Very basic equatorial mount for wide field photography. Opitions wanted.

[vespine]

I have a nice 35mm SLR with a prime 50mm lens that hasn't had enough use recently so I thought playing around with some widefeld photography sounds fun so I've been reading a lot about different sorts of mounts and stuff: equatorials and barn doors and all that. There are also 2 hinge barn doors which are supposed to be much more accurate but a bit more complicated and involved...

Having just bought 12" LB I was just playing around and making a little cardboard model to try to visualise how feasible modifying the dobsonian into a equatorial position would be. What I worked out is that if you tilted the existing mount at a 53 degree angle (where I live) and pointed the telescope's zenith it at the SCP you would essentially have a equatorial mount right there. The obvious problems to overcome being that either the whole thing would tip over or the OTA would fall off the stand as the supporting bracket is designed to be flat on the ground.. Another issue would be not being able to point the scope lower then 37 degrees north. These things would probably not be too hard to overcome even without recreating a whole new mount from scratch, but if you did manage to work out those issues you'd still find that the eyepiece may be almost impossible to actually look through in some positions, as it would be pretty high and pointing straight up. Whatever; it was just a thought and felt like a good exercise to go through, it certainly opened my eyes to some of the issues involved in tracking and mounting.

Further to that, I realised that the most basic equatorial mount is just a lazy suzan on a wedge. I was reminded of the sites I'd seen about the accuracy of barn-door camera mounts and thought surely a well aligned little "turntable" would be about as accurate as you could get? I think creating the actual mount would be a LOT easier then making a 2 hinge barn door and I don't see why it would not be just as accurate. I guess the hard(er) part would be accurately spinning the turn table at 15 degrees an hour.

I've been looking around and can't really see anyone adapting this idea for a basic camera mount, is there an inherent flaw to this design that I have not considered yet? I understand it won't be as accurate as a precision computer guided mount but I'm talking about something as accurate as a 2 hinge barn door without nearly as much effort.

My initial thoughts about drive would be to have a belt around the outer circumference of the turntable with a smaller drive wheel, itself driven by a reduction gear set preferably with ratios worked out so that 1440 (minutes in a day) revolutions of the drive shaft result in one full rotation of the turntable, this is so that you could still operate it manually by turning the drive shaft matching the second hand of a watch. You could also then obviously replace it with a 1RPM motor.

Ok, so that would take a little bit of working out to do well, but you have to do that with a double barn door design and more, it seems... Has anyone heard of this type of mount being made? I'm going to keep playing with the idea and maybe try to put something together eventually. Might draw up a diagram in a bit if it is hard to visualise.

[janoskiss (Steve H)]

I suspect an EQ3 + RA motor would probably do the job for an SLR. Probably a lot less hassle than motorising barn doors, though that sounds mighty tempting too.

[iceman (Mike)]

I was investigating barn door trackers to do widefield stuff (with a digicam, mind you) before getting my EQ platform.

Now I just stick my camera on a tripod and put the tripod on the EQ mount. It's accurate enough for widefield stuff (under 50mm FL). Anything higher and the tracking needs to be made sure it's accurate (ie: by putting a telescope on it and do some drift aligning through the eyepiece).

[[1ponders] (Paul)]

Agree Steve. After a bit of tweaking and adjucting I found my EQ3 and RA motor did a commendable job for widefield. Actually I was quite pleasantly surprised after running the RA through K3 with a ToUcam in place at how good the periodic error was. Mind you there are EQ3s and then there are EQ3s. Mines the one that comes with the Celestron HD-102eq refractor which is basically the same as the C4-R package put together by celestron.

[RAJAH235]

Vespine, Just another thought. You could always go 'curved rod' camera tracker. Mine works very well. Fairly easy to make n set-up... L.
http://s2.photobucket.com/albums/y7/...nt=IMG0004.jpg
http://s2.photobucket.com/albums/y7/...nt=IMG0002.jpg
http://s2.photobucket.com/albums/y7/...nt=IMG0001.jpg

[Merlin66 (Ken)]

I've used a 2 hinge barn door for the last 15 years; excellent for up to 200mm lenses.
Even the basic barn door with a 1 rpm motor will meet your needs. Only takes about 1 hour to make!!! You can get a small stepper motor kit for $45 which would run it of a 12V battery.

[leon]

vespine

Hi there, i just joined the iceinspace community yesterday 9.4.2006 and already am lucky enough to be able to contribute something, hopefully it will be of some use.
By now i'm getting a bit old and grey haired, but ive used the curved bolt and 1 RPM motor idea for the last 20 years and found it to work great.
only last week i decided to build another one which i finished today which will carry a small telescope, as i have made it much heavier.
i got the idea from an the American Sky and Telescope magazine, it had an atricle of how to construct one of these drives (October 1980 edition page 333)
As i said i have used and used my construction over and over again, and , if properly alinged to the SCP it never fails to produce good quality images.
When Halley was around in 1986 it got a fair flogging, the proof is hanging on my study wall.
I would be happy to send you a copy of the formula needed to construct one of these mounts, plus some photo's of how the finished article looks.
They are simple to build if you are a bit handy, and great fun to use.
It was quite a conversation piece at the last Vic South Party and as well at the 2004 South Pacific Star Party, where it cracked it for a certificate for a unique Wide Field Astro-Photography Drive.
I have always use my film SLR's on it and now plant my Pentax IstD on it with good results, however i'm still not sure if i really like Digital Imaging, but thats probably because i'm from the old school of Astronomy.
To build one of these drives, and to see it work well will give you much pleasure.

Clear Skies, Leon

[Roger Davis]

Good book for reference is the "Handbook for Star Trackers", mines worn out.

[RAJAH235]

That's where I got mine from.....great little book..... L.

[vespine]

Thanks for the replies everyone. Well, now i'm undecided, seems like everyone thinks the hinge thing is the way to go but I'm still really curious about how well my turn table idea would work.

I'll make some time to get out to my dad's in the next week or two, he has a home workshop and loves doing stuff like this with me

[IanW]

Originally Posted by vespine

Thanks for the replies everyone. Well, now i'm undecided, seems like everyone thinks the hinge thing is the way to go but I'm still really curious about how well my turn table idea would work.

I'll make some time to get out to my dad's in the next week or two, he has a home workshop and loves doing stuff like this with me

I get the impression you're thinking about using a lazy-susan type bearning, which will work, though they aren't really that smooth in operation nor do they take axial loading well as they tend to bind. Axial load by the way is the load at 90° to the shaft. A motorised Scotch mount is the simplest and best solution.

See: http://www.mikeoates.org/mas/projects/scotch/ for a good simple and reliable design.