78 replies to this topic

[ccrunner84]

Hi Orlyandico,

 

I don't believe so, it's been awhile since I've had it all out.  Pretty sure I tried both being perfectly balanced, or slightly east-heavy.  Disabling corrections all together always seemed to produce a nicer graph but it eventually starts drifting.  PEC has been done using TheSky's built in tool too, which corrected the periodic error to ~1.5 arc seconds P-P.

 

Could this all just be seeing related, maybe I'm expecting the impossible for my location?

 

Thanks,

 

Nick

[orlyandico]

Seeing is also a possible culprit. I have terrible seeing as well.

[freestar8n]

All I know is that I get the best results when guiding at 1 sec, and last night the guiding was generally around and under 0.5" total rms which made me happy and for quite a while it was at 0.4.  

 

Plus everything worked!   For once!  Finally making progress, now I need to learn how to use PS.

 

Here's a graph from last night...I was so happy I had to take a photo, like a proud father.

 

That's great - congratulations.

 

Sorry if my tone can be impatient at times - but this stuff about "chasing the seeing" has been around for over ten years - and it's probably prevented people from trying things that could actually improve their images.  In your case you did give it a try - and you got good results.

 

So I hope more people experiment with shorter guide periods - to one second or so.  Whenever you change one of the guide parameters - like the guide period - you probably need to tune everything else - especially aggressiveness.  But if you study the guide plot you should have a sense when it is over-correcting - and you get oscillations - or when you are under correcting - and the rms is large.

 

And you do need a decent guidestar - ideally with OAG.  In order to get good guidestars with short exposures I recommend making sure the stars are as small and non-aberrated as possible - because that really spreads the light out and makes it hard to guide on faint stars.

 

But for experimentation purposes - you could even just guide on any very bright star and not try to image anything in particular.  Just see what the fwhm is in the images.

 

Guiding well like this is also beneficial for recording the PEC curves in the first place.

 

Frank

[prostcj]

Unfortunately, there are a lot of new imagers who just assume the shorter the guide exposures and the greater the aggressiveness the better.  I started out thinking that way.  Luckily, a club member sorted me out on the aggressiveness.    One night I lengthened my guide exposures to 3 seconds because of a dim guide star and was surprised at how much better my tracking got.  I had to look up why tracking could get better with a longer guide exposure before I even heard of the term "chasing the seeing". 

Either way, it is good advise to try different settings until you find what works best for you and your set up. 

[freestar8n]

I guess that does summarize things pretty well.  If you always think the centroid is perfect and represents the correction to make - that would push you to high aggressiveness values.  And that would be bad when doing 1s corrections - because of seeing and latency.  But it just means you should dial down the aggressiveness.  You have less confidence in the guide error - so you respond less to it.  But you do it more often.  If you made a bad move in the previous correction - your new correction will be larger and you act on it.

 

So - yes if you move to faster corrections you probably want to reduce aggressiveness.  But there can be a net win in terms of just how small the stars are in the images.  I would try different guide correction periods - and at each one try different aggressiveness and other parameters - to smooth out the guide error plot.

 

Frank

[freestar8n]

I post images all the time - and lately they are with cge-pro and 1s corrections - with PEC engaged.  The PEC is trained with the PECTool and about 8 turns of the worm.  Just simple averaging and loading into the mount.

 

Here is one of my recent postings:

 

http://astrogeeks.co.../antnebula.html

 

That shows raw results with 30m exposures and guide corrections every second - to yield about 1.7" fwhm at 0.4" per pixel.

 

I compare that result with *unguided* hyperstar images with PEC engaged - but those exposures are only 30 seconds instead of 30 minutes - because I used a low read noise ASI-1600 camera.

 

PEC is engaged in both images - and it probably helps at f/10 guided - but it definitely helps quite a bit at f/2 unguided - so that I can image unguided and get decent results.  The fwhm is not as good at f/2 as it is at f/10 - but it's still a nice result - and PEC is critical.

 

Frank

[Ken82]

How do gear teeth and worm period play a part in the periodic error ? I mean if you have a longer worm period is that a good thing ?

[james7ca]

...I have often wondered about putting an ND filter on the guide scope so that I can use 5-10 second exposures and still have non-saturated stars..

Rather than an ND filter, why don't you try a near-IR filter? That should help with the saturation and also steady the seeing.

 

... If your peak to peak PE is under 1", as long as you have a guide alignment, there would be no need to guide since a TEC 140 (.75 reducer) and KAF 8300 will sample at close to 2" / pixel. 

 

Just to state the obvious, even with a very low (or even zero) error in the tracking you'll still need to guide if you use exposures that are long enough to be affected by atmospheric refraction or flexure in your mount or optical system.

 

Further, in my opinion, when guiding and when using a separate guide scope differential flexure can be just as much of a problem as the quality of your mount or your seeing conditions. In fact, unless you have a very rigid optical path you may even see flexure with an OAG.

[james7ca]

How do gear teeth and worm period play a part in the periodic error ? I mean if you have a longer worm period is that a good thing ?

I think a longer worm period can be a good thing, particularly if your PE has more than just the primary frequency. I have a Mach1 mount that has this problem, it has a fairly low total range in its PE but it also has a significant secondary period so that the rate of change is quite high. A Mach1 has a worm period of just over 6 minutes and mine has a total PE of around 10 arc seconds. Normally, that could mean +/- 5 arc seconds every 3 minutes, but the mount also has a secondary that has a period of 90 seconds so the rate of change is several times as bad as you might expect given only the primary worm period.

[prostcj]

 

...I have often wondered about putting an ND filter on the guide scope so that I can use 5-10 second exposures and still have non-saturated stars..

Rather than an ND filter, why don't you try a near-IR filter? That should help with the saturation and also steady the seeing.

 

... If your peak to peak PE is under 1", as long as you have a guide alignment, there would be no need to guide since a TEC 140 (.75 reducer) and KAF 8300 will sample at close to 2" / pixel. 

 

Just to state the obvious, even with a very low (or even zero) error in the tracking you'll still need to guide if you use exposures that are long enough to be affected by atmospheric refraction or flexure in your mount or optical system.

 

Further, in my opinion, when guiding and when using a separate guide scope differential flexure can be just as much of a problem as the quality of your mount or your seeing conditions. In fact, unless you have a very rigid optical path you may even see flexure with an OAG.

 

Really?  Better not tell the guys using absolute encoders unguided then.  The guide exposures need to be long enough to not "chase" the seeing.  Not to mention a lot of people don't guide for exposures under 5 minutes.  Pretty sure 5 minutes is long enough to be affected by atmospheric conditions, if not, I need to put a telescope in that location!

Edited by prostcj, 12 February 2017 - 12:07 PM.

[james7ca]

 

 

...I have often wondered about putting an ND filter on the guide scope so that I can use 5-10 second exposures and still have non-saturated stars..

Rather than an ND filter, why don't you try a near-IR filter? That should help with the saturation and also steady the seeing.

 

... If your peak to peak PE is under 1", as long as you have a guide alignment, there would be no need to guide since a TEC 140 (.75 reducer) and KAF 8300 will sample at close to 2" / pixel. 

 

Just to state the obvious, even with a very low (or even zero) error in the tracking you'll still need to guide if you use exposures that are long enough to be affected by atmospheric refraction or flexure in your mount or optical system.

 

Further, in my opinion, when guiding and when using a separate guide scope differential flexure can be just as much of a problem as the quality of your mount or your seeing conditions. In fact, unless you have a very rigid optical path you may even see flexure with an OAG.

 

Really?  Better not tell the guys using absolute encoders unguided then.  The guide exposures need to be long enough to not "chase" the seeing.  Not to mention a lot of people don't guide for exposures under 5 minutes.  Pretty sure 5 minutes is long enough to be affected by atmospheric conditions, if not, I need to put a telescope in that location!

 

But, in your original post you said NOTHING about needing a mount with absolute encoders when doing unguided photography. But even then you need to build and then use a good sky model which is something else that was not yet mentioned in this thread. Sure, you can do short exposures without guiding if you have a reasonably good mount and are working at image scales that aren't too extreme and I even alluded to that in my original response ("...you'll still need to guide if you use exposures that are long enough...").

 

As further support, I can tell you that when I've talked to the technical support staff at Astro-Physics they say that you absolutely need to guide when using one of their mounts. That said, you only need to look at some of the posts I've done here on CN to see that you can do pretty good work unguided, I've done dozens of successful images unguided with mounts that have ranged from an AVX to a Mach1. In fact, I've done wide-field, narrow-band photography using several-minute-long exposures with a 105mm telephoto lens unguided on an AVX mount. Thus, there are a lot of issues at play here and having a good mount with low PE (or lowered with PEC/PEM) is only one of those factors.

[prostcj]

Here is what you said:

 

Just to state the obvious, even with a very low (or even zero) error in the tracking you'll still need to guide if you use exposures that are long enough to be affected by atmospheric refraction or flexure in your mount or optical system.

 

This is not only not obvious but wrong.  I brought up absolute encoders as an example of close to zero tracking error.  PEC is for mount errors, guiding is for tracking errors.  PEC has nothing to do with seeing and guiding can only be negatively affected by guiding unless the exposures are long enough to average out the refraction pattern or you are using adaptive optics with really, really short exposures to actually keep up with the movement of the guide star due to refraction.

 

Not sure how telling me that you proved astrophysics wrong by imaging unguided with one of their mounts bolsters your assertion that one has to guide with zero tracking error due to seeing conditions.

 

The point I was making originally is if someone has 1 arcsec/pix peak to peak mount error due to PEC, and is imaging at 2 arcsec/pix, they can achieve good results unguided if they tackle the other tracking error (mainly polar alignment). 

Edited by prostcj, 12 February 2017 - 09:09 PM.

[Jerry Hubbell]

Periodic error is just the periodic component of the tracking error. It isn't all of the error. Generally, it is only half of the error, maybe even less. As you could see in the raw data, while the periodic error of my CGE Pro is 9", the actual error cycle over cycle is more like 15". But the error within a cycle is usually around 9".

 

The same applies to the data after correction was applied. Even though the PE was reduced to less than 2" after PEC was turned on, the raw data had more like 10" of variance, but within a single worm cycle the variance was much smaller. Less than 3" for some periods of time.

 

Is native PE meaningful? Yes, because it does generally indicate the tightness of the mount's tracking. Mounts with lower native PE are generally tighter in all of their components and track better than mounts with higher native PE. It was very hard to get results approaching the M33 shot with my CGEM, even while guiding, and it had a native PE of around 25". 

 

Is corrected PE meaningful? If we are talking unguided imaging, yes, but keep in mind that it isn't going to eliminate all tracking error, but if the native PE is low enough, and the PEC then reduces that further, it can work for short periods with short focal lengths. In my example above, shooting 2 minutes unguided at 1960 mm, while it was "ok", I would have to throw away too many exposures, which is a lot harder than guiding and keeping all of the exposures.

 

Is corrected PE meaningful when guiding? You will hear different answers from different people on this. This is because if you look at the raw data and the combination of little errors, like seeing, the auto-guider is making corrections every 1 to 5 seconds, and at that time scale, the little errors are much larger than the slowly moving periodic error. Nonetheless, I do think I get a 10% to 20% improvement in my guiding when I enable PEC. And the data certainly shows periods of time that would require less corrections. But keep in mind that I have conquered many of the other subtitles of guiding, like guide camera focus and balancing. Those aspects would cover up a small 10% or 20% improvement if not tended to.

I think what you have demonstrated here is the argument I have been making for several years that PEC and PPEC only handles up to 50-60% of the problem on 90+% of the mounts out there that people are using. The non-periodic components make up a large chunk of it also. That is why I advocate for the use of high-resolution incremental encoders on the RA axis. This takes care of both Periodic Error and Non-Periodic error as measured at the RA shaft. What's left is seeing, flexure, and atmospheric refraction, which can be modeled and accounted for also. Auto-guiding really takes care of anything that PPEC would resolve and therefore renders it a bit redundant and really not worth the effort in my opinion. That is why I decided not to include PEC in the Explore Scientific PMC-Eight controller system. I have had some push back on that, but I think it is a good engineering decision. Thanks for the thread.

[Jerry Hubbell]

"I am not sure what you mean by it's not all the error? Do you mean it's not all the error in the mount or considering PA error, cone error and stuff like that all together to do unguided imaging?"

 

I was only showing that periodic error is the error that is repeatable, predictable and can be negated with periodic error correction, but that it is only part of the "mechanical error" in a mount. There are other causes such as bearings, flexure and imprecise gearing that is not predictable and periodic error is generally only half of the mechanical error. The terms you mention are also causes of tracking error, but are longer term and can be predicted and modeled to some extent.

 

Many users tend to believe that periodic error makes up more of the mechanical error than it actually does. No doubt, the industry uses the term like they use to use 600X magnification. The reality is that you have mechanical error and only part of it is periodic and the split is often 50/50, even with premium mounts. Nonetheless, premium mounts are made more precisely, have much lower native PE, and much lower mechanical error. It isn't their low native PE that makes them better, they are simply made better, and as a result of that, have lower native PE as well as lower mechanical error overall.

 

Also, I was showing that while using PEC to negate the periodic component can improve unguided imaging, it can only go so far, and it offers very little improvement for guided imaging. This is mainly due to the short window of time (< 5 sec) in which guiding operates and in that time frame, the main errors are seeing and random mechanical errors. The only way to improve that is to make a better mount with lower mechanical error and image on better nights of seeing. Periodic error correction doesn't greatly affect what is happening in the short window of time an auto-guider operates. This is why schemes to make premium mounts out of regular mounts by correcting periodic error using encoders do not actually result in premium performance.

 

While native PE can be a useful measure of a mount's performance, it is not a guarantee. You can have low PE and yet have large non periodic mechanical error. Just because a manufacturer claims PE numbers that are as low as what a premium mount achieves, that doesn't mean the manufacturer has addressed all of the other mechanical errors similarly. The only real proof is how well the mount images. And keep in mind the focal length (pixel scale) you intend to use it at.

I disagree with your statement here "This is why schemes to make premium mounts out of regular mounts by correcting periodic error using encoders do not actually result in premium performance." You forget that high-resolution incremental encoders running at 5 samples per second easily correct the non-periodic component along with the periodic component in mid-range mounts. It's the non-periodic components that cause a large part of the out-of-round stars even when auto-guiding. That is why I moved from auto-guiding to using the TDM 6 years ago. The only problem with mid-range mounts as compared to premium mounts is this non-periodic error in the gear train. It's very frustrating that even when auto-guiding with an EQ6 or ATLAS you still have to throw out up to 30-40% of your frames due to poor quality worms and mesh issues. No amount of smoothing of the gears in these mounts takes care of 100% of the problem.

[james7ca]

Here is what you said:

 

Just to state the obvious, even with a very low (or even zero) error in the tracking you'll still need to guide if you use exposures that are long enough to be affected by atmospheric refraction or flexure in your mount or optical system.

 

This is not only not obvious but wrong...

Craig, I'm not sure what else I should say, but my original statement was certainly not "wrong."

 

When I say "atmospheric refraction" I'm NOT talking about what is commonly termed the "seeing conditions" (I don't believe that those two terms are generally applied as representing the same condition). Rather, I'm talking about the fact that the tracking rate you need to use when following the motion of a star varies over time depending upon the altitude of the star and the resultant refraction that displaces said star from the location at which it would appear based upon a fixed sidereal rate of motion. Thus, even with a "perfect" sidereal rate of tracking with zero periodic error and a good polar alignment you're going to see an apparent drift or displacement of the star if you attempt to do any long exposure imaging. Furthermore, flexure in your mount or optical system is also going to potentially cause a drift even if your mount has absolute encoders and is tracking at a "perfect" sidereal rate.

 

In fact, this is exactly why you need to create an accurate pointing model to allow unguided, long-exposure imaging. If you don't correct for atmospheric refraction and for flexure in your system then you could see literally arc minutes of movement in a star's position over a period of several hours of tracking. Now, scale that down to an exposure time of 15 or 30 minutes and you can still see displacements that can easily be measured in multiple arc seconds. This is one reason why short exposure, unguided imaging can work, because you can mostly ignore the effects of atmospheric refraction since the star's position or altitude is changed very little if your exposure times are short. Similarly, short exposures are unlikely to be affected that much by flexure in your optical system.

 

 

 

...Not sure how telling me that you proved astrophysics wrong by imaging unguided with one of their mounts bolsters your assertion that one has to guide with zero tracking error due to seeing conditions.

Quite frankly, I'm confused by the reason or logic behind this statement.

 

 

Let me see if I can rephrase both of our original statements to eliminate any further confusion.

 

You said:

 

If your peak to peak PE is under 1", as long as you have a guide alignment, there would be no need to guide since a TEC 140 (.75 reducer) and KAF 8300 will sample at close to 2" / pixel.

Well, that's true but only in limited cases (those being short exposure times). However, if you had said (bold statement is my addition):

 

 

If your peak to peak PE is under 1", as long as you have a guide alignment, and a mount with absolute encoders with a good pointing model, there would be no need to guide since...

I would pretty much agree with that modified statement.

 

 

Then I said:

 

Just to state the obvious, even with a very low (or even zero) error in the tracking you'll still need to guide if you use exposures that are long enough to be affected by atmospheric refraction or flexure in your mount or optical system.

Which I maintain is certainly true for the vast majority of users and which could stand alone as a valid rebuttal to your previously unqualified statement.

 

 

But maybe I could have eliminated any potential for disagreement by saying the following:

 

Just to state the obvious, even with a very low (or even zero) error in the tracking you'll still need to guide if you use exposures that are long enough to be affected by atmospheric refraction or flexure in your mount or optical system. Unless, you have a mount with absolute encoders and have a good pointing calibration so as to try and eliminate the effects of atmospheric refraction and flexure.

 

In fact, it was only after our original statements that you brought in the qualification of using a system with absolute encoders and even then you failed to mention that you'd still need a pointing model to allow relatively unrestricted, unguided imaging.

Edited by james7ca, 13 February 2017 - 01:58 AM.

[james7ca]

Craig (and anyone else following the discussion), I see that Jerry just posted the following:

 

...That is why I advocate for the use of high-resolution incremental encoders on the RA axis. This takes care of both Periodic Error and Non-Periodic error as measured at the RA shaft. What's left is seeing, flexure, and atmospheric refraction, which can be modeled and accounted for also...Thanks for the thread.

 

IMO, he has made an fully accurate statement, and note the section concerning flexure and atmospheric refraction which he separated from just "seeing" and periodic error. Am I just crazy or did he just say basically the same thing that I've been representing?

[syscore]

If the TDM had actually worked well, wouldn't we all have them? At least all of us without premium mounts. The reason it failed is that if the mount itself isn't precise, it can't react precisely to the encoders. It was an idea years ago, but it has been throughly tested now, and it is history.

[prostcj]

Craig (and anyone else following the discussion), I see that Jerry just posted the following:

 

...That is why I advocate for the use of high-resolution incremental encoders on the RA axis. This takes care of both Periodic Error and Non-Periodic error as measured at the RA shaft. What's left is seeing, flexure, and atmospheric refraction, which can be modeled and accounted for also...Thanks for the thread.

 

IMO, he has made an fully accurate statement, and note the section concerning flexure and atmospheric refraction which he separated from just "seeing" and periodic error. Am I just crazy or did he just say basically the same thing that I've been representing?

Ah, gotcha.  I was confusing terms.  Seems like maybe even modeling would not fully account for atmospheric refraction since I would imagine it would change throughout the night.

[spokeshave]

If the TDM had actually worked well, wouldn't we all have them? At least all of us without premium mounts. The reason it failed is that if the mount itself isn't precise, it can't react precisely to the encoders. It was an idea years ago, but it has been throughly tested now, and it is history.

I think that one of the primary drawbacks of the TDM was that by the time you bought everything necessary to attach it to your mount, you were approaching the cost of a premium mount. $1700 for the encoder and electronics and another $500 for the mount-specific adapter. 

 

Tim

[galaxy_jason]

It is funny to me that people talk so much about chasing seeing. If a mount doesn't try to keep the star centered during turbulent seeing, the the star image will still suffer. The star will continue producing light and that will show up as smear or multiple areas of multiple exposure. So, if the mount could keep up with seeing, that could actually help IMO.

 

Roland told me once, that he was getting better results with 1 second guide exposures and high aggression settings, kind of counter to what is considered the case by many.

Blueman 

This depends of the latency of sending a guide command to the motor controller via the guide port or ASCOM. 
In the case of Meade's starlock system the guide scope takes over direct control of the motor controller. The 

brighter the guide star, the faster the corrections and the the better it guides. On the same telescope, an 

external guider might be better suited by a 1 or 2" correction "cadence". 

 

On a mount with low native PE, PEC may make things worse because most people use the default setting

of PEMPro which, even with FFT will try to correct the high order fundamentals. I usually turn off all but the

first 1-4 fundamentals when training. 

 

On a mount with 10" or more PEC is a must. Remember that PE and non PE errors add. You can only get rid

of non-PE drive roughness by lapping (which Meade now does), but you can reduce the component of drift

that PE adds with PEC. This assumes long focal length of course. If you are shooting with a short FL refractor

then who cares. 

[DuncanM]

If the TDM had actually worked well, wouldn't we all have them? At least all of us without premium mounts. The reason it failed is that if the mount itself isn't precise, it can't react precisely to the encoders. It was an idea years ago, but it has been throughly tested now, and it is history.

Not trying to be argumentative, but I assumed that they did work pretty well.

[syscore]

I followed the yahoo user group for a couple years, till it became deserted. The expense (as Tim pointed out) was one thing, but after spending that kind of money, you still didn't have the performance of a premium mount, nor even the performance of a well guided mount. That to me is "not working". Especially if we are talking about a $1,700 piece of equipment. And you still had to guide because the TDM only affected RA, not DEC, which was a gut punch to the folks obsessed with unguided imaging, and getting guiding and TDM working together was quite problematic. Some say that the TDM was simply not accurate enough and all it could really do is give unguided like (in RA at least) performance with a short focal length scope, which is probably true when you consider that real absolute encoders cost much more ($6k to $7k). But I also think the issue is with the lack of precision in the mounts. In any event, even at $1,700, if it did work pretty well, we should be awash with them, and the user group busy. I certainly would have bought one. $1,700 is a lot easier to swallow than $6,000 for a premium mount.

[JohnH]

This is why stacking is suck a boon to a lot of astrophotographers.

 

The mounts drive is well behaved for minutes at a time

[Jerry Hubbell]

I followed the yahoo user group for a couple years, till it became deserted. The expense (as Tim pointed out) was one thing, but after spending that kind of money, you still didn't have the performance of a premium mount, nor even the performance of a well guided mount. That to me is "not working". Especially if we are talking about a $1,700 piece of equipment. And you still had to guide because the TDM only affected RA, not DEC, which was a gut punch to the folks obsessed with unguided imaging, and getting guiding and TDM working together was quite problematic. Some say that the TDM was simply not accurate enough and all it could really do is give unguided like (in RA at least) performance with a short focal length scope, which is probably true when you consider that real absolute encoders cost much more ($6k to $7k). But I also think the issue is with the lack of precision in the mounts. In any event, even at $1,700, if it did work pretty well, we should be awash with them, and the user group busy. I certainly would have bought one. $1,700 is a lot easier to swallow than $6,000 for a premium mount.

The TDM works fine, just as advertised. I have used one for 6 years now. You have to be smarter than your equipment to understand what it is good for and what it isn't. For the purpose it was designed for, correcting for mid to lower grade mount worm and wheel defects, it is awesome. You can't auto-guide out problems that occur over time frames of 100-500 milliseconds with an auto-guider. As far as correcting for declination drift while using the TDM, the current version works fine with an auto-guiding camera to correct for that. I found that with a very good polar alignment (no detectable dec drift over 5 minutes), then you can take unguided photos with the TDM operating just fine. Here is one from last year with TDM on a 20 year old Meade 12" LX200 fork mount. This is an image of NGC2907 taken April 12, 2016. This is a stack of 12 120 sec exposures. The focus isn't perfect, but it shows the performance of the mount with the TDM. This was taken with a ten year old SBIG ST2000XM. The focal length of the imaging train for this image was 1583 mm.

 

 

The big issue for me was the time and effort it took to babysit the auto-guiding and even then you could only expect maybe 60-80% good frames. I don't have a huge amount of time to waste doing astrophotography so I wanted something that would just work, and make my mount disappear. I want all my equipment to be transparent and not present any barrier between me and the object I am gathering data for. That is my personal philosophy on how equipment should behave. I should not have to think about it once it is tuned up and working. You have to know your equipment inside and out for that to happen though.

 

This may not be politically correct, but for those that cannot afford the high end mounts that have very smooth PE and get bullied into only showing images with perfectly round stars, the TDM contributes to that for a reasonable price. Those that have used high-resolution encoders to correct their mounts tracking rate understand what it is good for and what it is not.

Edited by Jerry Hubbell, 18 February 2017 - 04:25 PM.

[Jerry Hubbell]

If the TDM had actually worked well, wouldn't we all have them? At least all of us without premium mounts. The reason it failed is that if the mount itself isn't precise, it can't react precisely to the encoders. It was an idea years ago, but it has been throughly tested now, and it is history.

I thought I would revive this thread with an image of M27 The Dumbbell Nebula. This was taken with the 6-inch ES 152 ED APO CF scope with the 3-inch ES 0.7x FR/FF and SBIG ST2000XM camera. The mount is a Losmandy G11 with PMC-Eight and Telescope Drive Master (TDM) drive correction system. The FOV is 48x36 arc-min and the FL is 851.2 mm (as measured with a plate solve). This provides and effective f/5.6 focal ratio. This image is 6 x 5-min frames calibrated and stacked for a total exposure of 30 minutes.

 

To be clear, this is an UNGUIDED image only relying on the accurate polar alignment and TDM.