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Equipment Discussions >> Mounts

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wolfman_4_ever
Pooh-Bah


Reged: 07/15/11

Loc: El Segundo, Ca, So. Cal
Re: Mount Upgrade Time! AP, SB, 10Micron, etc new [Re: dawziecat]
      #5910469 - 06/08/13 09:36 PM

Quote:


So, Dave, what is it that made you decide to spring for the encoders?




I'm still trying to ask myself the same question.. I talked with AP about it. They recommend the encoders for remote observatories. The encoders always know where the mount is so this option helps a lot for remote. For a backyard observatory, what's the point... except I use my backyard OB as remote with everything automated., so I guess that is why.. Is it still worth that price? Unknown I guess. AP holds their value very well.. will the encoders option allow for the markup when it comes to to sell the mount? Unknown since I will probably keep the mount long enough until it's only worth the price of it's metal.


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orlyandico
Postmaster
*****

Reged: 08/10/09

Loc: Singapore
Re: Mount Upgrade Time! AP, SB, 10Micron, etc new [Re: Mike7Mak]
      #5910681 - 06/09/13 12:32 AM

1) I have not seen any 10-million tick encoders at a reasonable cost. The cheapest I've seen are absolute, 22-bit Hengstler at about $500 (I'm trying to justify buying some of these because using incremental encoders in my project has hit a dead end of sorts). 22 bits is 4 million ticks, but absolute accuracy is only +/- 35" (2048 ppr).

Heidenhain also has the 25-bit ECN425, but even though that's 32 million ticks, the repeatable accuracy is only +/- 10 arcseconds. Hence I am still puzzled how the TDM manages to do +/- 1" to +/- 2" with the (lower class) Heidenhain ERN180/480 incremental encoder - a feat I have not managed to duplicate.

The problems with encoder swash, eccentricity, etc. (all issues I haven't solved in my DIY encoder) are addressed in the AP mounts using multiple read heads. I fully expect the AP mounts use Renishaw encoders with the inner diameter of the RA and DEC housings as the encoder tracks.

2) Ed isn't saying this because he sells them, but the 10Micron HPS1000 is the cheapest dual absolute encoder mount that you can get. It weighs more than a Mach1, has the capacity of a Mach1, costs about $1000 more, but has dual absolute encoders.

Incidentally... Ed, if you could share the manufacturer and model number of the encoders on the GM1000HPS I would be enlightened.


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vdb
sage


Reged: 12/08/09

Re: Mount Upgrade Time! AP, SB, 10Micron, etc new [Re: Mike7Mak]
      #5910900 - 06/09/13 05:45 AM

When buying the mesu 200 we considered high res encoders ...
We've seen plenty of shots that are unguided at 20 minutes and over 2 meter ... Partner even has an ASA and doesn't need guiding ...
In the end we decided to buy without as the mesu's error is really slow and smooth and has no backlash.

Another advantage is that guider sees other errors's the encoder does not, flex, mirror movement etc ... yes you can model for those errors, but then they need to be 100% identical time after time.


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freestar8n
Post Laureate
*****

Reged: 10/12/07

Re: Mount Upgrade Time! AP, SB, 10Micron, etc new [Re: wolfman_4_ever]
      #5910910 - 06/09/13 06:03 AM

My take on encoders is the following:

If you want to do unguided imaging with a permanent setup, they would be a big win. If the mount is well modeled, then even if it is perfectly aligned there will still be a need for some motion in dec., so accurate dec. encoders would have value.

If you have such a setup then I expect good guiding with OAG alone would do better than unguided - especially if the seeing is good. If a low end mount has encoders then they may smooth out the motion and improve guided images - but I don't know why it would help with a high end mount. Either way - I don't know any examples where someone did tight oag autoguiding with and without encoders enabled - and saw an improvement in the resulting images due to encoders.

I can see the benefit of absolute encoders on a remote mount to give you a firm indication of where it is pointing - but even crude encoders would do the job to know your approx. location - and plate solves would then let you know exactly.

I can imagine that with mid-range equipment, a high res encoder would help on the dec. axis to reduce the effects of backlash and stiction. But even then that would only really be a need if stars tend to be oblong in dec. - indicating it's a limiting factor.

Overall - I would much rather have them than not have them, ignoring budget, and if I had a high end mount in a permanent observatory I would want encoders also - for accurate fast tracking of asteroids and satellites for example. But I wouldn't expect it to improve guided images.

Frank


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orion69
scholastic sledgehammer


Reged: 05/09/10

Loc: Croatia
Re: Mount Upgrade Time! AP, SB, 10Micron, etc new [Re: EFT]
      #5910957 - 06/09/13 07:57 AM

Quote:

What this type of system relies on is a very accurate model over a very accurate polar alignment. In essence, the mount can be set up, a roughly polar aligned and a group of modeling stars quickly centered and off you go. Polar alignment does not have to be perfect, ...




Frankly, I don't quite understand this...
Let's say we have a mount with absolute encoders and using most accurate model, but polar alignment is not perfect, it's just a little off.
So, how would encoders with model compensate field rotation which will most certainly happen in 30 min or longer subs if polar alignment is not accurate enough?
I'm not aware of any mount that is able to correct polar alignment in "run time" and that assumes alt-az correction of the mount.


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freestar8n
Post Laureate
*****

Reged: 10/12/07

Re: Mount Upgrade Time! AP, SB, 10Micron, etc new [Re: orion69]
      #5910974 - 06/09/13 08:25 AM

I think there can be a middle ground where the polar alignment is very accurate and would not show field rotation - but it would still benefit from very accurate and well modeled corrections in dec. A mid-range mount without encoders could not blindly make corrections in dec. that would be beneficial - but with high-res encoders I think it is possible.

But again - tight guiding with OAG would be better - and then you don't need encoders, and you don't need to model the mount and polar alignment with a bunch of stars. To me that is much more of a "set up and go" system.

Frank


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Per Frejvall
sage


Reged: 09/28/12

Loc: Saltsjöbaden, Sweden
Re: Mount Upgrade Time! AP, SB, 10Micron, etc new [Re: freestar8n]
      #5911449 - 06/09/13 02:37 PM Attachment (28 downloads)

Guys and gals, a few comments from a real absolute buff

I have two of them, a GM1000HPS and a GM2000HPS. It isn't just the fact that it can do unguided that thrills me. In essence, the totally hassle-free operation of these babies is absolutely wonderful.

I haven't calculated how much field rotation you get with, say, 15 minutes of polar error and 20 minutes of exposure, but it can't be much.

I have my balcony setup on a pier. With the GM1000 I did the following:

1. Set correct altitude on mount
2. Aimed at the chimney that I know is about N
3. Ran a 15 star alignment
4. Consulted handset and got "0.5Lf, 0.7Up"
5. Adjusted Az screw by half a turn to the left
6. Adjusted Alt screw by 0.7 turns further up
7. Ran a 55 star model followed by a model optimization

The handset now reports as shown in the image. "Running a model" means invoking my model making software - avilable for free or donation - and reading a book while the stuff is being done. "Model optimization" is a function of that software and it involves removing the worst point repeatedly until the desired target RMS error is achieved.

This is all super cool and I love technical gadgets

Now, as for guiding. Considering the fact that I shot a couple of one-hour subs with my GM2000HPS with a 190MN (1000mm FL) unguided and got round stars, I am thinking of trying with only an adaptive optics thing and no guide inputs to the mount. It may work!

Encoders. 10Micron will not tell you what encoders are in their mounts. Renishaw is one candidate and there are others. I have noticed that absolute encoders differ in terms of how absolute they are. Most will need an initial movement in order to find out where they are, and the amount of movement required decreases with increasing encoder cost.

I believe most absolute encoder equipped mounts will do a slight movement back and forth per axis in order to get a fix on the encoders.

Now, I can do one hour subs with my NEQ6 too, but I will have to guide it and I will lose more subs. I do not like guiding. Sure, it can be pretty reliable but never totally hassle free. And you lose imaging time with all the calibration and stuff.

I guess hte final verdict is that encoders are a sure way to success but you do not really need them. But just like a car, where we all do fall victims to our vanity, it is more satisfying to have a cool mount (and leather upholstery, power this, power that, more cylinders and many extra horsepower that you do not really need in order to drive).

/per


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Alph
Carpal Tunnel


Reged: 11/23/06

Loc: Melmac
Re: Mount Upgrade Time! AP, SB, 10Micron, etc new [Re: Per Frejvall]
      #5911487 - 06/09/13 03:02 PM

Quote:


3. Ran a 15 star alignment
4. Consulted handset and got "0.5Lf, 0.7Up"
5. Adjusted Az screw by half a turn to the left
6. Adjusted Alt screw by 0.7 turns further up



So a 15 star pointing model and crude knob adjustments got you 39" from the NCP!? That's a luck not a method.

Quote:

Now, as for guiding. Considering the fact that I shot a couple of one-hour subs with my GM2000HPS with a 190MN (1000mm FL) unguided and got round stars,



That tells me nothing. What's your plate scale?


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wolfman_4_ever
Pooh-Bah


Reged: 07/15/11

Loc: El Segundo, Ca, So. Cal
Re: Mount Upgrade Time! AP, SB, 10Micron, etc new [Re: Per Frejvall]
      #5911638 - 06/09/13 04:44 PM

Quote:

I am thinking of trying with only an adaptive optics thing and no guide inputs to the mount. It may work!




Better hurry up before I try it!

Nice software in those 10Micron Mounts.. To bad they don't offer the 3000 in the US yet. I really considered it. A US based manufacture was the final decision with their top notch support.

Thanks for the info Per. I believe JWalk has a 1600 with encoders using the beta ver of APCC in NM Skies.


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Per Frejvall
sage


Reged: 09/28/12

Loc: Saltsjöbaden, Sweden
Re: Mount Upgrade Time! AP, SB, 10Micron, etc new [Re: Alph]
      #5912735 - 06/10/13 10:21 AM Attachment (19 downloads)

Quote:

Quote:


3. Ran a 15 star alignment
4. Consulted handset and got "0.5Lf, 0.7Up"
5. Adjusted Az screw by half a turn to the left
6. Adjusted Alt screw by 0.7 turns further up



So a 15 star pointing model and crude knob adjustments got you 39" from the NCP!? That's a luck not a method.

Quote:

Now, as for guiding. Considering the fact that I shot a couple of one-hour subs with my GM2000HPS with a 190MN (1000mm FL) unguided and got round stars,



That tells me nothing. What's your plate scale?




Polar alignment after 15 model points is usually sub minute. No luck involved. The adjustment instructions, as well as the knobs, are very precise and works well with both the GM1000 and the GM2000. No crudeness involved there...

Plate scale should be easy to guess, at least. Most cameras seem to be around five to seven microns pixel size. In this case it was an SX M26C, so image scale was 1.24"/px. It was a test and, naturally, the subs were useless with burned-out stars.

Single sub M106, one hour unguided, SW 190MN, SX M26C, GM2000HPS.


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orion69
scholastic sledgehammer


Reged: 05/09/10

Loc: Croatia
Re: Mount Upgrade Time! AP, SB, 10Micron, etc new [Re: Per Frejvall]
      #5912767 - 06/10/13 10:40 AM

Quote:


Polar alignment after 15 model points is usually sub minute. No luck involved. The adjustment instructions, as well as the knobs, are very precise and works well with both the GM1000 and the GM2000. No crudeness involved there...





If I understand correctly, first you set 15 model points (not using alt-az knobs I presume) and after that polar alignment is sub minute?
How can that be if you didn't use alt-az knobs and mount isn't permanently setup?

Or is it after 15 model points and adjusting alt-az knobs?

Edited by orion69 (06/10/13 10:43 AM)


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orlyandico
Postmaster
*****

Reged: 08/10/09

Loc: Singapore
Re: Mount Upgrade Time! AP, SB, 10Micron, etc new [Re: orion69]
      #5912881 - 06/10/13 11:49 AM

I think he builds a 15-star model. The mount then figures out how far off the pole he is. Then it tells him how many turns to turn the Alt and Az knobs to get on the pole.

Seems like a supercharged version of ASPA. I believe it..


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EFT
Vendor - Deep Space Products
*****

Reged: 05/07/07

Loc: Phoenix, AZ
Re: Mount Upgrade Time! AP, SB, 10Micron, etc new [Re: freestar8n]
      #5913036 - 06/10/13 12:55 PM

Quote:

I think there can be a middle ground where the polar alignment is very accurate and would not show field rotation - but it would still benefit from very accurate and well modeled corrections in dec. A mid-range mount without encoders could not blindly make corrections in dec. that would be beneficial - but with high-res encoders I think it is possible.

But again - tight guiding with OAG would be better - and then you don't need encoders, and you don't need to model the mount and polar alignment with a bunch of stars. To me that is much more of a "set up and go" system.

Frank




This might help from the manual of a dual absolute encoder mount:

Quote:

Notes on autoguiding

When properly aligned, and using the dual axis tracking option, the [dual absolute encoder] mount is able to track stars with extreme accuracy. Usually autoguiding is not required, and may easily be counter-productive if the seeing is worse that about 1 arcsecond FWHM; in that case, guiding corrections will likely “correct” what are really movements of the guide star image due to the atmosphere.

With very long exposure times (tens of minutes) tracking errors could show nevertheless. In this case you will need autoguiding. In order to minimise the possible errors introduced by the guiding corrections, you will need to use a very low “aggressiveness” setting in your autoguiding software, and/or very low autoguide speed (down to 0.1x) and integration times up to several seconds. If your alignment is not very good, you may use higher autoguider speeds and faster integration times in order to track objects, but accuracy will suffer accordingly.

Note that if you need to autoguide on the declination axis, you may obtain a slightly better result if you disable the dual axis tracking option, so that the declination corrections will be always in the same direction.




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EFT
Vendor - Deep Space Products
*****

Reged: 05/07/07

Loc: Phoenix, AZ
Re: Mount Upgrade Time! AP, SB, 10Micron, etc new [Re: orlyandico]
      #5913060 - 06/10/13 01:12 PM

Quote:

I think he builds a 15-star model. The mount then figures out how far off the pole he is. Then it tells him how many turns to turn the Alt and Az knobs to get on the pole.

Seems like a supercharged version of ASPA. I believe it..




What he is doing is a combination of alignment procedures including one of two possible polar alignment routines as follows:

Iterative Alignment of the Polar Axis

This procedure uses a star (chosen among a small list) and Polaris to align the polar axis. The accuracy of the alignment will depend of the orthogonality error of the optical axis: the larger the orthogonality error, the worse the polar axis will be aligned.

1. If you have not done it before, select MENU – Alignment – Clear Align and press ENTER.
2. Select MENU – Alignment – Polar Iterate.
3. The system will provide a small list of suitable stars. Choose the star you prefer and press ENTER.
4. Now the system will ask to slew to the star. Press ENTER to confirm the slew.
5. Centre the star with the keypad and press ENTER.
6. Now the system will ask to slew to Polaris. Press ENTER to confirm the slew.
7. Centre Polaris moving the polar axis with the altitude and azimuth movements as described in paragraph 4.13 and press ENTER.
8. Repeat the procedure from step 4 until both stars are centered with a good precision, i.e. you can slew between the two without significant errors.
9. Press ESC to terminate the procedure.


Aligning the Polar Axis with the Polar Align function

This procedure uses the data computed in the alignment step to align the polar axis. If done after at least a three-stars alignment, or a two-stars alignment followed by at least one refinement, it is insensitive to orthogonality error; moreover it can be done without using Polaris, so it is very useful if you can't see Polaris from your point of view.
The first thing to do is a two-stars alignment or a three-stars alignment.
After the alignment, the mount will know the polar axis misalignment. Now do the following:
1. Select MENU – Alignment – Polar align and select a star from the list.
2. The system will ask to slew to the star. Press ENTER to confirm.
3. The scope will slew to the star and miss it. Now move the polar axis of the mount with the altitude and azimuth movements until the star is accurately centered in the field of view and press ENTER.
The polar axis now is correctly aligned, and the mount will point correctly. However, it is advisable to make a new three-stars alignment in order to improve the pointing accuracy. At this point you can't use directly the Refine 2-stars function, since you would obtain a wrong model.
In order to obtain optimal performance, it is not required to correct physically the orthogonality error; if you wish to do so, however, the mount will help you with the procedure.


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freestar8n
Post Laureate
*****

Reged: 10/12/07

Re: Mount Upgrade Time! AP, SB, 10Micron, etc new [Re: EFT]
      #5913617 - 06/10/13 05:23 PM

I don't see anything there that would explain why good oag autoguiding without encoders would be inferior to autoguiding with encoders. Encoders would allow you to guide with less aggressiveness - but what matters in the end is the actual size of stars you get for the given conditions. I haven't seen results or even anecdotal evidence that "My stars are much smaller now." But I don't doubt the unguided imaging would be better.

Frank


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psandelle
professor emeritus


Reged: 06/18/08

Loc: Los Angeles
Re: Mount Upgrade Time! AP, SB, 10Micron, etc new [Re: freestar8n]
      #5913881 - 06/10/13 08:02 PM

Per - what's the average time it takes you to run through your setup? (I don't know if you're a fast reader, or a slow reader. )

Thanks,

Paul


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Alph
Carpal Tunnel


Reged: 11/23/06

Loc: Melmac
Re: Mount Upgrade Time! AP, SB, 10Micron, etc new [Re: Per Frejvall]
      #5914445 - 06/11/13 02:06 AM

Quote:

Polar alignment after 15 model points is usually sub minute. No luck involved. The adjustment instructions, as well as the knobs, are very precise and works well with both the GM1000 and the GM2000. No crudeness involved there...



It is simply unbelievable. I have never heard anyone touting this type of performance from TPoint and the Paramount ME. TPoint has been used for a couple of decades in many professional observatories and the author of TPoint published many papers on mount pointing models. On the other hand, 10micron can’t even make the user manual available for download. They have zero credibility. I don’t buy for a minute that a 15 point model plus two knob turns can result in the polar alignment accuracy of less than one arc-minute.

Quote:

Plate scale should be easy to guess, at least. Most cameras seem to be around five to seven microns pixel size



Not at all. Pixel sizes in commercial cameras range from 3 to 26 microns.


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Per Frejvall
sage


Reged: 09/28/12

Loc: Saltsjöbaden, Sweden
Re: Mount Upgrade Time! AP, SB, 10Micron, etc new [Re: Alph]
      #5914520 - 06/11/13 04:33 AM Attachment (30 downloads)

Alph,

If you choose not to belive what I say, that is quite alright by me. I do agree that 10Micron should make the manual available, but the argument I have heard put forward against it is that they have such novel technology in their mounts that they do not want to give the competition the benefit of peeking into it.

T-point may model the sky perfectly (and I think it probably does) but it cannot read any positional feedback from the mount axis and thus can only apply corrections on a blind basis. 10Micron has the absolute encoders connected in the feedback control loop (along with the less accurate motor encoders) and produce a much tighter control pattern.

I found a technical note on this on their site: http://10micron.de/informations/differences-between-mount-drives/?lang=en


Frank,

Guiding is guiding, I agree to that. I have never had serious issues with guiding but it is still something that I have learned to enjoy being without. So, for me unguided imaging is a nifty technical feature that I enjoy, and on top of that I have very good unguided performance. Unguided imaging also gives more imaging time which I will utilize this fall when my stuff will be running ACP Scheduler.

As for the notes on 0.1x guiding speed and agressiveness, I believe those are valid notations. I have yet to try serious guiding with my 10Micron mounts, and I plan to do that this fall when darkness returns. Right now I do not even have nautical dusk and just barely civil (-7° tonight). Experiments will be with the smaller mount, the GM1000HPS that will be in a small obsy on an island in the Stockholm archipelago. The GM2000HPS will be in Provence and thus more difficult to get to.

Paul,

Time varies and as I have a fixed setup on a pier on the balcony I only have to do something when I change scope or something.

Generally speaking and looking at model run logs I can deduce that with 4 seconds exposure and 2 seconds of slew settle time, a 100-point model takes 48 minutes. That is with a slew rate of just 5°/s (my neighbors complain when I run the GM1000HPS at full speed, which us 15°/s, when the sound travels in the concrete floor of the balcony to their ceiling...)

I do not perform the iterative polar alignment described in the manual nor the "center star" polar alignment. I simply let my own software clear the model, enter new refraction data and then create plate solved alignment points in the mount. After that I just follow the info shown in my post above (the screen dumps from the handset). If it below two minutes on the first try I let it bee, else I redo once. The latter situation is somewhat unusual and the result is mostly sub minute. Somebody at 10Micron apparently calculated the effect of a knob-turn very accurately.

Attached is a log from a model run a while ago. I have since developed the model builder further and it can now do an iterative point deletion in the mount with a target expected RMS error. Usually, the optimization zaps 10-15 points for a target RMS error of 4-5".

I actually made a small screen-dump clip of my new model optimization at work. For those interested, it gives an insight as to how the model builder works and looks, and also shows the terrible terrain mask I have from the balcony

http://filer.frejvall.se/Optimize.swf.html



In general, mount technology has taken a giant leap forward with the introduction of high accuracy absolute encoders in generally available commercial products. My personal belief if that encoders are needed in modern products and that they should be included in the modeled internal control loop for best results. My take. My 2 cents

All the best from sunny Sweden,

Per


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Per Frejvall
sage


Reged: 09/28/12

Loc: Saltsjöbaden, Sweden
Re: Mount Upgrade Time! AP, SB, 10Micron, etc [Re: Per Frejvall]
      #5914521 - 06/11/13 04:38 AM

Ed,

See my post above. I do not do either of the suggested methods. I just follow the knob advise. The center star polar alignment works very well too but involves repeated exposures and time out in the cold with an iPad or something I do not have, nor have I ever had, an eypiece on my scope so the CCD is my only eye.


/per


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famax
member


Reged: 07/01/07

Re: Mount Upgrade Time! AP, SB, 10Micron, etc [Re: Per Frejvall]
      #5914564 - 06/11/13 06:41 AM

A simple test on the 10µ gm1000 hps on this french website :
http://www.astrosurf.com/ubb/Forum2/HTML/038986.html


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