Hi All

I was thinking of making a motorised Dobson 'rocker cradle' for my 12" reflector to allow star tracking.
It it worth the trouble and can I use it for astrophotography?
I have looked at several designs and have a fair grasp of what is required.
I also suppose using good quality bearings instead of 'skateboard wheels' might allow smoother running!

Or should I just invest in a good motorised EQ mount?

PK

I would go with the EQ mount. The problem with a motorized dob for photography is "field rotation". Since neither axis of an alt-az dob is aligned with the earth's axis, the stars off center will streak (small arcs). There are "work arounds" involving short exposures and stacking software.

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Any day you wake up on "the right side of the dirt" is a good day.

T. Anderson

If put on a platform, as I think PKAY is suggesting, then there is no field rotation. But they are good only for short, say 30 second to one minute exposures. So you end up stacking hundreds of frames to get the exposure time needed. Or your use track and accumulate. Problem with T&A is that it includes good and bad frames and often uses less than optimal flats and darks.
So yes it can be done but a good EQ mount will serve you better for deep sky.

For planetary photography either a platform or a dob driven on both axes would work quite well. Field rotation is not an issue and a slight drift will actually help resolution if the images are "drizzle" processed.

Also, if deep sky is your interest and you've never done it before, start with something a lot smaller and with a much shorter focal length than a typical 12" dob tube. You are facing a much steeper learning curve with a big scope. The shots you can get with a small scope will amaze you. Being that a typical location results in 3" stars even a 4" scope has as much resolution as a 12". Longer exposure compensates for aperture quite well. The mount needed is far cheaper and easier to control as well.

But if planetary imaging is your interest your idea should work quite well.

Rick

I thought this was simple! This is my take on what is happening, so if I am wrong please enlighten me!

For a 60 minute exposure, the object will rotate approx. 15 degrees.
That comes from 360 deg in 24 hours (approx.)
So the cradle would only have to rotate 7.5 deg. for a 30min exposure. That is easy to achieve ( I think).

Next, the cradle pivoting axis is aligned at the same angle as your latitude, pointing at the pole, also aligned with the center of gravity of the scope.
This means you only need one drive motor to keep up with rotation.

The procedure would be
1: Align the cradle level and pointing at the pole.
2: Set the Alt, AZ of the Dobson to center the target and then lock those pivots.
3: Turn on the cradle motor.

Am I missing something?

PK

That's the principal of a poncet platform and yes it gets rid of field rotation IF you are accurately polar aligned. Within a couple minutes of the pole. But that's only one problem you face.

You will need some way to guide it, on both axes as even a very tiny alignment error will screw up an image on declination as well as RA. Also you will find you have periodic error of the main drive gear to remove. This isn't easy at all. Mounts that limit this by design to only about 3" of arc start at $12,000!!! So you will need to guide that out. Computer controled PEC helps but still you have to guide. Then there's atmospheric refraction that changes the effective speed of the drive depending where in the sky you are pointed. Lower has more drift from this. An object near the horizon is a couple minutes or more higher in the sky than it really is. At the meridian it is correct east to west but still closer to the zenith by a bit, again depending on declination. Then on the other side it again is high by the time it sets. The result is an object moves slower than siderial rate! Some actually adjust the pulse rate of the drive motor for this but it changes with declination. Hence another reason you have to have a way of guiding the exposure when taking through a scope. Shoot near the meridian to reduce this problem.

The best way around this is to limit individual exposures to 1 minute or less. Then throw out those where the tracking was poor (probably about one half), then align and stack the remaining 100 or so images. Using a low read noise camera is a necessity here.

I've seen it done but those who do it mainly do it to show, yes you can do it if you want to go through all the work needed. Also they used commercial platforms of high accuracy (and cost).

About the only way to align such a platform is by the drift alignment method which takes a lot of time at first but if the system has fine slow motions to adjust the alignment on both axes it can be done in 30 minutes once you are familiar with the process. That would get you close enough for 1 minute shots if your gears are up to it. The precision necessary is very high.

Now if all you want to do is put a camera on it to track the sky with say the equivalent of a 100mm lens or less then the platform should track well for 5 or 10 minutes but through a scope of any focal length at all the errors will show far quicker than you think they will.

There's a good reason you don't see deep sky shots taken this way very often!

Rick

Thankyou for the advice.
I really had no idea, thought it was easy!

I have decided to ditch the cradle idea and research home made EQ mounts that I can simply 'plonk' the 12" reflector onto.
Have seen some good designs based on Mt Palomar I think?

Any other links or advice on that type of EQ mount for large reflectors?

Soon I will be hounding for advice on guide scopes and how they work!

Regards

PK

Planetary photography is easy with a platform or EQ mount, at least the drive part is easy. It's long exposure deep sky shots that puts the heavy requirements on the mount.

Unfortunately, nearly all the problems with building a platform apply to a standard EQ mount as well except the EQ mount has a way to guide in declination and has room for a much larger drive gear and bearings. The larger these are the easier it is to get the precision needed. Still it isn't the average home machinist's project. The precision needed in the bearing alignment alone is beyond all but the most experienced. This is why you'll see few deep sky astrophotos on home built mounts of any type other than those taken with very short focal lengths such as a camera lens. Though for that Orion's very inexpensive EQ-1 does very well and is likely cheaper than home made. But that's mainly for cameras only, not through the scope shots.

Probably the cheapest way to make an accurate home brew EQ mount is to use a sector arm drive. These can be quite accurate as the radius of the drive gear is measured in feet rather than inches. A friend of mine built a very good mount (weighed 200 lb however) for carrying his 12" f/5 scope using this system when in high school. Getting backlash out of the gears was his biggest problem.

In any case starting a lot smaller than a 12" scope will greatly reduce the problems you face. See the many great astrophotos posted to the astrophotography section for examples such as to beautiful 7 Sister's shots with 4" scopes of less than 600mm focal length. There's a heck of a lot to image at that scale that will keep you busy for years.

Off axis guiding (using the same scope) is usually the easiest to master. Trying to use a second scope for guiding has the added problem of flex. If one flexes even slightly and the other doesn't exactly match then your guiding is off. Using the same scope removes this stability problem! It doesn't matter if the guide star looks like a comet or not.

The only time I found a second guide scope useful was when tracking a comet's motion. But then I used the main scope as the guider and the guide scope to take the image as it's much lower image scale was needed to capture the comet and its tail. But comets are so bright with a CCD today you can use short stacked CCD exposures tracking the stars, stack them aligning to the comet's core and get the same result. Stacking programs will do much of the work for you.

Join a local club, there are usually a few telescope makers in one that can give you the knowledge you need.

Rick