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Image not in focus need help

astrophotography beginner Celestron dslr imaging equipment eyepieces
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#1 FarSideoftheMoon

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Posted 23 January 2020 - 05:34 AM

Hopefully I am posting this in the right place.

 

Just a quick question. I align and then physically focus in on what I am seeing in my optical tube. I then take the eyepiece off and attached my Canon Rebel XS with the t-ring adapter to the telescope, look at the display screen and everything out of focus. Why?

 

It's easy to focus back in when I am looking at the moon but not so much when there is only faint stars. 

 

The end goal here is that I am trying to do long exposures and that can't really happen when the image isn't in focus. 

 

Any suggestions?


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#2 Cal D

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Posted 23 January 2020 - 05:46 AM

Different eyepieces and cameras will have different focal points, the best bet with your camera is a batinov mask and focus on a bright star and leave it as is, once in focus you can move around and take your pics. Of course temp can change things but just move back to your bright object and refocus.

Hopefully I am posting this in the right place.

 

Just a quick question. I align and then physically focus in on what I am seeing in my optical tube. I then take the eyepiece off and attached my Canon Rebel XS with the t-ring adapter to the telescope, look at the display screen and everything out of focus. Why?

 

It's easy to focus back in when I am looking at the moon but not so much when there is only faint stars. 

 

The end goal here is that I am trying to do long exposures and that can't really happen when the image isn't in focus. 

 

Any suggestions?


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#3 the Elf

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Posted 23 January 2020 - 06:27 AM

For focus set ISO to a high value, exposure time to a few seconds and turn on life view. Point the telescope to a bright star. Focus slowly and all of the sudden all the stars will popp in when they are close to focus. Next switch to 10x zoom and use a Bahtinov-Mask. If you don't have one reduce the exposure time setting in life view/10x until the star gets dimmer. Play with the focus. You will probably come to 1/500s when the star is dim in focus and invisible if you are just a few microns out. If you have a B-mask, take a test shot at ISO H, 3sec and verify the image using the zoom function. Don't forget to remove the mask!!!!!! Now set to ISO 800 and start imaging. Redo the focus check after the first hour and then when temperature changes more then 5°C or so. I have a fridge thermometer at my tripod to check temp. 

Good luck!

 

I wrote an article for newbees with a few books I recommend and a few videos linked at the end. Maybe it helps you not to stumble into all pitfalls on the way:

http://elf-of-lothlo...ingStarted.html


Edited by the Elf, 23 January 2020 - 06:30 AM.

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#4 BQ Octantis

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Posted 24 January 2020 - 06:17 PM

Every setup has a different focus point—if you didn't pay extra for parfocal eyepieces, you'll even need to adjust focus between eyepieces. So I turn the focuser all the way in (clockwise) and count turns from that end to bring a bright star into focus for my eyepiece setup—it's 9 full counterclockwise turns of the focuser. I then replace the eyepiece with the camera and count clockwise turns until it's in focus. My camera is 4 full clockwise turns of the focuser from the eyepiece focus.

 

The reason for this is simple optics: for a target at infinity (or close to it), the focus point of the objective for a compound lens is at the focus of the eyepiece (opposite your eyeball), whereas the focus point for a simple lens is at the focal length of the objective (so on the camera sensor itself).

 

eyepiece.png

Source = https://www.physicsf...escopes.853822/

 

telescope.png

Source = https://steemit.com/...maging-basics-5

 

BQ



#5 Bill G.

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Posted 25 January 2020 - 10:20 PM

Keep in mind you cannot focus accurately and reliably by eye when looking through the viewfinder anyway on a DSLR. Even with a bright object. Depth of field is very narrow and your eye will correct the image to some extent. So, while you see it in focus because your eye corrects it (Read somewhere up to 3%), it may not be on the sensor. If you can use the live-view method mentioned (don't know that camera) you will have a much better and more reliable focus. You might hit it right on visually..... and you might not.... Just not a reliable method. The lower the focal ratio, the more likely it will not be dead on.

Bill G.


Edited by Bill G., 25 January 2020 - 10:29 PM.



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