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Newbie here... how to do drift alignment

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#1 Bc0428

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Posted 08 July 2020 - 12:55 AM

Hello, just bought my cem25p but i have no idea how to do polar alignment...

i live in low latitude and bortle 8 skies, polaris is blocked by mountains and stars are not highly visible with naked eyes, but jupiter is easy to spot, can it be done with jupiter?

i have been searching for the tutorial like using phd2 but it seems i just cant get the idea behind it... is there any tutorials that are short and precise could be recommended ?

besides, is there any easier way to do it with cem25p? It seems ioptron mounts have functions like iterate, 2/3 stars alignment etc, but im confused by it, so what should i do specifically.


Edited by Bc0428, 08 July 2020 - 01:17 AM.

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#2 brian32672

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Posted 08 July 2020 - 01:41 AM

Easiest way to find Polaris (as I live in bortle 8 ~ 8.5) Is to use a tablet, or smartphone (that has gps) and use the free program Stellarium.

And make sure you can not see it due to blockage. This is assuming you can even see the big dipper. I can not (most times).

You can also look at most houses that run east/west the highest point to the north from behind house looking over roof, will be aligned within 2 degrees of Polaris.



#3 FlankerOneTwo

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Posted 08 July 2020 - 04:24 AM

Not too hard with PHD2. I've recently just started doing this, because I cannot see Polaris from my house either. I haven't used the iOptron Polar Align Iteration, couldn't tell you if it's easier or faster.

 

1. Roughly align scope to true north; I just use a compass, accounting for my location's magnetic variation to align to true (as opposed to magnetic) north.

2. Select Tool / Drift Align

3. PHD will prompt you to slew scope to roughly meridian and celestial equator - this is counterweight bar horizontal (scope on meridian, which is the north-south line), and Dec axis at zero (celestial equator). Scope will be pointing up at (90 - your latitude) degrees, if in Northern Hemisphere.

4. Find a star in the field of view, calibrate PHD, and start guiding.

 

5. Click Drift. PHD will select a guide star, and track with guide outputs off. Watch the Dec trend line (red by default) for some period of time. I find a minute or two is adequate Any drift is due to azimuth (horizontal) error.

6. Click Adjust and adjust azimuth to reduce slope of Dec trendline. May need to experiment to figure out which direction of rotation move trendline in which direction. Once alignment is close enough, you will notice a purple circle around the guide star - this is the error circle. To eliminate the error, rotate in azimuth in the appropriate direction to put the guide star on the circle. If alignment is way off, you will not see this circle at first because it will be larger than the screen.

7. Repeat 5 and 6 until error is within desired tolerance (Dec trendline essentially flat).

 

8. Click "Altitude"

9. PHD will prompt you to slew to close to either eastern or western horizon

10. Click Drift. Watch Dec trendline. Any drift is due to altitude (vertical) error.

11. Click Adjust, adjust elevation to reduce slope, star on the purple circle as above.

12. Repeat 10 and 11 until done.

 

Hope this helps, pretty quickly really once you get the hang of it.



#4 Bc0428

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Posted 08 July 2020 - 04:50 AM

Not too hard with PHD2. I've recently just started doing this, because I cannot see Polaris from my house either. I haven't used the iOptron Polar Align Iteration, couldn't tell you if it's easier or faster.

 

1. Roughly align scope to true north; I just use a compass, accounting for my location's magnetic variation to align to true (as opposed to magnetic) north.

2. Select Tool / Drift Align

3. PHD will prompt you to slew scope to roughly meridian and celestial equator - this is counterweight bar horizontal (scope on meridian, which is the north-south line), and Dec axis at zero (celestial equator). Scope will be pointing up at (90 - your latitude) degrees, if in Northern Hemisphere.

4. Find a star in the field of view, calibrate PHD, and start guiding.

 

5. Click Drift. PHD will select a guide star, and track with guide outputs off. Watch the Dec trend line (red by default) for some period of time. I find a minute or two is adequate Any drift is due to azimuth (horizontal) error.

6. Click Adjust and adjust azimuth to reduce slope of Dec trendline. May need to experiment to figure out which direction of rotation move trendline in which direction. Once alignment is close enough, you will notice a purple circle around the guide star - this is the error circle. To eliminate the error, rotate in azimuth in the appropriate direction to put the guide star on the circle. If alignment is way off, you will not see this circle at first because it will be larger than the screen.

7. Repeat 5 and 6 until error is within desired tolerance (Dec trendline essentially flat).

 

8. Click "Altitude"

9. PHD will prompt you to slew to close to either eastern or western horizon

10. Click Drift. Watch Dec trendline. Any drift is due to altitude (vertical) error.

11. Click Adjust, adjust elevation to reduce slope, star on the purple circle as above.

12. Repeat 10 and 11 until done.

 

Hope this helps, pretty quickly really once you get the hang of it.

It seems a guiding camera is needed to use phd2? Im not sure

but i dont have one, so this method not for me?



#5 Bc0428

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Posted 08 July 2020 - 04:51 AM

Easiest way to find Polaris (as I live in bortle 8 ~ 8.5) Is to use a tablet, or smartphone (that has gps) and use the free program Stellarium.

And make sure you can not see it due to blockage. This is assuming you can even see the big dipper. I can not (most times).

You can also look at most houses that run east/west the highest point to the north from behind house looking over roof, will be aligned within 2 degrees of Polaris.

Hmm... i doubt the accuracy of using a phone app, especially near metallic objects that severely affects the compass in my phone, even using google maps!



#6 brian32672

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Posted 09 July 2020 - 08:33 PM

How do you figure that? I get less than 1 degree of accuracy using this method.

With a tablet that is over 5 years old.

The internal gyro, works quite well.

 

It is VERY easy. Just take a small (1" x 2") piece of plexiglass, or clear plastic.

Tape it to the back side of the tablet/phone, towards top.

Put some crosshairs on the plastic.

Using something you can see clearly, example Jupiter, get it in the crosshairs. Using basic math, or even better just get a level (nevermind this)

 

Even easier is the house method, same concept. Basic math, or better use a 0.59 cent protractor. The peak of the house works best, but you can also just go to a side of the house. You will need to know what degrees Polaris is from your position (there are plenty of websites for this) Looking straight on, side of house towards north, using protractor as guide.

 

Example, where I live I know Polaris is 42.5 degrees in the sky. (might be 32.5 have not needed to look up that figure in a long time). This is only to see if you can even see Polaris, from your position, or are actual mountains in the way?????

 

If mountains are in your way, The magnetic north and a cheap compass as poster stated, would be a easy way (but not accurate also, however within 2~Degrees of where you need to be). Assuming you have a goto mount that works correctly, it would still be enough to get you in the ballpark.

 

The app, is VERY accurate, but I have found with my older tablet, I have to spin around 360 a few times, until Jupiter, or whatever I am looking at, gets in the crosshairs. If you have never used the app, I certainly would not knock it. And this is the free version I am talking about. I have used it about a half dozen times, and whatever I am looking for is always on target, with me looking straight at screen, and object just above screen (in exact place every time).



#7 Bc0428

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Posted 09 July 2020 - 09:14 PM

How do you figure that? I get less than 1 degree of accuracy using this method.

With a tablet that is over 5 years old.

The internal gyro, works quite well.

 

It is VERY easy. Just take a small (1" x 2") piece of plexiglass, or clear plastic.

Tape it to the back side of the tablet/phone, towards top.

Put some crosshairs on the plastic.

Using something you can see clearly, example Jupiter, get it in the crosshairs. Using basic math, or even better just get a level (nevermind this)

 

Even easier is the house method, same concept. Basic math, or better use a 0.59 cent protractor. The peak of the house works best, but you can also just go to a side of the house. You will need to know what degrees Polaris is from your position (there are plenty of websites for this) Looking straight on, side of house towards north, using protractor as guide.

 

Example, where I live I know Polaris is 42.5 degrees in the sky. (might be 32.5 have not needed to look up that figure in a long time). This is only to see if you can even see Polaris, from your position, or are actual mountains in the way?????

 

If mountains are in your way, The magnetic north and a cheap compass as poster stated, would be a easy way (but not accurate also, however within 2~Degrees of where you need to be). Assuming you have a goto mount that works correctly, it would still be enough to get you in the ballpark.

 

The app, is VERY accurate, but I have found with my older tablet, I have to spin around 360 a few times, until Jupiter, or whatever I am looking at, gets in the crosshairs. If you have never used the app, I certainly would not knock it. And this is the free version I am talking about. I have used it about a half dozen times, and whatever I am looking for is always on target, with me looking straight at screen, and object just above screen (in exact place every time).

Thanks a lot, this sounds promising and worth a try.

i used skysafari pro and even not close to metallic objects, it sometimes offset quite a lot and i thought phone apps is much more vulnerable than compass.

i recently found a way, dont know if you tried it before.

 

https://www.cloudyni...bert-vice-r2760

 

using this v shape drift method for azimuth, and setting my latitude to the altitude, it should be perfectly parallel to the rotation axis i guess?? 



#8 Dynan

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Posted 09 July 2020 - 09:22 PM

Here's a slick method:

 

http://www.cwjames.i...ignment_ccd.htm



#9 Bc0428

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Posted 10 July 2020 - 08:45 AM

Here's a slick method:

 

http://www.cwjames.i...ignment_ccd.htm

Ahhh exactly what i was talking about.

btw, just curious about the mechanism behind it, for better understanding and determine which direction of offset  (so to correct quicker but not by trial and error).

 

i get the idea of azimuth fine tuning but for altitude, whats going on? It doesn't seem that bad if the altitude is off for a couple of degrees.



#10 Dynan

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Posted 11 July 2020 - 04:48 PM

Not sure I understand your question. The process is counter intuitive to me at first...adjusting E/W for alt adjustment (and conversely adjusting Up/Down for az). But thinking it through, it makes perfect sense.

 

I don't know of any way to skip the trial and error of initial adjustments, but only one exposure, and subsequent adjustment, should put you on the right track...

 

Let us know how it goes!

CS!

 

EDIT:

I do disagree with the author of the method on one point...that of needing to be perfectly level for the method to work. The adjustments by their co-dependent nature correct for a non-level mount. I grant a level mount is easier to adjust, especially if you're doing both sides of pier (starting scope E or W of pier), for example, to assure accuracy after a meridian flip. But the method will work at ANY degree of level...just easier if close at the start.


Edited by Dynan, 11 July 2020 - 05:00 PM.


#11 Andrekp

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Posted 13 July 2020 - 03:22 PM

Easiest way to find Polaris (as I live in bortle 8 ~ 8.5) Is to use a tablet, or smartphone (that has gps) and use the free program Stellarium.

And make sure you can not see it due to blockage. This is assuming you can even see the big dipper. I can not (most times).

You can also look at most houses that run east/west the highest point to the north from behind house looking over roof, will be aligned within 2 degrees of Polaris.

 

 

Hmm... i doubt the accuracy of using a phone app, especially near metallic objects that severely affects the compass in my phone, even using google maps!

what, exactly, is within a few degrees of Polaris that is so easy to confuse with Polaris that you need accuracy better than a degree or two?



#12 Phil Sherman

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Posted 13 July 2020 - 06:28 PM

Thanks a lot, this sounds promising and worth a try.

i used skysafari pro and even not close to metallic objects, it sometimes offset quite a lot and i thought phone apps is much more vulnerable than compass.

i recently found a way, dont know if you tried it before.

 

https://www.cloudyni...bert-vice-r2760

 

using this v shape drift method for azimuth, and setting my latitude to the altitude, it should be perfectly parument allel to the rotation axis i guess?? 

While Chris' method in the referenced document is a great way to do a drift polar alignment, it can be significantly improved with some minor changes. If the top of the tripod isn't level, you'll need to do multiple iterations of azimuth and altitude adjustments because the adjustments will interact with each other. Every mount I know of understands the difference between slewing E & W. A slew rate of 1x sidereal will stop tracking when slewing E and will actually move the mount at 2x sidereal when slewing W. This speed works best for drift alignment because the gears always remain engaged to drive the mount westward. The two minute exposure time is, most likely, longer than necessary for mounts costing less than $5000. These mounts rarely have adjustments with the resolution to take advantage of the two minute exposure time.

 

1. There should be no need to locate a star. An image taken at the drift alignment points will rarely not have some stars visible in it. Any star in the image can be used for the drift measurement. The image can always be level adjusted if needed to make the stars visible.

 

2. I use a different timing scheme for the exposure. I use a 70 second exposure letting the mount track for 5 seconds then slew E for 30 seconds followed by a slew W for 35 seconds. The 5 second tracked portion produces a bright spot that makes a reference for identifying the return trail. The extra 5 seconds on the W slew makes the return trail pass through the bright spot, facilitating the determination that the alignment is "perfect".

 

If I want a more accurate measurement, I use a 130 second exposure with 60 and 65 second slews.


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#13 celeron787

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Posted 15 July 2020 - 07:19 AM

While Chris' method in the referenced document is a great way to do a drift polar alignment, it can be significantly improved with some minor changes. If the top of the tripod isn't level, you'll need to do multiple iterations of azimuth and altitude adjustments because the adjustments will interact with each other. Every mount I know of understands the difference between slewing E & W. A slew rate of 1x sidereal will stop tracking when slewing E and will actually move the mount at 2x sidereal when slewing W. This speed works best for drift alignment because the gears always remain engaged to drive the mount westward. The two minute exposure time is, most likely, longer than necessary for mounts costing less than $5000. These mounts rarely have adjustments with the resolution to take advantage of the two minute exposure time.

1. There should be no need to locate a star. An image taken at the drift alignment points will rarely not have some stars visible in it. Any star in the image can be used for the drift measurement. The image can always be level adjusted if needed to make the stars visible.

2. I use a different timing scheme for the exposure. I use a 70 second exposure letting the mount track for 5 seconds then slew E for 30 seconds followed by a slew W for 35 seconds. The 5 second tracked portion produces a bright spot that makes a reference for identifying the return trail. The extra 5 seconds on the W slew makes the return trail pass through the bright spot, facilitating the determination that the alignment is "perfect".

If I want a more accurate measurement, I use a 130 second exposure with 60 and 65 second slews.


Is there any way to modify this method for a star adventurer where the 2 slew E/W buttons moves at 12x speed?

#14 Phil Sherman

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Posted 15 July 2020 - 09:13 AM

Is there any way to modify this method for a star adventurer where the 2 slew E/W buttons moves at 12x speed?

Good news - you can do it.

Bad news - you'll have to make a controller to do it.

 

Your star adventurer has a guide port which should be usable for slow adjustment of position. The standard ST4 guide port normally controls both RA and DEC axis motors. Since your "mount" has only an RA motor, only three of the six contacts will be used. The ST4 port normally works by having a relay connect two of the wires. For the RA axis, the wires are: common, E, and W. All you need is a box with two push buttons connecting the wires going to a 6-pin connector. An alternative to two push buttons is a single on-off-on switch which is probably easier to use than push buttons.

 

A plastic electrical box with a cover from Home Depot will hold the switches. Amazon has a bag of 20 6S6P RJ connectors for less than $6. A length of 4-wire phone cord or ethernet cable can be used for the wire. You'll also need the special crimping tool to attach the connector to the cable. An alternative is to buy an ST4 cable from one of the astronomy vendors (~$10) and cut off one of the ends. The exposed wires can now be soldered to the switch/switches.

 

See: "https://stargazerslo...fferent-types/"

for a reference describing which wire does what.


Edited by Phil Sherman, 15 July 2020 - 10:37 AM.



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