vespine
05-04-2006, 11:37 AM
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.
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.