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Basic widefield with a Camera and Tripod


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After various discussions in a photo forum, thus the references to lens sweet spots etc, I wrote this up. All you need for this is a Camera and a Tripod. I've done these with my Z2 and the 450d. The Z

For shots like these it is often worth looking out for earlier Canon EF prime lenses. The older ones without usm are often optically superb and were designed for larger 35mm-sized sensors but sell for

Hi John Thanks for the write up. I have been using something similar to remove light pollution from my widefields. I wrote it up on my website, but your write up is much better and more comprehensive

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Since then Leo, I have a more expensive lens, and it too focuses past infinity, but also has a distance scale, albeit, doesn't appear to be so easy to use. Glad the processing info was helpful.

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  • 5 years later...

Hi everyone,

I'm sort of back after an absence of over a year or so.  Had problems with my eyes ... got two new implants and can see the stars again.

Having sold all my kit ages ago, I have a faint hankering to do some more wide field imaging.  I have a Canon D1000, a nifty-fifty lens and a Manfrotto tripod.  Also got a tablet and DSLR controller.  I'm all set to go ... however.  Just had a thought about something.  

Assuming I going to take 100 exposures of Orion's Belt at 2 secs per exposure.  I'm leaving 2 seconds between each exposure for the sensor to cool down etc.  That means there will be 300 seconds (100x2 + 50x2) or 5 minutes for which the stars to move across the field of view.   As I'm planning to use the nifty-fifty ... should I be worried about problems with the stacking of the images?

Many thanks

Pete

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Some math is in order to answer that question.

Don't know if I'm going to quote specs right, but I'll do the math so you can "fill" in the right values and redo calculations if necessary.

You'll be using 50mm focal length with sensor that has 5.7um pixels and 3888 of them in width.

First we need to establish working resolution in terms of arc seconds per pixel - and we use following formula for that:

resolution = 206.3 * pixel size / focal length, and when we swap in the values, we get ~23.5"/pixel

Next thing that we need to know is sidereal rate, and that is ~15"/s, this means that each 1 second, sky will move 15".

If you align your camera so that width is in direction of RA, we can then translate sky motion in number of pixels. In 300s at a rate of 15"/s, sky will move 4500". Since we have ~23.5"/pixel resolution we can now calculate number of pixels that sky will move in 300s - result is ~191 pixels.

Since your camera is 3888 pixels we can use this to get percent of frame displacement compared to width of frame. This turns out to be a bit less than 5% (100% *  191/3888).

This is very small movement between first and last frame and stacking software should easily pick up on this. You will not loose much of image due to stacking crop either - only about 5%.

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With that camera and lens you will get about 8 seconds exposure. before obvious star trails. Using Deep Sky Stacker and kappa Sigma Clipping this will help tidy you star shapes. Depending where in the sky you point the camera you may get longer or less, East or West up to 60 degrees is good. After DSS just crop the out put file to remove field rotation. The lens is best stopped down to say f4.

Using that camera and lens it is roughly 400/50 giving 8 seconds, adjust for personal tolerance of trails.

Have fun.

Edited by happy-kat
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  • 4 weeks later...

Finally got myself a dslr and i tripod so i can try out the tutorial at the begining of this thread.  Quick question, from what i have read the quick picture i took as a test last night has the stars as small discs rather than points of light.  Is this a focusing issue? I just set the camera (canon 200d) to manual and changed the iso and exposure settings. Iso was 400 and exposure was 30s.

 

20190211213151_IMG_0130.JPG

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That's great you have started. Yes focus is slightly off hence the stars are softly round. Good star colours though.

Infinity focus may not be right at the end of adjustment travel but slightly back from it.

 

Edited by happy-kat
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55 minutes ago, happy-kat said:

That's great you have started. Yes focus is slightly off hence the stars are softly round. Good star colours though.

Infinity focus may not be right at the end of adjustment travel but slightly back from it.

 

Thanks for confirming that for me. Just need some more clear skies now. I was amazed when i bought the camera and tripod, that usually means weeks of cloudy skies (it certainly was when i bought my scope). But last night was good for me for a couple of hours.

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