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My Polar Alignment Technique Yielding Poor Results

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#1 Orion68

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Posted 31 July 2020 - 07:57 PM

I have read some of the posts in other threads about polar alignment so sorry in advance for re-visiting this issue. My question involves something I haven't seen mentioned in the other posts; the position of the fork arms when doing the polar alignment.

 

This is what I have been doing (and not getting great results):

 

 1. Aligning on the NCP, not Polaris.

 2. Setting the declination circle to 90 degrees.

 3. Rotating the ota tube in RA so that the eyepiece holder is pointing straight up.

 4. Using a Polar finder app to determine the location of Polaris in relation to the NCP at the time of the alignment.

 5. Using an eyepiece with a FOV of about 1.5 degrees at low power.

 6. Adjusting the tripod until Polaris is at the edge of the FOV in the eyepiece at the correct location shown in the Polar finder app. I'm eyeballing the placement of Polaris at the correct spot on the edge of the field of view. This would put Polaris 3/4 degree away from the NCP.

 7. Turn on the tracking motor.

 8. Use a known star like Vega and use the slow motion controls to place Vega in the center of the eyepiece field of view.

 9. Rotate the RA setting circle to match the RA of Vega.

 

The step I'm wondering about is #3. Should the eyepiece holder be pointing straight up when aligning to the NCP. Since all RA hour angles terminate at the NCP it doesn't seem like it wouldn't matter where the eyepiece holder is pointing, but not sure my logic is sound.

 

I can find objects after doing the alignment but usually they are off by 1-2 degrees. Hoping someone can confirm if this is normal or point out where my alignment process is going South.

 

Thanks in advance.

 

 

 

 

 

 

 

 



#2 RobertPettengill

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Posted 31 July 2020 - 09:56 PM

There are so many ways to do this. The angle of your eyepiece in your process doesn’t matter. What ever process you use  you will only be accurate to about a degree with the Q setting circles, unless you drift align.  then your alignment will be good, but you will still only be able to set coordinates to half a degree or so. 

 

Here is a fun way in only 5 steps:

 

1. Set your wedge to your latitude and the polar axis pointing approximately north. 
 

2. With your lens cap still on, use a level on the cap to point the scope straight up. 
 

3. Set your RA setting circle to your local sidereal time, there’s an app for that. 
 

4.  Move the scope to coordinate 2:57 RA, 89 deg 21 min for Polaris.

 

5. Adjust your wedge alt and az to center Polaris


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#3 NC Startrekker

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Posted 01 August 2020 - 07:43 AM

That's a great hack Rob. Thanks for sharing it.



#4 Orion68

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Posted 01 August 2020 - 11:41 AM

That does sound fun, what a neat trick! Will try it the next time out.

Many thanks for the advice.

#5 Toxo144

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Posted 01 August 2020 - 04:30 PM

Orion68,

 

Which eyepiece are you using to approximate the "1.5 degree" field of view (FOV)?

 

I ask this because it is easy to forget that an approximated calculated FOV is often different from the true field of view (TFOV) of a given eyepiece.  The TFOV is MEASURED, not calculated.  If doing just a "rough" polar alignment the calculated eyepiece FOV is good enough.  For very accurate initial polar alignment you may want to take a stopwatch with you next time you and your Questar are out under the stars, and actually measure the TFOV of the eyepiece in question.  I suspect you will be surprised at the results.

 

I find that the technique you outlined in your initial post works rather well if you are using the Brandon 32mm eyepiece parking Polaris just outside the FOV.  If you are using another eyepiece, I strongly suggest you measure a TFOV.  See your Questar manual pages 34-35 in my edition where it discusses the FOV, TFOV, and other approximations.  It gives instructions on how to measure the TFOV there.  It could very well be that the inaccuracies you are getting are the result of an underlying assumption that isn't quite correct....

 

Agree with the other posts above about drift alignment, accuracy of the setting circles, et cetera.

 

Cheers, clear skies, and enjoy that Questar!

 

Toxo


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#6 Orion68

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Posted 01 August 2020 - 05:21 PM

Good point on the field of view.

 

I'm calculating the FOV using a 32mm UO Konig with a 60 degree AFOV. If my calculation is correct, that would give very close to a 1.5 degree field of view. Have never measured the actual field of view, definitely should do that to be sure.

 

It's going to be clear tonight so I'll give Rob's technique a try.

 

Clear skies everybody.


Edited by Orion68, 01 August 2020 - 05:23 PM.


#7 Orion68

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Posted 01 August 2020 - 05:38 PM

Orion68,

 

Which eyepiece are you using to approximate the "1.5 degree" field of view (FOV)?

 

I ask this because it is easy to forget that an approximated calculated FOV is often different from the true field of view (TFOV) of a given eyepiece.  The TFOV is MEASURED, not calculated.  If doing just a "rough" polar alignment the calculated eyepiece FOV is good enough.  For very accurate initial polar alignment you may want to take a stopwatch with you next time you and your Questar are out under the stars, and actually measure the TFOV of the eyepiece in question.  I suspect you will be surprised at the results.

 

I find that the technique you outlined in your initial post works rather well if you are using the Brandon 32mm eyepiece parking Polaris just outside the FOV.  If you are using another eyepiece, I strongly suggest you measure a TFOV.  See your Questar manual pages 34-35 in my edition where it discusses the FOV, TFOV, and other approximations.  It gives instructions on how to measure the TFOV there.  It could very well be that the inaccuracies you are getting are the result of an underlying assumption that isn't quite correct....

 

Agree with the other posts above about drift alignment, accuracy of the setting circles, et cetera.

 

Cheers, clear skies, and enjoy that Questar!

 

Toxo

Toxo,

 

If you have used the method I described (sounds like you have), how do you make sure you are putting Polaris in the right spot.on the edge of the field of view? I'm having trouble visualizing that Polaris is in the right location. Are you using some kind of reticle eyepiece?

 

Many thanks for the feedback.



#8 Toxo144

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Posted Yesterday, 12:18 PM

Orion68,

 

I have used the method you outlined, with a slight modification.  After I level the mount (in my case, the Tristand), attach the Questar, position it exactly as you describe, the mount and scope should be pointing roughly at Polaris.  At this point I simply take a look at Kochab naked eye from the normal viewing position behind the scope.  Imagine a clock face from this position - just looking naked eye at Polaris - superimposed on the sky with Polaris at the center of the clock.  Note the "clock" position of Kochab.  If Kockab is directly over Polaris, this would be 12:00.  If directly below Polaris, this would be 6:00, et cetera.  Note this position.  I then look through the eyepiece - at normal viewing power, as you do also - and move the MOUNT until Polaris is just outside the FOV of the eyepiece of the Brandon 32mm eyepiece.  I usually accomplish this by getting Polaris very close to the edge of the appropriate "clock" position I noted Kochab in and then do the final Azimuth tightening of the Tristand.  This tends to move, for me, Polaris just out of the FOV.  This is my final mount "tightening" adjustment so at this point the mount is fixed, and pointed at the NCP. This will work with ANY mount, even a good tripod, that can be set up equatorially.

 

Then turn on the clock drive, find a suitably easy bright star, use the Questar scope RA and Dec controls to center it in a higher power eyepiece or an illuminated reticle, and then set the RA setting circle.  By this time the motors on the PG2 are warmed up and running, and you are ready to go.

 

If you want to "get a jump" on finding the clock position of Kochab, just turn on your favorite planetarium software and locate the "clock" position of Kochab just before you start.  If the sky is still too bright to see Kochab naked eye, but dark enough to see Polaris in the eyepiece, this will get you polar aligned and ready to go during twilight earlier than waiting for Kochab to be visible.  Then use any bright star or planet you can see to set your RA setting circle.  Good to go!

 

Note I can do this with the Brandon 32mm eyepiece because the TFOV is known - and I confirmed it by measuring it as per the Questar manual.  For YOUR eyepiece that you are using, you will have to measure it yourself, then estimate where in the visualized FOV you want to place Polaris.  As mentioned before, you might be very surprised at how different the MEASURED TFOV is from a calculated estimated FOV.  And, of course. if you want REALLY accurate polar alignment for astrophotography and the like, do a drift alignment.  The NICE thing about getting a good initial polar alignment is that it will save you some valuable time when you then go to do the drift alignment.

 

Hope this helps and is expressed clearly.  Let me know if any questions.

 

Cheers!

 

Toxo


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#9 Orion68

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Posted Yesterday, 12:56 PM

Orion68,

 

I have used the method you outlined, with a slight modification.  After I level the mount (in my case, the Tristand), attach the Questar, position it exactly as you describe, the mount and scope should be pointing roughly at Polaris.  At this point I simply take a look at Kochab naked eye from the normal viewing position behind the scope.  Imagine a clock face from this position - just looking naked eye at Polaris - superimposed on the sky with Polaris at the center of the clock.  Note the "clock" position of Kochab.  If Kockab is directly over Polaris, this would be 12:00.  If directly below Polaris, this would be 6:00, et cetera.  Note this position.  I then look through the eyepiece - at normal viewing power, as you do also - and move the MOUNT until Polaris is just outside the FOV of the eyepiece of the Brandon 32mm eyepiece.  I usually accomplish this by getting Polaris very close to the edge of the appropriate "clock" position I noted Kochab in and then do the final Azimuth tightening of the Tristand.  This tends to move, for me, Polaris just out of the FOV.  This is my final mount "tightening" adjustment so at this point the mount is fixed, and pointed at the NCP. This will work with ANY mount, even a good tripod, that can be set up equatorially.

 

Then turn on the clock drive, find a suitably easy bright star, use the Questar scope RA and Dec controls to center it in a higher power eyepiece or an illuminated reticle, and then set the RA setting circle.  By this time the motors on the PG2 are warmed up and running, and you are ready to go.

 

If you want to "get a jump" on finding the clock position of Kochab, just turn on your favorite planetarium software and locate the "clock" position of Kochab just before you start.  If the sky is still too bright to see Kochab naked eye, but dark enough to see Polaris in the eyepiece, this will get you polar aligned and ready to go during twilight earlier than waiting for Kochab to be visible.  Then use any bright star or planet you can see to set your RA setting circle.  Good to go!

 

Note I can do this with the Brandon 32mm eyepiece because the TFOV is known - and I confirmed it by measuring it as per the Questar manual.  For YOUR eyepiece that you are using, you will have to measure it yourself, then estimate where in the visualized FOV you want to place Polaris.  As mentioned before, you might be very surprised at how different the MEASURED TFOV is from a calculated estimated FOV.  And, of course. if you want REALLY accurate polar alignment for astrophotography and the like, do a drift alignment.  The NICE thing about getting a good initial polar alignment is that it will save you some valuable time when you then go to do the drift alignment.

 

Hope this helps and is expressed clearly.  Let me know if any questions.

 

Cheers!

 

Toxo

Excellent! Thank you for that great description Toxo.

 

Just one more question; as you place Polaris in the correct position on the edge of the field of view, do you keep your eyepiece holder pointing straight up when viewing through the scope.

 

In other words, if Polaris is above the NCP, do you have the eyepiece holder pointing straight up when viewing through the scope to confirm the placement of Polaris. And, when Polaris is below the NCP, do you also have the eyepiece holder pointing straight up as you view Polaris through the scope.

 

Again, many thanks for your feedback.

Clear skies, Tim



#10 Orion68

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Posted Yesterday, 01:02 PM

There are so many ways to do this. The angle of your eyepiece in your process doesn’t matter. What ever process you use  you will only be accurate to about a degree with the Q setting circles, unless you drift align.  then your alignment will be good, but you will still only be able to set coordinates to half a degree or so. 

 

Here is a fun way in only 5 steps:

 

1. Set your wedge to your latitude and the polar axis pointing approximately north. 
 

2. With your lens cap still on, use a level on the cap to point the scope straight up. 
 

3. Set your RA setting circle to your local sidereal time, there’s an app for that. 
 

4.  Move the scope to coordinate 2:57 RA, 89 deg 21 min for Polaris.

 

5. Adjust your wedge alt and az to center Polaris

Rob,

 

I tried your technique last night and it worked great. Had the best time with the Q last night - so nice to be able to find what you are looking for every time. The only things I failed on were some DSO's too close to the moon, silly to even try but I enjoy the challenge.

 

Unfortunately, I've discovered some slop in my RA setting circle that I was not aware of so I need to resolve that.

 

Kudos to you for an easy and effective polar alignment routinebow.gif

Best Wishes, Tim


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#11 Toxo144

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Posted Yesterday, 09:07 PM


Tim,

Apologies for not typing carefully this afternoon - I was rushing to get out the door to a "socially distanced" graduation party. Here is a link to a post
on CN describing the "Painless Polar alignment" procedure in detail, posted by Optics Patent. I forgot to mention that the Hour position of Kochab has to be
"inverted" for the apparent eyepiece position you want to put Polaris in just before nudging it out of the field of view of the 32mm Brandon eyepiece.

https://www.cloudyni...nt#entry9364183

What this means is that if Kochab appears at the noon position on a clock face superimposed on the sky with Polaris at the center, then when you look through
the eyepiece place Polaris just outside the 6:00 position. If Kochab is at 1:00, place Polaris just outside the eyepiece FOV at 5:00. If Kochab is at 2:00,
place Polaris at the 4:00 eyepiece position. If Kochab is at 3:00, Polaris is also at 3:00 through the eyepiece. If Kochab is at 4:00, place Polaris at 2:00.
If Kochab is at 5:00, place Polaris at 1:00. If Kochab is at 6:00, place Polaris at 12:00. If Kochab is at 7:00, place Polaris at 11:00. If Kochab is at 8:00
place Polaris at 10:00. If Kochab is at 9:00, also place Polaris at 9:00. if Kochab is at 10:00, place Polaris at 8:00. If K is at 11:00, place P at 7:00.

To answer your question, using this method means it doesn't matter whether the barrel of the scope is rotated or not. When you start out, the Questar Dec is
set at 90 degrees, and the MOUNT elevation is adjusted so the Questar scope barrel is pointed towards Polaris. At this point, the eyepiece will be pointed
towards the ground on the North side of the Questar forks, Just grab the forks and rotate the Questar Azimuth until the eyepiece is on the opposite side,
pointing towards the southern sky on the south side of the questar foks. From there, it doesn't matter - the eyepiece holder, scope, and mount are ready to look
at the apparent sky clock position of Kochab and then use the mount elevation and Az controls to put Polaris in the appropriate position just outside the FOV of
the Brandon 32mm eyepiece. It won't matter whether Polaris is above or below the NCP.

That being said, if Rob's method works well for you, use it! It is also easy and accurate. I use the Kochab method because I find the Questar setting circles
to be so tightly calibrated its hard for me to get the Dec circle to 89 degrees 21 minutes on the nose. Much easier for me to place Polaris in a clock
position. But be assured that BOTH methods work really well.

Cheers and Clears,

Toxo


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