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Durval Menezes
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Reged: 08/30/10

Loc: Rio de Janeiro, Brazil.
DSC precision troubles
      #4394885 - 02/18/11 12:46 PM

Hello CN folks,

After a few weeks getting my DSCs operational (long story), I'm still having problems... maybe someone here can help point me in the right direction.

My current setup uses optoelectronic encoders with 256-step resolution (which on quadrature mode yields 1024 steps) connected to a bluetooth version of David Ek's DSC box. I connect my Palm PDA (a Treo 680) running Astromist 2.6 to it and, after a 2-star alignment procedure (no initial "zenith-pointing" like on the Intelliscope is required on my setup) I'm off to observing.

CORRECTION: my encoders' resolution is actually 4096 steps (1024 with quadrature), *not* 1024 (256 with quadrature) as stated above. Sorry for my mistake.

What I don't like is the precision: even using my widest-field EP (a Baader 31mm Aspheric, which on my scope has a TFOV of 1.8 degrees) the object I'm trying to point to is almost never at the center of the field, and sometimes the object isn't even shown (and I have to scan around for it manually). My first question: Is this normal? What kind of pointing precision (in degrees) you folks usually get from your DSCs?

I've been experimenting with Astromist's "N-star alignment" procedure (where, after the 2-star alignment, you must align the scope with at least 4 other stars) and it certainly seems to improve precision, but it's a pain in the neck: I have a lot of obstructions at my main observing place (about a quarter of the sky visible, on a "corridor" running approximately from NNE to SSW), and to get 6 different widely spaced stars takes a lot of time (the other day I was up at 4:30AM for some observing and by the time I finished aligning the telescope, it was almost 5:30AM and the pre-dawn sun was already starting to spoil the sky).

When I got the scope, it had some skidding on the azimuth encoder (the encoder axle was slipping inside a rubber ring where it connects to the scope base) but I've managed to fix that and now I can guarantee that both the encoder axles moves in absolute sync with the scope itself. Also, I don't think that the DSC is losing pulses from the encoders.

My second question is, what do you folks do to increase the precision of your DSC setups? Any hints?

Thanks in advance,

Edited by Durval Menezes (02/19/11 09:09 AM)


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Skylook123
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Reged: 04/30/05

Loc: Tucson, AZ
Re: DSC precision troubles new [Re: Durval Menezes]
      #4395154 - 02/18/11 03:11 PM

In my experience with a variety of DSC packages, there are a couple of factors that can affect how accurately you are able to point. Here are a few, in no particular order.

Orthogonality of the setup. Are the two axes truly perpendicular, or is there some offset? This includes some looseness, or even some manufacturing or assembly misalignment.

Movement or shift in the setup if it is a dobsonian. Many dob OTAs slip side to side in the rocker box cradle, and this can introduce some error. When I have used a tube-type OTA, I've put furniture bumbers on the side of the OTA to keep it from shifting in the cradle.

Stability of the alignment of the optical axis. Does the refractor tube move a bit in the cradle or on the dovetail, or does the SCT mirror "flop" in going through the meridian, or does the mirror shift during focusing of an SCT, or does the focuser shift during the alignment process, or does the instrument need collimation? These all can lead to a warped sense of the sky in the computer. Ways to improve the performance are to make sure the optical tube is secure, check the collimation, never change focus once the alignment process is started, and if you have an SCT, check the user material or vendor to see if there is a way to either reduce the mirror flop or actually lock it down during alignment. And once the alignment process starts, if you have a newtonian on a GEM mount, don't rotate it in the rings no matter how uncomfortable, if at all possible.

Finally, the biggest error cause I've seen is in the human at the eyepiece. Many people don't actually align their pupil with the center of the eyepiece. This can give some parallax error such that if you change sides of the meridian in a two star alignment, you introduce a double offset from the optical axis. Another potential problem is too low a power causing a variance in position. The way I cured my alignment troubles was going to a high power reticle eyepiece that nailed down the center very well. Also, I use a defocused star, like a bagel. This is easier to center accurately than a pinpoint star.

I think that collimating, minimizing mirror shift on SCTs, and using a defocused star in a reticle eyepiece at very high power are great ways to improve the accuracy of star alignments.

Good Luck!


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Jon Isaacs
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Re: DSC precision troubles new [Re: Durval Menezes]
      #4395170 - 02/18/11 03:17 PM

Durval:

It sounds like you have system up and running reasonably well, not perfectly but pretty good and you just need to fine tune it. You have done a good job.

I think your questions are beyond the scope of the beginners forum and would probably be better suited for the mounts forum where people who have real experience with encoders and DSCs hang out...

Best wishes

jon


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RAKing
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Re: DSC precision troubles new [Re: Jon Isaacs]
      #4395179 - 02/18/11 03:22 PM

I agree with Jon and will move your thread.

Ron


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dale c
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Re: DSC precision troubles new [Re: Durval Menezes]
      #4395240 - 02/18/11 03:53 PM

at 1024 steps the best you can expect is about 0.35 degree accuracy. To increase resolution you have no choice other than using higher resolution encoders.
Dale


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gnowellsct
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Re: DSC precision troubles new [Re: dale c]
      #4395297 - 02/18/11 04:22 PM

There are a number of different sources of flexure and inaccuracy in both dobson and equatorial mounts. At 4096 tics (the old equatorial standard) the best one could typically hope for was about 20 arc minute pointing error.

In my current setups I have pointing errors of around 2-4 arc minutes (on the AP900 with 10k tics) and 3-6 arc minutes (on a G11 with 8192 tics).

Higher resolution encoders do "buy" some increased accuracy but they typically are about third on the list of things to do. 1.8 degrees pointing error is not a system I would consider working very well: I didn't like 20 arc minutes. However, how fine you need to go is a function of aperture and field of view, field of view compensates for a lot of pointing error and aperture helps making things more obvious to the eye (but limits field of view). In any case on my 5" refractor 3-5 arc jminutes of pointing error is overkill: twenty arc minutes is plenty of accuracy.

You have several ways to approach the problem I've tried them all. You can try to nail down the hardware accuracy as Floyd points out the orthagonality is important. So that means shimming and tightening etc. You can extract more accuracy from the encoders by upgrading them but if your primary source of error lies elsewhere it won't work too well.

You can get an Argo Navis computer or hook your encoders up to a laptop with software bisque. What we're getting at here is a pointing model which takes data from the sky and then analyzes how the star/object positions that you report differ from reality, and constructs a computerized compensation. Argo navis offers this. Software Bisque has a program that does it. It is built into Gemini. Other go-to systems have partial versions (not as many variables are modeled, such as Celestron).

In any case by far and away the most effective thing I did to improve pointing accuracy was to dump NGC Max and dump Sky Commander and get Argo Navis. You can go to their web site (wildcard-innovation.au) and download the pdf of their instruction manual and then look at the mount modeling/pointing error sections of the catalog. If you zap a few stars the computer will model the various sources of error and then compensate the coordinates.

Argo Navis' pointing model got my pointing error out of the 18 to 25 arc minute range down to the 5 to 10 arc minute range, and upgrading the encoders shaved those values approximately in half. So the biggest single thing I did was to upgrade the computer.

Pointing models for dobs are less complicated than pointing models for equatorial mounts. If you zap stars only in the areas that you can see the compensation model will be most accurate in the areas that you can see. If you have all-sky access you would get a better statistical all-sky performance if you zap all-sky stars.

How many stars you choose to start with is up to you, it is possible to add additional alignments as you observe (smaller tighter objects are best) so if you do some alignments and then do some observing on small planetaries or tight globs you can do some observing and improve the model as you do so.

there is a Yahoo group Argo_Navis_dtc and the aforementioned home page for wildcard innovations.

My friend Pete used to have pointing accuracy of about ten to twenty arc minutes with his Obsession servoCAT go-to mount. Part of the issue that affects pointing accuracy in go-to systems is backlash compensation. I was able to get greater accuracy because I tended to build 10 star models and because CNC machined aluminum is intrinsically more rigid than wood with inserted truss poles.

regards
Greg N


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Durval Menezes
super member


Reged: 08/30/10

Loc: Rio de Janeiro, Brazil.
Re: DSC precision troubles new [Re: gnowellsct]
      #4397086 - 02/19/11 01:37 PM

Hello Jim, Jon, Ron, Greg (in chronological order):

First of all, many thanks for the all the solid advice.

Second, I must confess I made a mistake: my encoders' resolution are actually 4096 (1024 with quadrature mode), not 1024 as I stated in my first post; I've just edited it to add that correction and avoid misleading future readers.

I will reply to each message following this one.

Cheers,


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Durval Menezes
super member


Reged: 08/30/10

Loc: Rio de Janeiro, Brazil.
Re: DSC precision troubles new [Re: Skylook123]
      #4397118 - 02/19/11 02:02 PM

Hello Jim,

Quote:

Orthogonality of the setup. Are the two axes truly perpendicular, or is there some offset? This includes some looseness, or even some manufacturing or assembly misalignment.




You are right, now that you mention it I remember that the encoder axle isn't perfectly orthogonal to the ground board, I'd say it's something like 5 or 10 degrees from a straight angle. That will take some work to rectify :-/


Quote:

Movement or shift in the setup if it is a dobsonian. Many dob OTAs slip side to side in the rocker box cradle, and this can introduce some error. When I have used a tube-type OTA, I've put furniture bumbers on the side of the OTA to keep it from shifting in the cradle.





I don't think I have that on my setup: I've done some experiments and determined that the bottom mirror box with its side attitude bearings fit very snugly onto the rockerbox.


Quote:

Stability of the alignment of the optical axis. Does the refractor tube move a bit in the cradle or on the dovetail, or does the SCT mirror "flop" in going through the meridian, or does the mirror shift during focusing of an SCT, or does the focuser shift during the alignment process, or does the instrument need collimation? These all can lead to a warped sense of the sky in the computer.
Ways to improve the performance are to make sure the optical tube is secure, check the collimation, never change focus once the alignment process is started, and if you have an SCT, check the user material or vendor to see if there is a way to either reduce the mirror flop or actually lock it down during alignment. And once the alignment process starts, if you have a newtonian on a GEM mount, don't rotate it in the rings no matter how uncomfortable, if at all possible.




Mine is a newtonian reflector on a dobsonian mount (so the refractor/SCT/GEM problems would not apply) and I try to keep it very well collimated (and always test it via a star test at the beginning of my observations, recollimating if I don't get the perfect bullseye-like Airy discs figure). On the other hand, I think I have some shift on the focuser (it's a HC-2 helical focuser). Will try your suggestion of not changing the focus during alignment.


Quote:

Finally, the biggest error cause I've seen is in the human at the eyepiece. Many people don't actually align their pupil with the center of the eyepiece. This can give some parallax error such that if you change sides of the meridian in a two star alignment, you introduce a double offset from the optical axis.




I think you really hit the head on this one: I have no reticulated eyepiece (will try to purchase one ASAP), and also I always align using my low-mag wide-field EP, thereby compounding the problem... will try aligning with my highest mag EP (Baader 5mm, at 249X) while I don't get the reticulated EP, in order to minimize these errors.


Quote:

Another potential problem is too low a power causing a variance in position. The way I cured my alignment troubles was going to a high power reticle eyepiece that nailed down the center very well.




OK, how much power would you say is good enough? I did a quick check and it seems I can purchase illuminated reticle EPs for 5mm, 12.5mm, 20mm and 30mm focal lenghts... which one do you think would fit the bill (my scope is a 10" f4.9 newtonian reflector on a dobsonian base, with a 2" HC-2 helical focuser)?


Quote:

Also, I use a defocused star, like a bagel. This is easier to center accurately than a pinpoint star.



This looks like a great idea! You mean defucused like I was doing a star test, right? That way, the Airy discs would make the star look like a bullseye target, very good for pinpoint precision...

Quote:

I think that collimating, minimizing mirror shift on SCTs, and using a defocused star in a reticle eyepiece at very high power are great ways to improve the accuracy of star alignments.




I will try first with my current high-mag unreticulated EP, then with a high-mag (how much?) reticulated EP, always on defocused stars and not moving the focuser during colimation. If this is not enough, I will try to make the encoder axle work on an orthogonal angle.

Thanks for the great tips, Jim!

Cheers,


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Durval Menezes
super member


Reged: 08/30/10

Loc: Rio de Janeiro, Brazil.
Re: DSC precision troubles new [Re: Jon Isaacs]
      #4397123 - 02/19/11 02:06 PM

Hello Jon,

Quote:

Durval:
It sounds like you have system up and running reasonably well, not perfectly but pretty good and you just need to fine tune it. You have done a good job.




Thanks for the encouragement! :-)


Quote:

I think your questions are beyond the scope of the beginners forum and would probably be better suited for the mounts forum where people who have real experience with encoders and DSCs hang out...




Good idea, and sorry if I should have figured it belonged on the mounts forum in the first place...


Cheers,


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Durval Menezes
super member


Reged: 08/30/10

Loc: Rio de Janeiro, Brazil.
Re: DSC precision troubles new [Re: RAKing]
      #4397125 - 02/19/11 02:07 PM

Quote:

I agree with Jon and will move your thread.





Thanks, Ron!

Cheers,


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Durval Menezes
super member


Reged: 08/30/10

Loc: Rio de Janeiro, Brazil.
Re: DSC precision troubles new [Re: dale c]
      #4397131 - 02/19/11 02:11 PM

Hello Dale,

Quote:

at 1024 steps the best you can expect is about 0.35 degree accuracy. To increase resolution you have no choice other than using higher resolution encoders.
Dale




I'm sorry but I made a mistake on my first post: my encoders' resolution is actually 4096 (1024 in quadrature mode). Does that mean my maximum nominal precision would be 0.35*1024/4096 = 0.0875 degrees (ie, are the number of steps and the precision linearly correlated)? If not, can you tell me what the formula is?

Thanks,


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Durval Menezes
super member


Reged: 08/30/10

Loc: Rio de Janeiro, Brazil.
Re: DSC precision troubles new [Re: gnowellsct]
      #4397171 - 02/19/11 02:38 PM

Hello Greg,

Quote:

There are a number of different sources of flexure and inaccuracy in both dobson and equatorial mounts. At 4096 tics (the old equatorial standard) the best one could typically hope for was about 20 arc minute pointing error.




Would maximum nominal precision also be 20 arcmins on a dobsonian mount with the same encoder resolution? Or does it depend on the mount type?


Quote:

In my current setups I have pointing errors of around 2-4 arc minutes (on the AP900 with 10k tics) and 3-6 arc minutes (on a G11 with 8192 tics).




This sounds like a great resolution!


Quote:

Higher resolution encoders do "buy" some increased accuracy but they typically are about third on the list of things to do.




Which would be the first two?


Quote:

1.8 degrees pointing error is not a system I would consider working very well: I didn't like 20 arc minutes




I heartily agree :-)

Quote:

However, how fine you need to go is a function of aperture and field of view, field of view compensates for a lot of pointing error and aperture helps making things more obvious to the eye (but limits field of view). In any case on my 5" refractor 3-5 arc minutes of pointing error is overkill: twenty arc minutes is plenty of accuracy.




I would love to get 20 arcmins: at 0.33 degrees, it would be almost 6 times better than what I have now...


Quote:

You have several ways to approach the problem I've tried them all. You can try to nail down the hardware accuracy as Floyd points out the orthagonality is important. So that means shimming and tightening etc.




Sure thing, as I realized I really have that problem (see my response to him). That will not be very easy for me (I'm not experienced at all in telecope mechanics), so I will try that last (or next to last).

Quote:

You can extract more accuracy from the encoders by upgrading them but if your primary source of error lies elsewhere it won't work too well.



As they are already working at 4096 steps (see above), I think this would be overkill right now... but I may try that later.

Quote:

You can get an Argo Navis computer or hook your encoders up to a laptop with software bisque. What we're getting at here is a pointing model which takes data from the sky and then analyzes how the star/object positions that you report differ from reality, and constructs a computerized compensation. Argo navis offers this. Software Bisque has a program that does it. It is built into Gemini. Other go-to systems have partial versions (not as many variables are modeled, such as Celestron).
In any case by far and away the most effective thing I did to improve pointing accuracy was to dump NGC Max and dump Sky Commander and get Argo Navis. You can go to their web site (wildcard-innovation.au) and download the pdf of their instruction manual and then look at the mount modeling/pointing error sections of the catalog. If you zap a few stars the computer will model the various sources of error and then compensate the coordinates.

Argo Navis' pointing model got my pointing error out of the 18 to 25 arc minute range down to the 5 to 10 arc minute range, and upgrading the encoders shaved those values approximately in half. So the biggest single thing I did was to upgrade the computer.




Good info! I'm using Astromist, which on its manual says it uses a model based on work from a certain "M. Toshimi Taki" for the 2-stars alignment, and then a "mathematical model" for the N-stars alignment. Possibly they are not as good as the Argo Navis... Does anyone here has experience with Astromist so as to deny or confirm that?

Quote:


Pointing models for dobs are less complicated than pointing models for equatorial mounts. If you zap stars only in the areas that you can see the compensation model will be most accurate in the areas that you can see. If you have all-sky access you would get a better statistical all-sky performance if you zap all-sky stars.





OK, good tip. In fact, Astromist's manual say that in a different way, which I hadn't understand at first... now that I reread it after reading your message, it makes a lot more sense ;-)

Quote:

How many stars you choose to start with is up to you, it is possible to add additional alignments as you observe (smaller tighter objects are best) so if you do some alignments and then do some observing on small planetaries or tight globs you can do some observing and improve the model as you do so.




Good idea! that way I won't waste a lot of time with alignment prior to observing, but start with a basic alignment and improve it as I go. In fact, I've checked
and Astromist supports doing exactly this via the "Improve pointing" menu option (it will add the object selected to the N-star alignment list).

Quote:

there is a Yahoo group Argo_Navis_dtc and the aforementioned home page for wildcard innovations.

My friend Pete used to have pointing accuracy of about ten to twenty arc minutes with his Obsession servoCAT go-to mount. Part of the issue that affects pointing accuracy in go-to systems is backlash compensation. I was able to get greater accuracy because I tended to build 10 star models and because CNC machined aluminum is intrinsically more rigid than wood with inserted truss poles.




All good info, Greg. Thanks again!

Cheers,


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dale c
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Reged: 03/30/09

Loc: Sedro-Woolley, Wa.
Re: DSC precision troubles new [Re: Durval Menezes]
      #4397518 - 02/19/11 05:18 PM

Durval,
If your encoders rotate at a 1 to 1 relationship with the axis involved, then 360/counts per encoder rotation, will be the the finest angular measurement. 360/4096=0.08789 degrees = 5.27 arc minutes degrees per step. The pointing accuracy depends on factors mentioned by others.

On my mount the dec encoder is driven directly, and my dec encoder gives me 8096 counts per revolution of the dec axis. The same model encoder is geared to my RA axis at a 2.5 to 1 rate resulting in 20,240 counts per revolution of my RA axis.
I have been using a skycommander DSC using 1 star alignment and usually point to within 20 arc minutes. If my polar alignment is closer to being on I point to within 5 to 10 arc minutes.
Dale


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Durval Menezes
super member


Reged: 08/30/10

Loc: Rio de Janeiro, Brazil.
Re: DSC precision troubles new [Re: dale c]
      #4397654 - 02/19/11 06:27 PM

Hello Dale,

Quote:

If your encoders rotate at a 1 to 1 relationship with the axis involved, then 360/counts per encoder rotation, will be the the finest angular measurement. 360/4096=0.08789 degrees = 5.27 arc minutes degrees per step. The pointing accuracy depends on factors mentioned by others.




It makes a lot of sense. Thanks for the explanation.

Quote:

On my mount the dec encoder is driven directly, and my dec encoder gives me 8096 counts per revolution of the dec axis. The same model encoder is geared to my RA axis at a 2.5 to 1 rate resulting in 20,240 counts per revolution of my RA axis.




Very well. On my case, both decoders are directly driven, so I have 4096 steps on both axis.

Quote:

I have been using a skycommander DSC using 1 star alignment and usually point to within 20 arc minutes. If my polar alignment is closer to being on I point to within 5 to 10 arc minutes.




So you have a polar mount. Anyway, that's the kind of precision I aspire to: something less than 1/3 of a degree would mean a lot less time searching for DSOs and a lot more time actually observing them...

Cheers,


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lucy
journeyman


Reged: 01/02/09

Re: DSC precision troubles new [Re: Durval Menezes]
      #4397848 - 02/19/11 07:41 PM

Hello Durval,

You can also try PalmDSC from http://palmdsc.dougbraun.com/

It is a lot easier to use it than Astromist if you just want a DSC. If you get the same kind of precision with PalmDSC then you will definitely need to address your telescope's mount fabrication issues.

Best regards,
Lucy.


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John Carruthers
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Loc: Kent, UK
Re: DSC precision troubles new [Re: Durval Menezes]
      #4398655 - 02/20/11 05:36 AM

a couple of points not mentioned so far, are the encoders perfectly concentric with the axes? does your optical axis coincide with the ota mechanical axis?

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Skylook123
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Loc: Tucson, AZ
Re: DSC precision troubles new [Re: John Carruthers]
      #4398883 - 02/20/11 09:34 AM

Lots of good material here. I was away for a few days, so I'll jump back to a question asked early on, about the reticle eyepiece power.

My two workhorse telescopes are 10" f/10 SCT, and 18" f/5 newtonian, so upon initial thinking I was a bit concerned about too small a field of view to get the target star into the field, balanced with the more power, the better. But my decision was taken away by my best friend and spouse for 44 years, who bought me an 8mm reticle eyepiece for Christmas four years ago. I guess that's why I married her! She saw I was looking over choices, and made it for me.

Since I don't use this eyepiece for any purpose other than star alignment, it works fine at 286X in the big newt and 318X in the SCT.

One lesson I learned with the one I have is that when there is a reticle position adjustment capability for off axis guiding with the reticle, you need to make sure that the reticle figure remains centered. If it moves off center through use or any other reason, then the alignment suffers as the tube rotates to move from star to star. I find I need to eyeball the back side before I put it in the focuser to make sure the reticle plate is still centered and hasn't shifted.


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rmollise
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Reged: 07/06/07

Re: DSC precision troubles new [Re: Durval Menezes]
      #4398994 - 02/20/11 10:20 AM

Quote:

Hello CN folks,

After a few weeks getting my DSCs operational (long story), I'm still having problems... maybe someone here can help point me in the right direction.






Assuming your encoders are OK (nothing slipping, encoders properly mounted and concentric), the probable reason is your alignment procedure. It's hard to say much more since you don't mention the brand of DSCs, but, in general, stay away from stars near the horizon. 20 - 30 degrees up, minimum. And you want the two stars to be as far apart as possible. This time of year I use Polaris and Procyon.

Leave Astromist out of the mix till you get going. And be aware that DSCs will, on most scopes, won't always put an object dead center. If you are getting 10' accuracy, that's about what to expect.


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rmollise
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Re: DSC precision troubles new [Re: Durval Menezes]
      #4398998 - 02/20/11 10:21 AM

Quote:

Hello CN folks,

After a few weeks getting my DSCs operational (long story), I'm still having problems... maybe someone here can help point me in the right direction.






Assuming your encoders are OK (nothing slipping, encoders properly mounted and concentric), the probable reason is your alignment procedure. It's hard to say much more since you don't mention the brand of DSCs, but, in general, stay away from stars near the horizon. 20 - 30 degrees up, minimum. And you want the two stars to be as far apart as possible. This time of year I use Polaris and Procyon.

Leave Astromist out of the mix till you get going. And be aware that DSCs, on most scopes, won't always put an object dead center. If you are getting 10' accuracy, that's about what to expect.


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Durval Menezes
super member


Reged: 08/30/10

Loc: Rio de Janeiro, Brazil.
Re: DSC precision troubles new [Re: lucy]
      #4399641 - 02/20/11 03:45 PM

Hi Lucy,

Thanks for the info, I will try PalmDSC and see if it affects matters.

Cheers,

Quote:

Hello Durval,

You can also try PalmDSC from http://palmdsc.dougbraun.com/

It is a lot easier to use it than Astromist if you just want a DSC. If you get the same kind of precision with PalmDSC then you will definitely need to address your telescope's mount fabrication issues.

Best regards,
Lucy.




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Durval Menezes
super member


Reged: 08/30/10

Loc: Rio de Janeiro, Brazil.
Re: DSC precision troubles new [Re: John Carruthers]
      #4399657 - 02/20/11 03:50 PM

Hello JC,

Quote:

a couple of points not mentioned so far, are the encoders perfectly concentric with the axes? does your optical axis coincide with the ota mechanical axis?




I think the encoders are perfectly concentric with the mnecanical axis, yes. Ditto about the optical/mechanical axis coincidence. But these are good points to check, nonetheless.

Thanks,


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Durval Menezes
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Reged: 08/30/10

Loc: Rio de Janeiro, Brazil.
Re: DSC precision troubles new [Re: Skylook123]
      #4399796 - 02/20/11 05:10 PM

Hi,

Quote:

My two workhorse telescopes are 10" f/10 SCT, and 18" f/5 newtonian, so upon initial thinking I was a bit concerned about too small a field of view to get the target star into the field, balanced with the more power, the better. But my decision was taken away by my best friend and spouse for 44 years, who bought me an 8mm reticle eyepiece for Christmas four years ago. I guess that's why I married her! She saw I was looking over choices, and made it for me.




Nice! Can you mention the make/model? Also, what TFOV does that EP get on your scope?

Quote:

Since I don't use this eyepiece for any purpose other than star alignment, it works fine at 286X in the big newt and 318X in the SCT.




To have similar magnification, I will have to resort to a 5mm EP (250X on my scope), the only one I can find is the Orion 5mm Illuminated reticle EP... but it seems expensive at $180... and the TFOV will be really small at 12.5 arc minutes, which will make finding the stars for the alignment difficult (I have a no-magnification red-dot laser finder, which will make it necessary to use at least one intermediate EP to go from it to the 5mm EP).

Do you think that much magnification is really needed? Or would a less-powerful, bigger FOV EP like the Orion 12.5mm Illuminated Reticle EP (100X mag and 24 arc min TFOV on my scope) or even the Orion 20mm Illuminated Reticle EP (62X and 67.7 arc minutes) do the trick?

Quote:

One lesson I learned with the one I have is that when there is a reticle position adjustment capability for off axis guiding with the reticle, you need to make sure that the reticle figure remains centered. If it moves off center through use or any other reason, then the alignment suffers as the tube rotates to move from star to star. I find I need to eyeball the back side before I put it in the focuser to make sure the reticle plate is still centered and hasn't shifted.




Good tip, I did not know that the reticle could get shifted. Thanks!

Cheers,


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Durval Menezes
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Reged: 08/30/10

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Re: DSC precision troubles new [Re: rmollise]
      #4399822 - 02/20/11 05:22 PM

Hi,

Quote:

Assuming your encoders are OK (nothing slipping, encoders properly mounted and concentric), the probable reason is your alignment procedure. It's hard to say much more since you don't mention the brand of DSCs, but, in general, stay away from stars near the horizon. 20 - 30 degrees up, minimum. And you want the two stars to be as far apart as possible. This time of year I use Polaris and Procyon.




Astromist picks the stars for me, always at 45 degrees or higher alt, and the 2nd star is always far apart from the 1st (the combo I've been using is Acrux and Denebola).

Quote:

Leave Astromist out of the mix till you get going. And be aware that DSCs, on most scopes, won't always put an object dead center. If you are getting 10' accuracy, that's about what to expect.




My DSC (David Ek's bluetooth version) has no keyboard/display and so depends on external software to be usable... but I will try PalmDSC as recommended by Lucy.

Thanks,


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Skylook123
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Re: DSC precision troubles new [Re: Durval Menezes]
      #4399903 - 02/20/11 05:51 PM

Quote:

Nice! Can you mention the make/model? Also, what TFOV does that EP get on your scope?






It's an inexpensive Meade 8mm Illuminated Reticle Eyepiece. As these devices go, it's pretty bottom of the barrel (the reticle plate shifts around very easily), but I only need it for the alignment. The FOV is like a soda straw; around 8 arcminutes in the 18", 5 arc minutes in the SCT. I get around this tiny view by using a low power eyepiece to get initially centered, then installing the reticle eyepiece for the fine tuning, of course never touching focus during the process.

In this case, the reticle is more important than the magification; you want to repeat an accurate centering. So, yes, I think that a longer focal length reticle eyepiece would make the alignment a bit easier, without loss of precision. High power is necessary if no reticle is used; the smaller field and a defocused star can bring some precision to the process. A reticle eyepiece does the same thing, but better. Both high power and reticle may be a bit of overkill. In my opinion, of course!


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Durval Menezes
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Reged: 08/30/10

Loc: Rio de Janeiro, Brazil.
Re: DSC precision troubles new [Re: Durval Menezes]
      #4523573 - 04/17/11 04:50 AM

Hello Folks,

It's been almost 2 months, so I'm posting an update.

Thanks in large part to your help, I've been able to diagnose and mostly fix my alignment troubles. Here is a q


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Durval Menezes
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Reged: 08/30/10

Loc: Rio de Janeiro, Brazil.
Re: DSC precision troubles new [Re: Durval Menezes]
      #4523640 - 04/17/11 06:20 AM

Hello Folks,

It's been almost 2 months, so I'm posting an update.

Thanks in large part to your help, I've been able to diagnose and mostly fix my alignment troubles. Here is a quick summary.

The main problem, as Jim (Skylook123) correctly predicted, was innacuracy when centering the alignment stars; I've fixed that almost completely via his technique of using a (relatively) high-power EP and defocusing the star to cause it to almost fill the field.

For this, I've tried at first my highest-power EP (a Baader Hyperion 5mm, which on my scope gives 249x on a 16' field) but found it difficult to find the star, as my finder is a simple 1x red led; I've settled on using my Nagler T6 9mm (138x, 36" field) with the focuser almost all the way out, which makes it relatively easy to find a bright star after sighting it on the finder, and causes it to fill 3/4 or so of the field. Also, I keep to stars away from the horizon, like Rod (rmollise) recommended.

Another spot-on tip I'm using is Greg's (Gnowellsct): instead of trying to use the star pairs pre-selected by Astromist (which, due to the many obstructions on my observing place, is always time-consuming and sometimes even impossible), I first choose any bright star approximately in the middle of the visible area of the sky, and then bypass Astromist's list for the 2nd star and directly pick another one nearby: I've found this gives me very good precision (certainly under 10') in that area of the sky, while wasting very little time.

If I then start observing on another area of the sky, it's enough to use the "Improve pointing" Astromist menu command to add 4 more alignment stars in that region (it doesn't matter how far apart they are) and then command a "N-Star Alignment": this quickly gives me not only very good precision on that new area of the sky, but also on the previous area, and on anything in between. Just by repeating this procedure as the sky turns over my head for each new region, I keep getting great precision from the DSC the whole night.

These two techniques together allow me to have very precise positioning with almost no hassle: I've just finished a quick observation session (less than one hour, from just after the Cheese Devil was low enough on the sky to stop spoiling it, to just before the pre-dawn light made it impossible to continue), and I was able to observe a lot of small DSOs (mostly "sub-Messier" NGCs) in the Libra-Scorpio-Ophiucus area: with the Nagler T6 9mmm on the focuser almost all the way out, first I used Jim's technique on Antares and Sargas in Scorpio for Astromist's "2-Star Alignment", then I did as Greg suggested and added 4 more bright stars on Scorpio itself and did a "N-Star Alignment"; from that point on, I just refocused the eyepiece (the Nagler never left the scope) and then dialed each DSO on Astromist: after pushing the scope to the indicated position, the DSO was always on the field and, most often than not, at its very center.

I didn't need to fix my azimuth encoder's non-orthogonality, and neither to replace Astromist with better error-modeling software: just the above two techniques did it for me.

I'm planning on acquiring a illuminated reticle EP on my next trip to the US, so as to get even more precise alignment (I will post a thread about that later, on the Eyepieces forum), but this is almost perfectionism on my part, as I already have the problem mostly fixed.

Thanks again for all the great help. Jim, Greg, Rod and all the others, you rock, folks!

Cheers,


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skybsd
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Re: DSC precision troubles new [Re: Durval Menezes]
      #4523658 - 04/17/11 06:44 AM

I've been following your thread, here.,

So pleased to hear that you've worked out the kinks - hope you now get some great nights to work your magic

Congrats!!!! :waytogo;

Regards,

skybsd


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