Telescope Drive Master - The Truth of the Matter

For those of you doubting the capabilities of a lower end mount (EQ6 Pro) coupled with the TDM, please take a look at my full size image of NEO Minor Planet 2001 LO7 taken on 02 July 2011.

http://www.pbase.com...454649/original

You can see a closup negative of 2001 LO7 here

http://www.pbase.com...image/136454702

These images are 10 x 180 sec unguided images with my ES 127 APO ED Carbon Fiber on my EQ6 Pro with the TDM active.

Based on my experience, and what I have seen posted from higher end mounts, I would find it interesting for anyone to say that my results do not match or even exceed the performance of higher end mounts unguided. I understand that there are those that would like to see everyone upgrade their cheap astrophotography mounts to the higher end $5000-$10000 mounts, but that isn't going to happen 9 times out of 10. So what is the alternative? I think I have found it with the TDM. I have attached below my response to a previous thread, because since it is at the bottom of a long thread nobody would probably bother reading it there.

Here is a link to a paper I wrote about my experience with the TDM

http://www.mda-teles...vestigation.pdf


I hope this helps the conversation.

Jerry Hubbell
Lake of the Woods Observatory

Hello Tassilo, members,

I just now ran across this thread. I have read through these comments with a lot of interest, and indeed there is a lot of misunderstanding about how the TDM works, and how it is useful to the average amateur with the good but inexpensive mount i.e., EQ6 Pro, Orion Atlas, etc. I was fortunate enough to work with Dennis DiCicco on his review of the TDM (he mentioned me in the article) and I have also been working with Attila Madai, and Istvan Papp of MDA-Telescoop to get my TDM working as best I can. I have also been working with Attila to improve the TDM. From my point of view, the TDM works exactly as advertised. I have had nothing but positive results using the TDM in my observing program. You can see the initial results of my testing and intitial observations here:

http://www.mda-teles...vestigation.pdf

You have to realize that, for me, the biggest goal I have for my observing program is the amount of time I spend imaging. I have (as we all do) weather issues, available time to observe, etc. I want to maximize my time and efficiency while imaging.

I found that over the past 2 years, although I have mastered using my guide camera and PHD for guiding, it still took time away from my imaging to manage it. Also, since I have a portable system, not a permanent observatory, the time spent setting up and taking down the equipment can be significant (2 hours total). So anything I could do to minimize the time and effort, yet still get high quality images, was going to be worth the expenditure. Also, the only other alternative to the TDM is to purchase a mount in the $5000 to $8000 range. This was not going to happen.

The total cost of the TDM with the Orion Atlas, or Sky-Watcher NEQ6 Pro mount is about $3500. I dare anyone to find a mount and drive corrector system that matches the performance for the price. I have found that the only real reason to purchase a more expensive mount is for the load capacity. The EQ6 provides a very capable load capacity of 40 lbs., 30 lbs. for astrophotography purposes.

Another thing to think about is, how practical is 20 minute subs to those who live with marginal to good sky brightness. I have found that although I can manage 10 minute exposures, for most of the time, the sky starts to fog the image. I would prefer to stick to 3-5 minute subs for deep sky work.

My main work is in Minor Planet astrometry and photometry, so being able to setup my single scope with the TDM EQ6 combo with only my main camera to manage, really goes a long way towards simplifying my setup, and making the most of my imaging time. In this configuration I am very happy with 1-3 minute subs.

The only other complaint about having perfect tracking but still having to use a guide camera, is the fact that polar alignment takes a long time with the standard drift method. This has always been the fallback argument against the TDM. I have found that, just like anything else in astrophotography, you have to work a little to get results. To make things easier, I have found that Alignmaster ( http://www.alignmaster.de/ ) is probably the fastest, most consistent way to get an excellent polar alignment in the least amount of time.

So bottom line, I have a system that with the help of TDM and Alignmaster allows me to set it up in 30-40 minutes, start my imaging within 10 minutes of completing my calibration frames and polar alignment, and for the remainder of the session, worry free tracking and imaging. After proper syncing, I find I can select my objects with ease with every one falling in the field of view of the camera.

That to me is the bottom line. I have done this with the most cost effective equipment I have found available thus far.

I hope that helps those understand my reasons for purchasing the TDM.

Jerry Hubbell
Lake of the Woods Observatory (MPC I24)
Locust Grove, VA

Hi Jerry,

Excellent images and details! Thanks for posting this. I have an interest in doing similar work to what you are doing and can see the benefit to the simplified setup (on less thing to do or worry about). Especially in the case of visiting many objects on a single night.

I'm not experienced in this type of imaging, but I would expect there to be some time fiddling with getting the guiding setup with every new target. Nice to be able to just center and go.

BTW, your first image link isn't coming up for me, but the second one is. How faint is that NEO?

Clear skies,

Jerry,
That is a great and very informative write up (the linked one) and a great post. I think the images speak volumes. Thanks very much for putting this more in the spotlight. I am a huge fan of real world examples with some amplifying text. Great job and thank you for posting in on this.

Hey Gord, CounterWeight (that can't be your real name LOL) I appreciate the comments. The original size image is very large of 2001 LO7, so that may be causing you some issues Gord. 2001 LO7 at the time of imaging was at magnitude 15.1 the faintest stars you can see in that image are around magnitude 19. This is a V-band image.

Jerry Hubbell
Lake of the Woods Observatory (MPC I24)
Locust Grove, VA

Hey Gord, CounterWeight (that can't be your real name LOL)

I think he is looking for a counter weight shaft. Oops!

Thanks for the feedback. I do have a problem with the price though, $2200 with a mount adapter.

Hey Alph, I understand the issue people have with the price, but you have to put it in context with what the alternative is, and what the results will be. For my observing program, I think spending a total of around $3500 for the EQ6 Pro/ Atlas plus the TDM is not out of line at all based on the resulting performance.

The alternative would be to spend anywhere from $5000 or more to get the same unguided performance.

If you read my post, and the linked paper, you will understand that my main goal was to obtain high quality images with the minimum amount of setup and fuss. That eats into the available imaging time, which to me is worth quite a bit of money.

Having said all that, I fully understand your position. It all depends on your goals with your imaging program.

I appreciate your comments

Jerry Hubbell
Lake of the Woods Observatory (MPC I24)
Locust Grove, VA

my main goal was to obtain high quality images with the minimum amount of setup and fuss.

I tried to use PemPro for polar alignment. It just takes too long.

Yes Alph, I found the polar alignment was the biggest issue with being able to minimize the amount of setup time and equipment. There is no doubt about it, using a guide scope and camera can minimize the time you spend setting up the mount. All you have to do is make sure it is pointing somewhat close to North and then rely on your guiding to make up for that and your inexpensive mount's PE. It is very attractive. Having said that, you still have to deal with the continuous maintenance of the guide scope during your imaging session.

What I found though after trying to increase the quality of my images, was that if you start with a weak foundation (poor polar alignment, poor tracking/PE) then no matter how good your guiding, you still had to deal with pointing accuracty, field rotation, etc. plus you can only get up to a certain quality in your imaging.

If you are really after the highest quality images, you need to start with solid performance in polar alignment, an d tracking. This enables very accurate pointing which minimizes your tiime going from one target to the next (which is what I need). Once you sync your mount control software, with a very well aligned mount, then you are good to go all night long. I find this preferable than having to recalibrating my guider and having to resync my mount for every target I want to image. This goes a long way towards minimizing the frustration level, and allows you to concentrate on the other issues with the very highest quality imaging, i.e. focusing and pointing accuracy to place your object in the center of the field. This is very important when imaging a moving object such as minor planets.

To answer your question directly, I have found that AlignMaster (http://www.alignmaster.de/) has been the best method I have found to do a quick polar alignment. I believe that this uses a similar method that Celestron has built into their hand controller. I believe I have read that somewhere.

Jerry Hubbell
Lake of the Woods Observatory (MPC I24)
Locust Grove, VA

Hi Jerry,

These images are 10 x 180 sec unguided images with my ES 127 APO ED Carbon Fiber on my EQ6 Pro with the TDM active.

Based on my experience, and what I have seen posted from higher end mounts, I would find it interesting for anyone to say that my results do not match or even exceed the performance of higher end mounts unguided.
I hope this helps the conversation.

Jerry Hubbell

Here is an image created from six images in a row, 30 minutes each (that's 1800 seconds each, 10x as long as your images), unguided, similar image scale, AP1200GTO, UltraTrack Software:

http://www.gralak.co...P105-ST10XE.jpg

There are lots of other long exposure unguided images out there, some at long focal lengths. 180 second unguided images in my opinion is not even close to the level that a high end mount can obtain.

-Ray Gralak

Hey Ray, thanks for the image, that is impressive. I would expect nothing less from a $10,000 mount that is probably mounted permanently with a very accurate polar alignment and located at a good dark sky site.

I have done subs up to 10 minutes but my sky is too bright to handle that amount of time, In order to get there I made sure my polar alignment was spot on. The stars in those images were basically the same as in the image I posted a link to previously. I believe I could accomplish similar results if I were at a pretty dark site that did not fog up the image. I hope to be able to image sometime at such a site. This would be a good challenge that I am willing to tackle. If I am not mistaken, the AP105 has a focal length of 610mm versus my scope with a 950mm focal length. The camera differences do make the field of view very similar.

I don't believe this should be a competition between a high end mount and a cheaper mount. For me it seems the primary need for higher priced mounts comes down to load capacity while maintaining accurate tracking. It should be easier to maintain accurate tracking with lower capacity mounts than with higher capacity mounts.

With that said, I think that for those folks that cannot afford the higher end mounts just to get the unguided tracking capability necessary to maximize their imaging time at the scope, the TDM is a cost effective solution.

Jerry Hubbell
Lake of the Woods Observatory (MPC I24)
Locust Grove, VA

Hi Jerry,

Hey Ray, thanks for the image, that is impressive. I would expect nothing less from a $10,000 mount that is probably mounted permanently with a very accurate polar alignment and located at a good dark sky site.

I believe I could accomplish similar results if I were at a pretty dark site that did not fog up the image. I hope to be able to image sometime at such a site. This would be a good challenge that I am willing to tackle. If I am not mistaken, the AP105 has a focal length of 610mm versus my scope with a 950mm focal length. The camera differences do make the field of view very similar.

I don't believe this should be a competition between a high end mount and a cheaper mount. For me it seems the primary need for higher priced mounts comes down to load capacity while maintaining accurate tracking. It should be easier to maintain accurate tracking with lower capacity mounts than with higher capacity mounts.

With that said, I think that for those folks that cannot afford the higher end mounts just to get the unguided tracking capability necessary to maximize their imaging time at the scope, the TDM is a cost effective solution.

Jerry Hubbell
Lake of the Woods Observatory (MPC I24)
Locust Grove, VA

Actually, I would be very surprised if you could do the kind of exposure lengths without software tracking rate correction for refraction, polar alignment errors, flexure, etc. The *real* truth is having a perfect tracking rate is not good enough in most areas of the sky for long duration unguided exposures because the *required* tracking rate to follow an object will vary as the scope tracks across the sky.

The AP1200/AP105 was setup in my back yard on a pier and not particularly well aligned. I had to use an HAlpha filter to prevent sky glow from overwhelming the image.

Here's another couple 5-minute images at a longer focal length that show what software tracking rate correction can do. These images are of M33's core at 0.55 arc-sec/pixel, which was obtained with my old ST10XE CCD camera at 2449mm focal length (AP155 refractor with a barlow). As you can tell the scope is not perfectly polar aligned yet PEC+tracking rate correction fixes this.

Without tracking rate correction (PE, flexure, alignment errors show):
http://www.gralak.co...m-5min-NoUT.jpg

With tracking rate correction:
http://www.gralak.co...9mm-5min-UT.jpg

Is there any tracking rate correction software for the TDM?

-Ray Gralak

Hi Ray,

Thanks again for the images. I am not sure what you mean by Software Tracking Rate Correction. The TDM is designed to correct the RA rate either to absolute Sidereal Rate, or to the King Rate that is optimized to a median altitude of around 45 degrees. This is what I use. This will correct for atmospheric refraction around those altitudes. I ensure my scope and mount are very tightly bound to minimize any flexure, I seem to have managed that okay. The only real error that is introduced that is left is polar alignment which of course you know causes field rotation and declination drift. I have used the standard drift alignment method over the past couple of years. That takes too long for me so I have started using AlignMaster to minimize the time I use to polar align. Once I have done these things, and done a good 2 star sync with EQMOD, then I have a very accurate and stable platform for imaging.

I am curious to learn how this software tracking rate correction works.

Thanks

Jerry Hubbell
Lake of the Woods Observatory (MPC I24)
Locust Grove, VA

Hi Jerry,

Hi Ray,

Thanks again for the images. I am not sure what you mean by Software Tracking Rate Correction. The TDM is designed to correct the RA rate either to absolute Sidereal Rate, or to the King Rate that is optimized to a median altitude of around 45 degrees. This is what I use. This will correct for atmospheric refraction around those altitudes. I ensure my scope and mount are very tightly bound to minimize any flexure, I seem to have managed that okay. The only real error that is introduced that is left is polar alignment which of course you know causes field rotation and declination drift. I have used the standard drift alignment method over the past couple of years. That takes too long for me so I have started using AlignMaster to minimize the time I use to polar align. Once I have done these things, and done a good 2 star sync with EQMOD, then I have a very accurate and stable platform for imaging.

I am curious to learn how this software tracking rate correction works.

Thanks

Jerry Hubbell
Lake of the Woods Observatory (MPC I24)
Locust Grove, VA

In addition to polar alignment errors and refraction (which varies by temperature and pressure), there are other errors from flexure that cause effective tracking rate errors (and pointing errors as well). If you are using alignment software that counts on centering stars it probably does not account for flexure which can be significant. If your mount has significant uncorrected periodic error that will also lead to positioning errors when centering stars, thus limiting polar alignment accuracy again.

BTW, flexure comes not only from the OTA but from the mount, tripod/pier, and camera/OTA connections. No matter how solid you think your setup is flexure exists at some level (unless you are in space where there is no gravity!).

Tracking rate correction software measures errors in the pertinent parts of the sky and then continuously adjusts the scope's RA and Dec tracking rates to match the apparent rate the sky is moving through the telescope. It requires that the mount be capable of setting RA and Dec tracking rates to a high level of precision. The result is near perfect unguided tracking despite alignment errors, refraction, flexure, etc.

-Ray

Here is an image I took on the second night of testing. This image was taken with my AT8RC and QHY9m.

http://www.pbase.com...217447/original

This is a 600 second image (see fits header below)with the TDM tracking enabled. Unfortunately, I took it with the cooler turned off, just prior to that image, I had restarted MaximDL and neglected to make sure the cooling was set at -20c which is my nominal setpoint during the summer. You can imagine my surprise when I saw all the "snow" in the image, and realized what the problem was. Well after that fiasco, I decided to go on to some real imaging with smaller exposure times.

Anyway, if you look at the image, you can still see how the stars are nice and round, although my focus was a little off.

Jerry Hubbell
Lake of the Woods Observatory (MPC I24)
Locust Grove, VA

SIMPLE = T
BITPIX = 32 /8 unsigned int, 16 & 32 int, -32 & -64 real
NAXIS = 2 /number of axes
NAXIS1 = 1674 /fastest changing axis
NAXIS2 = 1248 /next to fastest changing axis
BSCALE = 1.0000000000000000 /physical = BZERO + BSCALE*array_value
BZERO = 2147483648.0000000 /physical = BZERO + BSCALE*array_value
DATE-OBS= '2011-06-28T04:43:08' /YYYY-MM-DDThh:mm:ss observation start, UT
EXPTIME = 600.00000000000000 /Exposure time in seconds
EXPOSURE= 600.00000000000000 /Exposure time in seconds
CCD-TEMP= 24.724226079875109 /CCD temperature at start of exposure in C
XPIXSZ = 10.800000000000001 /Pixel Width in microns (after binning)
YPIXSZ = 10.800000000000001 /Pixel Height in microns (after binning)
XBINNING= 2 /Binning factor in width
YBINNING= 2 /Binning factor in height
XORGSUBF= 0 /Subframe X position in binned pixels
YORGSUBF= 0 /Subframe Y position in binned pixels
FILTER = 'None ' / Filter used when taking image
IMAGETYP= 'Light Frame' / Type of image
FOCUSPOS= 1362 /Focuser position in steps
FOCUSTEM= 30.000000000000000 /Focuser temperature in deg C
OBJCTRA = '18 30 20' / Nominal Right Ascension of center of image
OBJCTDEC= '+26 11 34' / Nominal Declination of center of image
OBJCTALT= ' 81.2165' / Nominal altitude of center of image
OBJCTAZ = '142.5231' / Nominal azimuth of center of image
OBJCTHA = ' -0.4157' / Nominal hour angle of center of image
PIERSIDE= 'WEST ' / Side of pier telescope is on
SITELAT = '38 20 35' / Latitude of the imaging location
SITELONG= '-77 46 10' / Longitude of the imaging location
JD = 2455740.6966203703 /Julian Date at start of exposure
JD-HELIO= 2455740.7035019537 /Heliocentric Julian Date at exposure midpoint
AIRMASS = 1.0157437575451260 /Relative optical path length through atmosphere
FOCALLEN= 1610.0000000000000 /Focal length of telescope in mm
APTDIA = 203.50000000000000 /Aperture diameter of telescope in mm
APTAREA = 27321.089037689566 /Aperture area of telescope in mm^2
EGAIN = 1.0000000000000000 /Electronic gain in e-/ADU
SWCREATE= 'MaxIm DL Version 5.15' /Name of software that created the image
SBSTDVER= 'SBFITSEXT Version 1.0' /Version of SBFITSEXT standard in effect
OBJECT = ' '
TELESCOP= ' ' / telescope used to acquire this image
INSTRUME= 'ASCOM QHY9 Driver'
OBSERVER= 'Gerald R Hubbell'
NOTES = ' '
FLIPSTAT= ' '
CBLACK = 21087 /Initial display black level in ADUs
CWHITE = 17875 /Initial display white level in ADUs
PEDESTAL= -100 /Correction to add for zero-based ADU
SWOWNER = 'Gerald R Hubbell' / Licensed owner of software
INPUTFMT= 'FITS ' / Format of file from which image was read
SWMODIFY= 'MaxIm DL Version 5.15' /Name of software that modified the image
HISTORY Flat Field (Flat None 2, None, 1674 x 1248, Bin2 x 2, Temp -20C,
HISTORY Exp Time 2s)
CALSTAT = 'F '
END
----------------------
History added since image was opened...
Original image: E:\Astrophotos\27June2011\600sec_TDM_test.fit
Flat Field (Flat None 2, None, 1674 x 1248, Bin2 x 2, Temp -20C,
Exp Time 2s)

Hi Jerry,

Here is an image I took on the second night of testing. This image was taken with my AT8RC and QHY9m.

http://www.pbase.com...217447/original

This is a 600 second image (see fits header below)with the TDM tracking enabled.

SIMPLE = T
BITPIX = 32 /8 unsigned int, 16 & 32 int, -32 & -64 real
NAXIS = 2 /number of axes
NAXIS1 = 1674 /fastest changing axis
NAXIS2 = 1248 /next to fastest changing axis

Is that image binned 2? It looks like it could be from the NAXIS1 and NAXIS2 values (assuming the CCD is a Kodak 8300??).

-Ray

Yes, Binned 2x The nominal pixel scale for my camera with the AT8RC of 1610mm FL is about 0.69 arcsec. So with a bin 2x it would be 1.38 arcsec. The QHY9m has a Kodak 8300 chip with a nominal size of 3348 x 2496 pixels.

Jerry Hubbell
Lake of the Woods Observatory (MPC I24)
Locust Grove, VA

Hi Jerry,

How good does your polar alignment get with Alignmaster? Does it quantify the error?

Cheers,

Tommy

Will the modest mount still be limited in what it can achieve compared to a very precise and rigid mount if you are willing to combine technologies: PEC + TDM + autoguiding + adaptive optics? Is it easy to add TDM to the mix and still get an incremental improvement?

Gale

Hey Tommy, Gale,

The AlignMaster Program does give you the pointing error from the North Pole in degrees, minutes, and seconds. In this way you can iterate on the adjustment to get as close as you like. I have only used it in the field a couple of times, and have been generally satisfied when I get to withn a few arc minutes of the pole. You can spend a lot of time with it to get within 30 arcseconds, but that is generally not necessary for the exposure times I am using.

To answer your question Gale, you can generally layer on correction systems, but you have to understand how they work to understand what the interactions might be. It is good to understand the time constants or frequencies each technology is designed to correct errors in. For example, The TDM is correcting errors in the range of 0.2 to 1 second time interval, the PEC is only designed to correct long term worm error in the 480 second range. When you incorporate autoguiding into the picture, I would set it to correct for errors (mainly DEC drift due to innaccurate polar alignment) in the 10-60 second range. Finally, Adaptive optics will correct for errors in the <0.2 second range down to how fast you can get your camera to make exposures.

Based on these requirements, I would consider TDM a replacement for PEC, and use a guide camera only for correcting DEC drift set to 10 second exposures. Adaptive optics would correct for seeing, and scintillation at a frequency of from 5-20 hz.

I hope this answers your questions

Jerry Hubbell
Lake of the Woods Observatory (MPC I24)
Locust Grove, VA

Yes, Binned 2x The nominal pixel scale for my camera with the AT8RC of 1610mm FL is about 0.69 arcsec. So with a bin 2x it would be 1.38 arcsec. The QHY9m has a Kodak 8300 chip with a nominal size of 3348 x 2496 pixels.

Jerry Hubbell
Lake of the Woods Observatory (MPC I24)
Locust Grove, VA

OK, my 30-min AP105 unbinned is about 2.0 arc-sec/pixel, and my 5-min AP155 barlowed is about 0.55 arc-sec/pixel. I think that in both cases the software corrected tracking has better (rounder) stars when you account for image duration and image scale. Would you not agree?

-Ray

Hi Jerry,

A couple additions to this...

To answer your question Gale, you can generally layer on correction systems, but you have to understand how they work to understand what the interactions might be. It is good to understand the time constants or frequencies each technology is designed to correct errors in. For example, The TDM is correcting errors in the range of 0.2 to 1 second time interval, the PEC is only designed to correct long term worm error in the 480 second range. When you incorporate autoguiding into the picture, I would set it to correct for errors (mainly DEC drift due to innaccurate polar alignment) in the 10-60 second range. Finally, Adaptive optics will correct for errors in the <0.2 second range down to how fast you can get your camera to make exposures.

By the Nyquist theorem you can only correct to 1/2 the frequency. So if sampling is occurring at 5 Hz, the maximum frequency that can be corrected is 2.5Hz.

Also, some PEC systems have corrections that occur 3-5 times a second, so they are capable of correcting frequencies in the same frequency range. But despite what PEC is capable of correcting you are correct that the purpose is to correct the slower worm error and possibly gear errors.

-Ray

To answer your question Gale, you can generally layer on correction systems, but you have to understand how they work to understand what the interactions might be. It is good to understand the time constants or frequencies each technology is designed to correct errors in. For example, The TDM is correcting errors in the range of 0.2 to 1 second time interval, the PEC is only designed to correct long term worm error in the 480 second range.

PEC is the only one of the correction methods able to predict an error and begin the correction in time to prevent it. As Ray points out it can correct errors much shorter than the 480 second range, for example gear box errors that are harmonic with the worm period. The problem is most modest mounts have non-harmonic errors. So PEC can take the already small error of a precision mount and reduce it by a larger percentage than the less precise mount.

The TDM corrects at a much higher frequency, so maybe the lack of a predicting capability does not matter. Perhaps the biggest advantage of the encoder technology is that it will improve over time and become cheaper because it can be adapted to many other products in addition to astronmical tracking mounts.

So given the interactions with the mount, if TDM is installed on both a precision mount and lower tier mount of the same weight class, will both track equally well unguided? Will both guide equally well? I would think the precision mount would have an edge - but how much I don't know.

Gale

OK, my 30-min AP105 unbinned is about 2.0 arc-sec/pixel, and my 5-min AP155 barlowed is about 0.55 arc-sec/pixel. I think that in both cases the software corrected tracking has better (rounder) stars when you account for image duration and image scale. Would you not agree?

-Ray

Hi Ray,
I have not measured the FWHM and elongation of the stars in my 600 sec image, or in your images, so it is hard for me to agree that your stars would be rounder without looking closer at the images, although you may be correct. Also, based on your description of the software tracking correction you are either modeling the correction for flexure and refraction based on previously acquired imaging, or it is being done in real time. Either way it sounds an awfully lot alike to using a guide camera to adjust the pointing of the scope to stay locked on the stars. The point of the TDM (for me) is to not have to rely on feedback from the stars in order to minimize the amount of extra equipment used to make excellent images. What is your software tracking correction using as a reference to correct to?

Thanks

Jerry Hubbell
Lake of the Woods Observatory (MPC I24)
Locust Grove, VA

as an aside.. the notion of using a high-precision encoder on the RA axis for corrections is not unique to the TDM (it is a $5000 option on the El Capitan).

so.. i tried looking for such encoders, the idea being "hmm.. maybe i can roll my own." only Gurley and Renishaw seem to make encoders of the required precision (>1 million pulses per revolution). maybe Johannes Heidenhain does as well but all their docs are in German.

the usual encoder makers (BEI, US Robotics, HP, Allen-Bradley..) don't make million-count encoders, they top out at around 200K PPR. and, the million-count encoders cost at least $500 (used).

so I don't really see how the TDM price can go much lower. Maybe (just maybe) it can hit $1000, but ultra-high count metrology encoders aren't exactly a mass-market item, they've been around for decades, and the astronomy market isn't really a mass market.

Hi Jerry,

OK, my 30-min AP105 unbinned is about 2.0 arc-sec/pixel, and my 5-min AP155 barlowed is about 0.55 arc-sec/pixel. I think that in both cases the software corrected tracking has better (rounder) stars when you account for image duration and image scale. Would you not agree?

-Ray

Hi Ray,
I have not measured the FWHM and elongation of the stars in my 600 sec image, or in your images, so it is hard for me to agree that your stars would be rounder without looking closer at the images, although you may be correct. Also, based on your description of the software tracking correction you are either modeling the correction for flexure and refraction based on previously acquired imaging, or it is being done in real time. Either way it sounds an awfully lot alike to using a guide camera to adjust the pointing of the scope to stay locked on the stars. The point of the TDM (for me) is to not have to rely on feedback from the stars in order to minimize the amount of extra equipment used to make excellent images. What is your software tracking correction using as a reference to correct to?

Thanks

Jerry Hubbell
Lake of the Woods Observatory (MPC I24)
Locust Grove, VA

The reference is plate-solved images precessed to JNow coordinates. The rate corrections are made by offseting the tracking rate in RA and Dec slightly from sidereal... enough to compensate for refraction, polar alignment errors, flexure, etc.

Although you could try FWHM it is probably not a good comparison because our sites are completely different and you said your image was a little out of focus.

Instead, please take a look at the stars in the AP155's 5-minute image at 0.55 arc-sec/pixel, which would be equivalent to a 10-minute image at 1.10 arc-sec/pixel. Then compare them to the stars in your 10-minute image (at 1.38 arc-sec/pixel). As I mentioned before I think I can see a slight bit of trailing in stars in your image (from upper left to lower right). It's slight, but it's there. I don't think there is any trace of trailing of stars in my AP155 image.

Now I realize that is just one image so if you have better images then my conclusion might be different.

-Ray