Partial Tracking Exposure Calculation

[Langy]

I've ordered a MD for my Astromaster 130EQ, I now that they are cheap and I may not get the smoothest and precise tracking speeds as it's all hit and miss. However assuming that the tracking runs at a reasonable pace and isn't too far out I have a question on exposure times.

Currently when I view with my 20mm EP (1 deg FOV). Jupiter should take around 4 minutes to travel through the EP. Now roughly for every 3 times Jupiter travels through the EP (3 degrees) I then have to alter the Dec by less than half a degree for where the scope is not properly polar aligned.

On my Canon EOS 300D with a 300mm lens sat on a static tripod I can get away with 1.3 seconds without trailing stars.

The 600 rule for this works out as 600 / (1.6 x 300) = 1.25 Seconds

So if the tracking on the RA does move in time (maybe allow 10-25% allowance) with my Dec not being polar aligned, what sort of exposure times could I get away with?

Now on the scope the 600 rule works out as 600/ (1.6 x 650) = 0.57 Seconds

I know the easiest way will be to just test this, but with the current weather it adds another topic and gets peoples brains thinking, preparation is of great benefit before I go and test it.

[Langy]

OK I've done some rough calculations, after all it's Friday and this is far more interesting than the work on my desk, even though it't took nearly an hour on a spreadsheet and looking up calculations on wiki.

My EOS 300D will have a FOV of 4.33 degrees with the 300mm lens on that hits the CMOS. Now according to the 600 rule I should take a maximum exposure of 1.25 seconds to stop any trailing stars.

It will take an object 17.32 minutes to travel from one side of the FOV to the other where 1.25 seconds is required to stop this still on the recording device.

If the camera is in the scope at 650mm FL I get a 2 degree FOV. Using the 600 rule I should be exposing at 0.58 seconds.

With working out how long it takes for the object to travel across the entire FOV and dividing that by the exposure time I end up with a movement of 0.96% whilst the shutter is open.

So if my tracking motor is not too far out the DEC is losing its tracking by around 0.5 degrees every 12 minutes that gives me a movement of 0.04 degrees.

Now dividing the normal 0.25 degrees a minute movement by the new movement with tracking tells me that the earth is now seemingly moving 6 times slower than it actually is.

The 6 times slower can then be multiplied by the suggested speed from the 600 rule.

So if my camera with its 300mm lens piggy backed on the scope the exposure time can be altered from the 1.25 seconds to 7.5 seconds which should still only register 0.98% movement in the sky.

Straight in the scope goes from 0.58 seconds to 3.46 seconds

Scope with 2x barlows goes from 0.29 seconds to 1.73 seconds.

Hopefully a mathematician will correct me where my sums are wrong.

How sad am I on a Friday? 

[c3po]

May I throw a spanner in the works? I have the Celestron 130 and accompanying motor drive, and sadly, it's not as easy as attaching the drive to the RA shaft and switching it on. The MD has, in addition to its North/South switch, a speed control knob that you turn to adjust the tracking rate of the motor. Getting this set perfectly took me far too long, and as the battery voltage wore down, so the speed decreased and further adjustment to the speed controller was required....

[Langy]

I know that they are not brilliant, may even connect it to the mains supply if it causes too many problems, but it was more of a hypothetical calculation to see if the worst tracking loss was due to the Dec not being polar aligned what sort of reasonable exposures I could get.

Even just piggy backing the scope with the camera at 300mm to get the exposures up from 1.25 would bring in a lot more detail that what I can currently capture.

Thanks for the point though, it will get me thinking about a mains voltage. I have an old CB transformer in the loft that you can wind the voltage down to about 6v and up to nearly 20v which may eventually come out.

[jgs001]

I've done some testing with an EQ1 with a similar sounding drive motor. I only tried this with the kit 18-55 and a 50mm lens. I was able to get up to something around 2 to 3 minute exposures with the 18mm. And about 1 minute at 50mm. A lot will depend on the accuracy of the drive. First things first though, you should aim to get as accurate a polar alignment as possible. One way to do this, attach a finder scope to the Dec axis, and use that to get a better polar alignment. You can use the scope to improve the motor speed setting... aim low in the east, and use a higher power eyepiece to find a star, centre it (ish), it doesn't matter too much, as long as you can recall where in the FOV the star is. Check frequently, and adjust the motor speed until the star remains in the same spot in the FOV. I suspect that actually achieving this for any long period of time is unlikely, but aim for the best you can. I'd also suggest removing the scope to lighten the load, and just attach the camera directly to the mount. The lighter it is, the less strain on the mount and motor. If the mount uses a standard dovetail, then just buy a medium length one (it'll help with balancing) and fix the camera directly to that.

[c3po]

I know that they are not brilliant, may even connect it to the mains supply if it causes too many problems, but it was more of a hypothetical calculation to see if the worst tracking loss was due to the Dec not being polar aligned what sort of reasonable exposures I could get.

Even just piggy backing the scope with the camera at 300mm to get the exposures up from 1.25 would bring in a lot more detail that what I can currently capture.

Thanks for the point though, it will get me thinking about a mains voltage. I have an old CB transformer in the loft that you can wind the voltage down to about 6v and up to nearly 20v which may eventually come out.

Having tried widefield piggy-back AP with the Celestron the RA tracking motor, i know what you're attempting, albeit at a higher zoon with the 300mm lens. However, the theory is the same, and I quickly became tired of the restrictively short exposures i could expect with the DC motor - i use a Sony A200 with a CCD at a 1.5x crop factor and i was getting 30-50 seconds at best, no thanks to the DEC error and Polar Alignment tweaking needed and Periodic Error in the mount coupled with the faff of getting the tracking rate of the DC motor correct. 

After some disheartening results with the piggy-back AP, i looked for other solutions to the RA motor and came across this Stepper Motor drive unit designed by SGL's very own AstroTux. It takes the guess work out of the setting the RA motor and provides you with Sidereal, Lunar and Solar traking speeds. I've tested the unit briefly with my scope indoors to confirm correct tracking rate at 'solar' speeds, and have also tinkered with EQAlign and a webcam on the few clear nights we've had this year using the RA Stepper Motor to get my head around Drift Alignment. So far i think i'm getting there, and the next logical step would be to hook it all up and perfect the drift alignment with EQAlign; when that's sorted i'd attach the DSLR in place of the webcam and start shooting, how long an exposure i'd get is anyone's guess, but with the PE i've experienced with the mount, i'd hope for anything upwards of a minute or two given the mounts basic design - nothing extraoridinary, but certainly better than a handful of seconds.

[Langy]

I fitted the motor tonight and with the few minutes of being able to view the moon and Jupiter through the clouds the tracking was going reasonably well. The whole point of this post was to see how far I could expect to push the exposures. I know I won't get minutes to capture DSO properly with lots of detail, but I would like to be able to push the exposures to beyond what is possible with no tracking at all. If I can extend the 1.25 seconds on a tripod and piggy back to get at least 2.5 seconds, that's already a 100% improvement in capturing detail.

Proper DSO imaging costs money, lots of money, but with the calculations and plenty of tests I'm sure I can capture much more than without any form of tracking.