Optical encoders, thoughts on design, guiding and tracking
The application is closed loop, driving the RA axis only, to correct mechanical tracking errors.
After a lot of reading on the subject of quadrature encoding, PC- based and camera guiding, the simplest and most effective application seems to be to actively nail tracking accuracy along with refined polar alignment. Accurate pointing is not a goal.
I have a geared encoder assembly in mind, rather than a single wheel. I think the geared assembly has the advantage of lower latency.
The rough maths that I have done indicates that in order to control RA pointing to arc second tolerances, encoder resolution needs to be very high.
I'm not sure that resolution is critical for my application, providing there is adequate direction and speed feedback to schedule commands to the RA motor.
Combined with CCD drift alignment. Corrected tracking should allow for long exposures without guiding.
This seems to be the best and least costly application for a pretty old mount to improve its performance. If finding push-to was an issue, Argo Navis would be my choice, out of convenience.
Exploring encoder resolution further, given the limitations imposed by DIY toolkits - mine anyway. Printer resolution is an impediment to producing high resolution optical disks.
An alternative is a professionally made etched wheel - ideal. Alternatively, stuck with a printer, a double sided wheel, utilising reflective optics (in quadrature), with identical resolutions shifted a half phase out (if that's the right way of describing what I mean), effectively doubling the quadrature output.
This means more electronics. The A and B outputs from each side are conditioned by separate decoders (usdigital ls7184) and input to a microcontroller as direction and speed to schedule drive motor commands. Both signals could be sent to a single microprocessor input through high speed diodes, doubling the sample rate to that pin.
This seems like a workable solution to the limitations imposed by printers. And, with more sophisticated equipment, a way of easily doubling resolution.
I stuck a printed optical disk to each side of a CD, shifting the alignment on one side to dissect the pattern on the other.
I'm not necessarily claiming anything new here. The image below conceptualises the four phases as a % of the cycle for each pair of sensors; that is, 0, 50 on one side, intervening, 25, 75 on the other. I would like to say Octal, but in reality, seperate quadrature outputs overlapping by 45 degrees - 8 outputs off one wheel.
Hi Rowland,
I've been down a similar road, you could have a hi-res encoder on the RA axis and ensure tracking accuracy with a closed loop to make sure the tick frequency is a constant. but there is a problem with this.
the tracking rate varies based on where you're pointing due to refraction and other factors as explained by Gary in this thread http://www.usdigital.com/products/en...otary/shaft/S6
you'd have to track at a slightly amended rate from sidereal called the king rate.
geared encoders are prone to backlash errors, I'd suggest using a usdigital 10,000 cpr encoder on a 2:1 timing belt geared setup. so you'd get 20,000 ticks per RA rev with minimal backlash.
tHis resolution is still low for RA as you ideally need 1,296,000 ticks per rev.
But the easiest and most accurate is autoguiding as that corrects for a number of errors, not just tracking and its a lot easier to do.
so once you achieve a constant tracking rate through any method, it could be a stable resonator based clock source for the stepper or other methods and you achieve accurate polar alignment, all you need is autoguiding to correct either tracking errors or mechanical errors,
with accurate polar alignment, you may not even need dec guiding.
I'm using just RA guiding with my fork mount.
for a double sided quadrature system, the wheels will have to be aligned extremely precisely which I doubt is possible manually.
would be good if you post some pictures of your setup.
there is a software to generate encoder wheels, which you could print on a transparency sheet. But you'd need the right spaced sensor.
that's why the usdigital ones are ideal. http://www.iceinspace.com.au/forum/s...t=74481&page=2
I'm feeling out the options and really appreciate the feedback. Thanks again Alistair for comprehensive reply.
Other than the engineering difficulty of accurately aligning/offsetting the encoder lines the rest of the double sided encoder is not too difficult, even for me. I will post a working test sample out of interest - Christmas is upon us. A professionally made disk would be best. Some Chinese suppliers do small quantities. I saw a define your own design option as well.
In context. My mount is 15 years old and from what I can see and confidently turn my hand to, is not given to significant modification and requires either hollow shaft encoders such as those supplied by Takahashi or a geared assembly, a'la Argo Navis.
I understand the various tracking rates and accept the potential limitation of a siderial only system. Interestingly, the mount has potentiometers to adjust for other tracking rates - trial and error.
I am more inclined toward the guiding solution, but really want to simplify things - an encoder system to improve tracking is the minimalist option. Having exhausted all reasonable possibilities, I will move onto Alistair's recommendation of guiding.
Seriously, if not for the mental and creative stimulation, I would probably buy myself an ieq45-GT, all singing and dancing accurate little beast and be done with it. It's mostly cloudy down this way, so it wouldn't get much use anyway. With the Tak's raw tracking accuracy, I could get away with nothing but that.
I have gone from interfacing with a PC for tracking, to a closed loop system to iron out tracking errors (which depending on what you read is good or bad) to guiding, which is a completely new paradigm. Standby for more dumb questions.
Could you post some pics of your mount and hand controller? are you able to make manual corrections by depressing the hand controller buttons during sidereal tracking?
Does it have an RS232 port?
My suggestion for autoguiding was based on these facts
- that you already have a very sturdy mount that can track extremely accurately and already has low PE.
I've seen 5 min unguided shots and they're impressive.
So I don't see the need to alter or regulate the tracking rate directly using encoders. minor corrections from a guide camera would more than suffice.
- you're likely to stay within the 5 to 10 min exposure length for most objects unless you're going narrow band.
So all you're missing is the goto and the autoguiding.
to add autoguiding, your hand controller should be capable of manual guiding which I believe it is.
my understanding is that when the mount is tracking at sidereal rate, the e/w buttons would cause a 0.5x change in speed for the duration of the guide pulse if using st4 guiding or the duration of the button depress if using manual guiding.
so if your hand controller buttons do this, its quite easy to wire the optocoupler outputs from a guide camera to your hand controller.
The outputs from the ST4 port are normally high (+5v) and they're for the following
N = DEC+ = Up
S = DEC- = Down=
W = RA- = Right = Decrease track speed
E = RA+ = Left = Increase track speed
So all you do is work out the polarity of the switches, and wire the st4 output in parallel to the switches so they simulate a button depress.
the guiding software like phd will determine the length of the pulse based on the initial calibration routine.
As for goto, as mentioned, two 10k encoders, one for each axis, and a home built DSC (picaxe or arduino) or David Ek's DSC is ample.
Interestingly, the mount has potentiometers to adjust for other tracking rates - trial and error.
Other option is to add a digitally switched resistor ladder to the pot to get the desired 0.5x change in speed for auto guiding in RA
So you use the centre tap and an opto coupler to add a fixed resistor in series or parallel to get the two speeds
A picaxe or arduino can monitor the 4 st4 outputs and control the ladder. It's very simple and would only need a handful of components. No change to your mount controllers other than the pot tap. Only requirement is that the pot provides a sufficient change in tracking rate
Hi Alistair. I've attached the hand controller/ control panel page and the specifications page. Takahashi documentation is skant in detail.
I can order a separate guide cable from astronomiser in the UK. It's sold with a 6 pin RJ12 to 6 pin DIN connector in various lengths.
I set everything to siderial, except the high speed switch for dithering between images - a feature I must keep. However, slew rates are preset and control the rate at the hand controller.
Lots of reading to do and I'll follow up on Rajiva's page too.
well, that makes it easier. I'd suggest getting that cable or if you join the yahoo group, you can get the 6pin din connector pinout and make your own.
Also have a look through this forum for other Em200 owners who've added autoguiding. I'm sure there are others who've done it already.
Which guide camera do you have and which guide scope do you have?
the easiest is the finder guider but do check to make sure the pixel scale is suitable.
Else, you could use a webcam (not all brands, some are more sensitive than others), and then use the GPUSB interface from shoestring astronomy. that provides ST4 output using webcams as guidecameras normally have ST4 ports built in.
have you tried the CCD polar alignment method yet? do try it, very easy and very accurate.
Cheers.
i noticed you're keen on dithering, what camera are you using, does it have a lot of noise? phd can dither for you so you don't need to dither manually.
so can some dslr control software like backyard eos.
Thanks Alistair. I need to acquire 'all' the accessories for guiding. Until now I have avoided it to minimize complexity. Sorting through the guiding and tracking options has been an interesting journey. Suffice to say, that short of a course in C++ and writing my own drivers, interfacing with the Linux INDI protocol was something of a pipe dream.
Having said that, I am not about to lie down and buy my way out with all the latest gear. There is little to be learned by that. This is all new and fascinating. A hands on approach has taught me a lot. 2years ago I couldn't write an Arduino program at all.
Dithering is all important. While using a DSLR it was somewhat essential/critical. I developed a system interfacing the hand controller with an Arduino. I'm a convert and wouldn't dream of not dithering these days, even with my SBIG CCD.
No opportunity to get out imaging at all recently to try CCD alignment. I'll spend some time getting to the bottom of guiding and developing the electronics, software etc, while the days are long.
Going by what I can see, all you need is the cable, guide cam with an st4 port, and a guide scope. You won't need to build anything.
If you can get the cable, you could use my guide cam and scope for a test run if you don't have them. Mine is a qhy5.
Do you have details of what you've developed to interface with the hand controller?
Links to pics would be good as well
Thanks Alistair. I will tackle this in the new year. The electronics to replace the hand controller are just on paper. Dithering is mechanical, using a stepper roller arm to sequentially push the buttons. Guess it could be classed robotics. That was fun and in no way compromised the hand controller.
I wil keep you posted. Lot's of new ground to cover
