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GIR
super member


Reged: 01/02/10

Loc: Finland
Re: 10Micron new [Re: Tom Polakis]
      #6340087 - 01/28/14 01:45 AM

Quote:


Are the 5 to 10 points for low altitude modeled to deal with refraction? One thing that has bugged me about the concept of unguided imaging is that refraction is significant for large-scale imaging at moderately high altitudes, like 30 degrees. Even if you have perfect polar alignment and the mount can eliminate periodic error, how do these closed-loop mounts deal with refraction? Thanks.

Tom




Tom

Yes, the main reason for those extra 5-10 plate solves (for low altitudes) is getting rid of the remaining refractions, flexures, plate solving errors etc. It also shrinks the RA and DE tracking errors (which will always be present even in a large model) to practically 0. So it’s very handy to use even in connection with large models and especially with mobile setups. The beauty of the local model is that it takes plate solves from the path your imaging. So it’s VERY accurate and fast to do build (takes 2-3 minutes).

Playing with barometric readings, altitudes etc. is waste of time and not very accurate IMO. Especially when having all kinds of weather/atmosphere issues (temperature changes, reflections from snow, different seeing conditions etc.) Using plate solves from the path your imaging is far more accurate.

Another important factor is the time and place. Place is usually not a problem because you can get/input coordinates in many different ways (needed in any mount, not just mounts with encoders). Having an accurate time (sub-seconds accuracy) is bit more complicated because even the computer time is not very accurate. However, with a computer (and internet connection) you can use any simple (free) software to sync the computer time to some reliable time source (e.g. every 10 minutes). Or use a GPS unit to handle both the time and place automatically and accurately. I’m using a small 40 euro GPS device to do the job and haven’t had any problems even during the cold winter months.


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Per Frejvall
sage


Reged: 09/28/12

Loc: Saltsjöbaden, Sweden
Re: 10Micron new [Re: GIR]
      #6340127 - 01/28/14 02:41 AM Attachment (4 downloads)

Firmware is updated, as Ed correctly states, with an Updater application that is installed on your computer. It has the firmware embedded, so there are no separate firmware files. You install the updater of the version you want and simply run it.

Connection can be anything; wired RS232, wired Ethernet or WLAN (if you have the WLAN option).

The updater application is also used to add/remove/edit data in the comet and satellite databases in the mount. Once you have transferred your TLE files with the orbital parameters, they are stored within the mount (actually box) and are available without a computer.

As for refraction or not, I beg to disagree, GIR. Refraction is a well researched subject with a very reliable formula taking the temperature and ambient pressure as inputs. Science and research have determined that it is the pressure and temperature at the actual site that matters most.

10Micron handles the refraction calculations in-mount. This means that when you build a model (in any way you want), the refraction parameters need to be correct. Prior to entering the synced coordinates into the model, the mount firmware will nullify the refraction so that the refraction at the time of actual use of the model can be applied. Therefore it is imperative to have the correct parameters.

Should one choose the plate solve "force" method for low altitudes, the net result would be poor tracking when refraction does change to something other than what was present at the time of model building. I am quite sure that ASA also nullifies refraction prior to entering it in the model.

Some final notes on refraction are:

1) The formula most generally used is accurate to about 0.07', or 4.2"

2) Ambient pressure changes refraction by 1% for every 0.9 kPa

3) Ambient temperature changes refraction by about 1% for every 3°K

4) Increased pressure decreases refraction, increased temperature increases refraction

5) At 10° alt, the refraction is about 5', 5° 10', 30° 2'

As per point 5, you can see that even at 30° alt you are still looking at a good 120" of light bending. Use a temperature error of 10°C and you add or subtract 12" to that. You clearly HAVE TO use refraction in this game if you want to be able to use your model on more than one night (or two hour span if a high pressure system is passing by).

Time, then...

We all know that 1 second off in time yields an error in Ra of about 15", which is a lot when we expect sub 5" RMS performance. A GPS communicates at 4800 bps, normally with one start bit and one stop bit. Net performance is thus 480 characters per second, or 2.1 ms per character. Below is the spec from IEC 61162-1 specifying the NMEA 0183 protocol, and more specifically the sentence used to send the GPS time (the shortest one, not necessarily the one your GPS uses). For your GPS it would be like this:

$GPGLL,5916.25,N,01818.18,E,192831.22,A,D*3F

That constitutes 46 characters (including line endings) and takes 96 milliseconds to transfer to your computer. Before that, the GPS has to calculate the position fix with its hardware maths thingy (correlators), assemble the sentence and get it out the door, adding a few milliseconds to the game. Then your computer has to set the clock, a fast process, but nonetheless. In the end, you are at least 1.5" away at that time, most likely much more. Add to that the fact that the time is only specified with 10 ms of precision...

Using a good NTP client (Meinberg is good) via the internet can get your PC clock running at sub millisecond precision, so that is a lot better. Other factors that may or may not drown the errors here have to do with sidereal time of the plate solve. I haven't researched that yet

Given this time trouble, I wrote my time setting software so that it waits for a 2 ms slot at an even second, then sends it to the mount and records that time. When the mount responds to the time set request, it is reasonable to assume that the TCP/IP communication is negligible and that the time of response arrival closely matches when the mount set the time. The software reports this and you can reissue the command until you are satisfied with the precision. I usually settle for 30 ms or so. Of course, this is much better handled in ASA's software as it runs entirely in the PC and doesn't have to communicate the time to the mount.


Enough of a morning rant

All the best,

Per


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Hilmi
Post Laureate
*****

Reged: 03/07/10

Loc: Muscat, Sultanate of Oman
Re: 10Micron new [Re: Per Frejvall]
      #6340140 - 01/28/14 02:49 AM

So how does the mount handle permanent observatory mounting. Do you need to keep on constantly updating the temperature and pressure readings? Sounds counter intuitive.

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Skunky
sage


Reged: 09/16/13

Re: 10Micron new [Re: Hilmi]
      #6340175 - 01/28/14 04:18 AM

Any modeled mount will have that issue. But i bet someone will write a program to allow a sensor to update the mount automatically.

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Tonk
Postmaster
*****

Reged: 08/19/04

Loc: Leeds, UK, 54N
Re: 10Micron new [Re: Skunky]
      #6340216 - 01/28/14 05:09 AM

Quote:

Playing with barometric readings, altitudes etc. is waste of time and not very accurate IMO.




Per has succinctly answered the RI issue re the math.

If you look into it the refraction problem is not that sensitive over the pressure change rates likely to be experienced during a imaging session. The reality is you just need a reasonable fix as to the pressure/temp at the time you start imaging.


In my practical experience inputting barometric pressure/temp works well enough for the real world. I still get ROUND stars in long unguided exposures at 20 degrees alt.

Now I'll point out that rapid pressure changes are usually a symptom of a front passing over which invariably is accompanied by clouds and rain, making this whole scenario rather moot.


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famax
member


Reged: 07/01/07

Re: 10Micron new [Re: Tonk]
      #6340233 - 01/28/14 05:25 AM

I have a question about temperature and refraction :
We input temperature usually measured at 1m from the ground.
But temperature profile change a lot with altitude
and so should the refraction index...no ?
Sometime in winter they are temperature inversion between hign and low altitude for example , so the refarction parameters seems a bit too simple regarding reality...


I do agree with Gir that a part from direct drive techno
the local tracking thing is a real value for ASA mount, thanks to Mr Keller


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GIR
super member


Reged: 01/02/10

Loc: Finland
Re: 10Micron new [Re: Per Frejvall]
      #6340234 - 01/28/14 05:29 AM

Quote:



As for refraction or not, I beg to disagree, GIR. Refraction is a well researched subject with a very reliable formula taking the temperature and ambient pressure as inputs. Science and research have determined that it is the pressure and temperature at the actual site that matters most.

10Micron handles the refraction calculations in-mount. This means that when you build a model (in any way you want), the refraction parameters need to be correct. Prior to entering the synced coordinates into the model, the mount firmware will nullify the refraction so that the refraction at the time of actual use of the model can be applied. Therefore it is imperative to have the correct parameters.

Should one choose the plate solve "force" method for low altitudes, the net result would be poor tracking when refraction does change to something other than what was present at the time of model building. I am quite sure that ASA also nullifies refraction prior to entering it in the model.






Per,

If you really think that making the refraction calculation based on some formula and manually inputted parameters on e.g. temperature and ambient pressure …and trying constantly updating those parameters, is a good way to go, we really do disagree completely.

I’m using a good all sky model, which can be supplemented with a local model (done at the time you’re actually imaging), and using that combination to solve some of the remaining issues included when using only the sky model. There is no need play with any refraction parameters and the tracking accuracy is simply astonishing.

If you assume that the refraction parameters will change so much during the imaging session, you can always make another local model (takes only couple of minutes and it’s fully automated). However, I have never had a need to do that. Neither have I ever heard that someone would start updating and inputting new refraction parameters manually when imaging


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GIR
super member


Reged: 01/02/10

Loc: Finland
Re: 10Micron new [Re: famax]
      #6340236 - 01/28/14 05:30 AM

Quote:


I do agree with Gir that a part from direct drive techno
the local tracking thing is a real value for ASA mount, thanks to Mr Keller




Thanks famax...


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Tonk
Postmaster
*****

Reged: 08/19/04

Loc: Leeds, UK, 54N
Re: 10Micron new [Re: famax]
      #6340237 - 01/28/14 05:31 AM

Quote:

Sometime in winter they are temperature inversion between hign and low altitude for example , so the refarction parameters seems a bit too simple regarding reality...




Do you really want to image stuff below 20 degrees altitude?

Where refractive index issues are significant is largely in the "no-go" imaging area of the sky around the horizon so why are we worrying?


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Tonk
Postmaster
*****

Reged: 08/19/04

Loc: Leeds, UK, 54N
Re: 10Micron new [Re: Tonk]
      #6340240 - 01/28/14 05:36 AM

Quote:

If you really think that making the refraction calculation based on some formula and manually inputted parameters on e.g. temperature and ambient pressure …and trying constantly updating those parameters, is a good way to go, we really do disagree completely.




GIR - with your method do you re-run the local plate solve between each image? I.e. how do you constantly update your parameters? - Oh you say you never have had the need. That says that the problem isn't a sensitive one for your imaging durations. So ...

Quote:

…and trying constantly updating those parameters




Well if you don't have to .... we don't have to. So why are you raising this unlikely scenario?????




I largely think this plate-solving vs math refraction modeling argument is blown up out of reallity as my experience of using the barometric method gives excellent results - i.e. round stars. Surely thats the goal?

Per is quite right the mathematical modeling of atmospheric refration is accurate and practical enough not to be a problem. Frankly this is just a gearhead argument without recourse to testing reality and too full of "IMO" (actually MYGIBTYG "my gear is better than your gear") comments .

GIR - your ASA mount is wonderful and probably has the best and most accurate methods for everything under the night sky. However unless you can really tell me that you don't get accurate enough tracking with a 10Micron mount through practical hands on experience - then your "IMO"s are just guesses - right?


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GIR
super member


Reged: 01/02/10

Loc: Finland
Re: 10Micron new [Re: Tonk]
      #6340260 - 01/28/14 06:12 AM

Quote:



Well if you don't have to .... we don't have to. So why are you raising this unlikely scenario?????





Well, actually Per raised that scenario...

"Should one choose the plate solve "force" method for low altitudes, the net result would be poor tracking when refraction does change to something other than what was present at the time of model building".


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Hilmi
Post Laureate
*****

Reged: 03/07/10

Loc: Muscat, Sultanate of Oman
Re: 10Micron new [Re: GIR]
      #6340272 - 01/28/14 06:40 AM

I would still like some clarification, does one need to enter the temperature and pressure reading with every session in a permanently mounted set-up? If not, then this would tell me that the pressure reading is not that critical to the correct operation of the mount and can be safely ignored. If it makes a difference, then is there a possibly more streamlined solution of doing this?

I can imagine having to update the temperature and pressure readings then rebuilding the model would start getting old very fast in a permanent setup.

Do not take this as product bashing, this is a genuine question from a person truly interested in the product.

Edited by Hilmi (01/28/14 06:42 AM)


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Per Frejvall
sage


Reged: 09/28/12

Loc: Saltsjöbaden, Sweden
Re: 10Micron new [Re: GIR]
      #6340279 - 01/28/14 06:52 AM

OK, folks. Calm down. A few points:

Yes, the math of refraction is well understood. It is based on local temp and press.

If you tell your mount that it is +10° and 1030 hPa when it really is 0° and 997 hPa, and then run a model, that model is going to be really wrong. Period. It will only be good when you have +10° and 1030 hPa.

If you instead input the correct values, the model will work regardless of ambient temp and press if you input the values.

Slew to a star at 30° alt and then change the temperature by 10°C and watch it move. Naturally, refraction affects pointing more than tracking, but the further north you are the more the tracking is affected. And yes, ASA mounts can always do a local model, most likely valid unless the temperature or pressure changes significantly during the run. Autoslew tells you how much of refraction there is and displays coordinates as refraction compensated ones.

For my part, I let ACP send temperature and pressure to the mount at the start of every plan (1 hr). I am used to the temperature changing 10°C or more over a night's run so it matters to me.

So, there is nothing to it. The question was whether it matters, and the answer is that it does. You can also ask yourself why software packages like ACP has a check box that determines whether it or the mount does the refraction compensation...

Tonk, your variant is sufficient. You do not pay much attention to it, but you are using refraction and giving it data to work on. Good enough!

/per


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GIR
super member


Reged: 01/02/10

Loc: Finland
Re: 10Micron new [Re: Per Frejvall]
      #6340286 - 01/28/14 06:59 AM

Quote:


Time, then...

We all know that 1 second off in time yields an error in Ra of about 15", which is a lot when we expect sub 5" RMS performance. A GPS communicates at 4800 bps, normally with one start bit and one stop bit. Net performance is thus 480 characters per second, or 2.1 ms per character. Below is the spec from IEC 61162-1 specifying the NMEA 0183 protocol, and more specifically the sentence used to send the GPS time (the shortest one, not necessarily the one your GPS uses). For your GPS it would be like this:

$GPGLL,5916.25,N,01818.18,E,192831.22,A,D*3F

That constitutes 46 characters (including line endings) and takes 96 milliseconds to transfer to your computer. Before that, the GPS has to calculate the position fix with its hardware maths thingy (correlators), assemble the sentence and get it out the door, adding a few milliseconds to the game. Then your computer has to set the clock, a fast process, but nonetheless. In the end, you are at least 1.5" away at that time, most likely much more. Add to that the fact that the time is only specified with 10 ms of precision...

Using a good NTP client (Meinberg is good) via the internet can get your PC clock running at sub millisecond precision, so that is a lot better. Other factors that may or may not drown the errors here have to do with sidereal time of the plate solve. I haven't researched that yet

Given this time trouble, I wrote my time setting software so that it waits for a 2 ms slot at an even second, then sends it to the mount and records that time. When the mount responds to the time set request, it is reasonable to assume that the TCP/IP communication is negligible and that the time of response arrival closely matches when the mount set the time. The software reports this and you can reissue the command until you are satisfied with the precision. I usually settle for 30 ms or so. Of course, this is much better handled in ASA's software as it runs entirely in the PC and doesn't have to communicate the time to the mount.







Almost missed this one…

Just to be a bit more specific, I’m using the GPS module for determining the location and time for the computer. The GPS module doesn’t communicate directly with the mount. I would guess that the software is smart enough to understand that there is a certain lack with the GPS signal and is able to handle it. However, have never tested if there is some difference with that method or letting some software synchronize the computer clock to a good time source. Have to try that some day…


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Per Frejvall
sage


Reged: 09/28/12

Loc: Saltsjöbaden, Sweden
Re: 10Micron new [Re: GIR]
      #6340306 - 01/28/14 07:26 AM

Meinberg NTP - good stuff!

/per


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HowardK
Pooh-Bah
*****

Reged: 10/20/10

Re: 10Micron new [Re: Per Frejvall]
      #6340310 - 01/28/14 07:31 AM

Quote:

Firmware is updated, as Ed correctly states, with an Updater application that is installed on your computer. It has the firmware embedded, so there are no separate firmware files. You install the updater of the version you want and simply run it
Connection can be anything; wired RS232, wired Ethernet or WLAN (if you have the WLAN option).

The updater application is also used to add/remove/edit data in the comet and satellite databases in the mount. Once you have transferred your TLE files with the orbital parameters, they are stored within the mount (actually box) and are available without a computer.

Per ...where do i find the updater application to install on my computer?

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Per Frejvall
sage


Reged: 09/28/12

Loc: Saltsjöbaden, Sweden
Re: 10Micron new [Re: GIR]
      #6340318 - 01/28/14 07:36 AM

Quote:


Per,

If you really think that making the refraction calculation based on some formula and manually inputted parameters on e.g. temperature and ambient pressure …and trying constantly updating those parameters, is a good way to go, we really do disagree completely.





The mount calculates. You send it the data, just as in autoslew where you input it into dialog boxes...


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GIR
super member


Reged: 01/02/10

Loc: Finland
Re: 10Micron new [Re: Per Frejvall]
      #6340335 - 01/28/14 07:53 AM

Quote:



The mount calculates. You send it the data, just as in autoslew where you input it into dialog boxes...




Per,
Could you be a bit more specific ...input what data into which dialog boxes ?


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orion69
scholastic sledgehammer


Reged: 05/09/10

Loc: Croatia
Re: 10Micron new [Re: GIR]
      #6340340 - 01/28/14 07:57 AM

There is one question that comes to me reading all this posts about numerous variables that has to be addressed for mount to be able to do unguided imaging, why not just install OAG and everything is solved with one stroke?

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GIR
super member


Reged: 01/02/10

Loc: Finland
Re: 10Micron new [Re: orion69]
      #6340356 - 01/28/14 08:17 AM

Quote:

There is one question that comes to me reading all this posts about numerous variables that has to be addressed for mount to be able to do unguided imaging, why not just install OAG and everything is solved with one stroke?




Yes it might look like that but at least with the mount I'm using, there is very little manual input needed. And as a matter a fact, the main reason for starting to use unguided imaging (for me) was getting rid off all the hassle attached to guiding and using the mount in general.
There is no denying that the learning curve can be a bit steep but still very manageable.

When imaging nowadays, all I have to do is open the roof of my observatory (haven't motorized it yet), go back in house, sit on the sofa with my laptop and start the process. There are only few icons I have to click and the imaging is on. A bit more work when starting a new target but still can be done very nicely from the sofa


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