridian and I want to get away from the dreaded flip.

Umadog - thanks for the link interesting idea.

Merlin - That is another possibility I am looking into. I need to do some research into how they work with regards to the guiding.

The Newt I am looking into building is going to be around the 16 inch mark so this mount is going to be some considerable size in its own right.

The thought of a 16 inch newt doing a Meridian flip scares the hell out of me hence why I want another type of mount.

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Merlin66

Merlin66

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Seriously, I've built and used various Newtonians from 12" to 29" and the fork mounting was always the prefered EQ solution.

A "polar disk" gives some nice options of hardened roller drives with zero backlash and periodic error. My early mounts used the Bartel stepper motors.

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Vega

Vega

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Just for info, There are two solutions to avoid Meridian Flip...

1. Use EQASCOM (free software that controls your EQ mount 'through' your supplied Sysncan controller data cable) in the menu, untick the box for mount limits (effectively turns off meridian flip).

2. As long as you dont want to use GOTO, simply flip the scope in advance and find your target manually then turn on tracking. As long as your target isn't too far before the meridian otherwise your scope wont initially reach it (without colliding with the tripod legs). Works a treat

As a snazzy 3rd option, In EQASCOM software, I keep mount limits turned on but go in to the settings and set the meridian details to zero. However I set the Horizon limit to altitude zero. Basically this prevents meridian flip and it will also stop the mount from tracking once the scope is pointing to the horizon. Very useful for all-nighter imaging!

Cheers

Matt

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andrew s

andrew s

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Graham, if you do go Alt Az then remember it will not track through the zenith as the rate of change in drive rate changes too rapidly. Also I think Bright Star are bring out a reasonably prices derotator.

Most professional large telescopes now use Alt Az mounts but have cleaver software to keep the tracking accurate. I also think AWR were looking at doing an Alt Az drive system but they are quite expensive.

Andrew

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steelfixer

steelfixer

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Ron - Thanks for that. I love the yoke mount.

Merlin - You have me convinced. I dont suppose you have any pictures of one of those early mounts.

I intend to drive this with a couple of Nema 23 stepper motors I salvaged from a cnc milling machine.

I rearly want to make it with a direct drive belt system to negate all backlash. At least with the size I am planning the 170 to 1 gear ratio for use with EQMOD wont be a problem to achieve.

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Peter Drew

Peter Drew

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The Alt-Az type mount is inherently more stable than an equatorial, for a fork mount to compete it has to be one heck of a fork, Ken's suggestion of one fitted to a polar disk is a good option, most commercial forks are inadequately supported IMO. A very easy to make and also very stable with no meridian flip mount is the yoke type posted by Ron, there are several variants such as the "horse shoe" mount of the 200", should be right up your street as a "Steelfixer", they do take up a fair bit of space though! :D

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steelfixer

steelfixer

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Vega - I use EQMOD now.

My set up is obs based on a pier.

I never knew you could change the settings like that. I will try it out with my current set up sound like the perfect solution.

If you can set the horizon does that mean I can auto set a limit to stop the tracking when the trees get in the way?

I love this hobby you never stop learning.

Andrew - thanks for the input. What ever mount I end up deciding on I am going to build the whole thing myself.

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acey

acey

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Moonshane - I am trying to get away from the EQ because the location of my obs means most of my imaging is done near to the Meridian and I want to get away from the dreaded flip.

Moonshane was suggesting an equatorial platform, not a German equatorial mount. A dobsonian on an EQ platform has no meridian flip (though it does need periodically resetting).

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pete_l

pete_l

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Would it be possible to build a ALT AZI type mount with a built in facilaty to rotate the tube to overcome the field rotation.

The usual technique is to put your Alt-Az mount on a wedge.

This is an angled base, where the angle at the "sharp" end is 90° minus your latitude. So if you're at Lat 51° then the angle at the base would be 90 - 51 = 39° and when you place your Alt-Az mount on the wedge and point the sharp end to the north, the arms of your fork will point to Polaris.

You've then got a polar-aligned Alt-Az mount with no meridian flip. If you need to make fine adjustments to the polar angle, you can shim one end of the wedge or the other as needed. Just make sure everything is firmly screwed together so your scope won't tip over.

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steelfixer

steelfixer

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Acey - Many thank that will teach me to read things properly.

Moonshane - appolagies got the wrong end of the stick. The only problem with that is I have this massive pier in the middle of the obs and if I am going to change things I can see a lot of hard work to get it out again.

Peter - Just spent a while reading up on those horseshoe mounts interesting stuff.

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Merlin66

Merlin66

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An Alt-Az on a wedge IS a fork mounting!

The attached images show an assembly of the fork/ disk for a 18" Dobbie conversion. The Dobbie mirror box was replaced by the framed box; the trusses and mirror cell were re-used.

The stepper motors and g'box was used to drive one of the front support wheels on the disk..

I have other details of a fork disk used on a 12" f5, with a 359 tooth worm drive..

(Drop me a PM and I'll supply more info)

post-15261-133877725516_thumb.jpg

post-15261-133877725524_thumb.jpg

post-15261-133877725531_thumb.jpg

post-15261-133877725538_thumb.jpg

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