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Hello all,

I have just joined and have been looking around, and putting in various searches to find the answer to my question(s).

I have already found some valuable information, but i can't find a specific answer to a question i have relating to exposure times.

I have shot the milky way several times before, from a tripod and a wide angle lens. I am aware of and understand the "500 rule" and that worked fine for me at first when i was shooting with my Canon 6D Mark II. When i moved over to the Sony A7III i noticed significant trailing using the same rule and that led me to the NPF rule (Via the photopills app incase people dot know).

I am heading back to Tenerife once again in about 6 weeks time and want to buy a star tracker so i can get some really detailed images.

I have done a fair bit of research and in principle, the whole thing doesn't seem to be too daunting or difficult.

I have purchased the Polar Scope Align Pro app so i can align Polaris as accurately as possible, i will practise putting the unit together and familiarising myself with the different parts etc, but it is the exposure times that i do not understand.

My best glass is the Carl Ziess 50mm F/1.4 Planar, the 18mm F/2.8 Batis, the Sigma 35mm F/1.4 Art & the IRIX 15mm F/2.4 Blackstone.

I currently do not own, nor have i ever used a tracker, and I cannot find any information relating to which aperture, ISO and Shutter length any of these focal lengths should or could be shot at.

Is there anything similar to the 500 rule or NPF rule that relates to using a tracker with varied focal lengths? or is it just a case of stepping the lens down for sharpness and then trial and error?

Thanks in advance,

 

Matt.

 

 

 

 

 

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These are,I believe, just like a normal equatorial mount, where the exposure length is (in theory) unlimited. In practice, there is a lot to be said for taking a lot of shorter images and stacking them. Here is a page that gives some basic info (together with a video) on the mount, with some example results.

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When using fast lenses with a star tracker you will find you are using an exposure range of between 30 seconds and 2 or 3 minutes but if you want foreground detail too you can run some trackers at half speed and use the 500 rule or similar and double it.

The ISO selected will depend on the camera but for most Canons the sweet spot seems to be 800-1600.

Alan

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Learn your optimum lens settings before you go, that eliminates a significant variable:

https://www.lonelyspeck.com/a-practical-guide-to-lens-aberrations-and-the-lonely-speck-aberration-test/

 

Might be some optimal camera settings info here:

https://www.lonelyspeck.com/sony-a7iii-astrophotography-review/

http://www.photonstophotos.net/Charts/RN_ADU.htm

At the focal lengths you mention, anything like the star adventurer will track accurately enough to be sky limited even in dark skies. The included polar alignment scope will get you close enough with ease...

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Your tracker periodic error and RA rate, your polar alignment and balance and light pollution will be the limits, alongside thermal noise for very long exposures in hot climate. 

This being said, the habit is to avoid very long exposures because you lose a lot due to a plane or a gremlin, and shorter exposures set lower constraint on the mount. 

Also, few great lenses are really crisp and with flat field when used wide open, so you'll probably want to stop it down a bit to increase image quality. 

I imagine that with all the above, my suggestioni would be: don't get too analytic, find the best ISO value for your camera and expose in order to have the sky background about 1/4th to 1/3rd of the histogram. 

And enjoy your time in Tenerife! My family is longing to go back there... 

Fabio

Edited by FaDG
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8 hours ago, Demonperformer said:

These are,I believe, just like a normal equatorial mount, where the exposure length is (in theory) unlimited. In practice, there is a lot to be said for taking a lot of shorter images and stacking them. Here is a page that gives some basic info (together with a video) on the mount, with some example results.

Thank you for the help, I had read that article, but not seen the video.

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2 hours ago, jimjam11 said:

Learn your optimum lens settings before you go, that eliminates a significant variable:

https://www.lonelyspeck.com/a-practical-guide-to-lens-aberrations-and-the-lonely-speck-aberration-test/

 

Might be some optimal camera settings info here:

https://www.lonelyspeck.com/sony-a7iii-astrophotography-review/

http://www.photonstophotos.net/Charts/RN_ADU.htm

At the focal lengths you mention, anything like the star adventurer will track accurately enough to be sky limited even in dark skies. The included polar alignment scope will get you close enough with ease...

Thank you, I will read the articles in the link.

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40 minutes ago, FaDG said:

Your tracker periodic error and RA rate, your polar alignment and balance and light pollution will be the limits, alongside thermal noise for very long exposures in hot climate. 

This being said, the habit is to avoid very long exposures because you lose a lot due to a plane or a gremlin, and shorter exposures set lower constraint on the mount. 

Also, few great lenses are really crisp and with flat field when used wide open, so you'll probably want to stop it down a bit to increase image quality. 

I imagine that with all the above, my suggestioni would be: don't get too analytic, find the best ISO value for your camera and expose in order to have the sky background about 1/4th to 1/3rd of the histogram. 

And enjoy your time in Tenerife! My family is longing to go back there... 

Fabio

Thanks Fabio, I love Tenerife, especially Adeje. 

It seems that from the advice I have been given so far that a bit of trial and error is the way to achieve the best possible exposure.

There is no set rule like the 500 rule, but the histogram tip is going to be very useful.

 

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1 hour ago, Matty_C said:

There is no set rule like the 500 rule, but the histogram tip is going to be very useful.

The point there is to clearly separate the sky background from the read noise, as it will improve your SNR

We didn't stay long in Adeje, but El Duque was really fun due to the huge waves, and we also loved Medano, my son started surfing there. 

And my best Milky Way so far was shot from the southern shore of La Gomera: pitch black sky, the closest light in the south being Dakar (Senegal), 1500 km away! 🤣 

Edited by FaDG

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