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polar alignment when using an OAG

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#1 MakNewtMan

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Posted 17 February 2020 - 12:58 AM

I'm new in the world OAG's.  Typically, when I polar align, I use my 240mm FL guide scope with teh asi 120 mini mono camera and it works perfectly.  I've decided to upgrade my guide camera to a QHY5iii 174 mini mono camera for my 1600mm focal length RC. How do i polar align now since I won't have a guidescope to use with Sharpcap?  do you i use the 174 and OAG combo to polar via sharpcap or is that not going to work?  What's the best practice here for getting polar aligned when using an OAG and not a guidescope with Sharpcap?  Thank you!



#2 Dynan

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Posted 17 February 2020 - 01:57 AM

As long as your main imaging camera is compatible with SharpCap, just connect and run Polar Align. Your OAG guide camera isn't used. SC PA is pretty self-explanatory.



#3 mxpwr

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Posted 17 February 2020 - 03:09 AM

I doubt you will have any success doing polar alignment with Sharpcap using the 1600mm. I think the limit is somewhere around 400mm (what I use).
I also just upgraded to oag and pondering about the same question. My best idea so far was to have a quick release guide scope, just attached for the polar alignment and then remove before main session.

#4 Biggen

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Posted 17 February 2020 - 08:46 AM

Sharpcap needs a FOV of 1 - 2.5 degrees to do the polar align plate solve. Using a OAG with a longer FL scope can cause a problem with this.

 

I use an OAG but my FL is only 360mm so my FOV is quite large.



#5 MakNewtMan

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Posted 17 February 2020 - 08:53 AM

I figured the longer focal length might be an issue. I’m gonna try attaching a guide scope just for the polar alignment process like Mxpwr suggests. Thank you!

#6 Dynan

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Posted 17 February 2020 - 09:29 AM

I use the ASI533 camera on a 952mm F/L ED127 with an FOV of 0.68° x 0.68°. It works fine.

 

The instruction page at SharpCap doesn't mention the type of scope required...only the minimum recommended FOV of 1° to 2.5°. The limit discussed in the forum at SharpCap never says a guide scope (hence a large FOV) is needed.

 

https://www.sharpcap...polar-alignment

 

Robin mentions that 0.5° FOV on the shortest FOV side is about the limit. In the following thread even an FOV of 0.4° is mentioned as successful:

 

https://forums.sharp...php?f=13&t=2139

 

The results speak for themselves. Admittedly, it is VERY sensitive to adjustments, but I have learned to adjust accordingly. I do an alignment (sometimes two each side of pier for verification) on each side of the pier. I start SC PA with the scope rotated in RA approximately 90° E or W. When SC PA says to "Rotate the Scope ≈ 90°", I simply tell the CEM60 to 'GoTo Zero Position'...hence ≈ 90°.

 

2019_1230_SC_PA_E_1S.jpg

 

2019_1230_SC_PA_W_1S.jpg

 

 



#7 Dynan

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Posted 17 February 2020 - 09:36 AM

What imaging camera do you use? The FOV can be calculated to see if it's in the range that SC PA can use. admittedly, it would have to be a large sensor at 1600mm F/L to be useable.

 

I wouldn't like to think I needed to add gear just for PA. Of course, I divested my 2800mm F/L C-11, which would certainly need a guide scope for SC PA.

 

Here's a great alternative method that would require no extra gear:

 

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


Edited by Dynan, 17 February 2020 - 09:41 AM.


#8 mxpwr

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Posted 17 February 2020 - 09:59 AM

I use the ASI533 camera on a 952mm F/L ED127 with an FOV of 0.68° x 0.68°. It works fine.

The instruction page at SharpCap doesn't mention the type of scope required...only the minimum recommended FOV of 1° to 2.5°. The limit discussed in the forum at SharpCap never says a guide scope (hence a large FOV) is needed.

https://www.sharpcap...polar-alignment

Robin mentions that 0.5° FOV on the shortest FOV side is about the limit. In the following thread even an FOV of 0.4° is mentioned as successful:

https://forums.sharp...php?f=13&t=2139

The results speak for themselves. Admittedly, it is VERY sensitive to adjustments, but I have learned to adjust accordingly. I do an alignment (sometimes two each side of pier for verification) on each side of the pier. I start SC PA with the scope rotated in RA approximately 90° E or W. When SC PA says to "Rotate the Scope ≈ 90°", I simply tell the CEM60 to 'GoTo Zero Position'...hence ≈ 90°.

2019_1230_SC_PA_E_1S.jpg

2019_1230_SC_PA_W_1S.jpg


That's quite interesting.
How close to good polar alignment do you need to be with your setup?
I find 400mm with a 1/3 sensor difficult already, as I often don't get enough stars onto the sensor.

#9 Dynan

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Posted 17 February 2020 - 11:05 AM

That's quite interesting.
How close to good polar alignment do you need to be with your setup?
I find 400mm with a 1/3 sensor difficult already, as I often don't get enough stars onto the sensor.

I don't know how close I NEED to be for good tracking and guiding, but when I take valuable clear sky time out, about every couple months, I want it to be as close as possible to relieve any doubts if imaging errors come up. I used to be retentive about it, settling for no less than ZERO on alt or az with a compliment of less than 4. Found out that isn't necessary.

 

When you say a 1/3 sensor, what does that mean, if I may ask a noob question. The 533 sensor is 11.31 x 11.31 mm. Defined as a '1" sensor'. How does that relate?



#10 mxpwr

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Posted 17 February 2020 - 12:08 PM

I don't know how close I NEED to be for good tracking and guiding, but when I take valuable clear sky time out, about every couple months, I want it to be as close as possible to relieve any doubts if imaging errors come up. I used to be retentive about it, settling for no less than ZERO on alt or az with a compliment of less than 4. Found out that isn't necessary.

When you say a 1/3 sensor, what does that mean, if I may ask a noob question. The 533 sensor is 11.31 x 11.31 mm. Defined as a '1" sensor'. How does that relate?

Sensor sizes are a bit cryptic, I agree. 1/3 is about 5 by 4mm, give or take.

I was wondering how close to PA you need to be so that you can do a polar alignment with sharp cap given your setup.

Using the 1/3 on my 400mm guide scope, i have issues finding suitable stars that sharp cap thinks are ok for fitting. So I have to rotate around until I find a set that works.

Edited by mxpwr, 17 February 2020 - 12:09 PM.


#11 astrohamp

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Posted 17 February 2020 - 01:15 PM

I have polar aligned with SharpCap Pro in field many times with only one problem when the system thought I was in China or some other GPS location.  Frustrating till I worked it out.

 

990mm focal length f6.5 using an ASI294 camera,  1.11° x 0.76°.


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#12 Dynan

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Posted 17 February 2020 - 02:39 PM

The instructions say "Within 5° of the North Celestial Pole". You DID read the page of instructions, didn't you? whistling.gif



#13 mxpwr

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Posted 17 February 2020 - 03:12 PM

The instructions say "Within 5° of the North Celestial Pole". You DID read the page of instructions, didn't you? whistling.gif


I skimmed through them 2 years ago. Problem is, I can barely remember what I had for lunch yesterday.
And to answer your next question, yes, I'm also lazy :)
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#14 Dynan

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Posted 17 February 2020 - 08:33 PM

I skimmed through them 2 years ago. Problem is, I can barely remember what I had for lunch yesterday.
And to answer your next question, yes, I'm also lazy smile.gif

Memory entropy is quite relatable to me. I'm getting to where I can hide my own Easter Eggs...


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#15 t-ara-fan

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Posted 17 February 2020 - 11:58 PM

I doubt you will have any success doing polar alignment with Sharpcap using the 1600mm. I think the limit is somewhere around 400mm (what I use).

I did what OP did. Added an OAG and removed guide scope. Then when it was time to do PA I said "D'oh". 

 

However I was able to do SharpCap PA with my EdgeHD-8, 0.7x reducer, and ASI071MC Pro (APC sensor).  So it worked at 1422mm with a 57' x 38' FOV. I was pleasantly surprised. 
 


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