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1.25 or 2 inches barlow for astrophotography with DSLR on Skywatcher evostar 100 ED

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#1 dave2space

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Posted 19 January 2021 - 01:51 PM

Hi,

I wonder whether is better to have a 2" barlow for astrophotography with DSLR on Skywatcher evostar 100 ED, or it is sufficient a 1.25" barlow?

is there any difference between 2" and 1.25" barlows when I use the DSLR or eyepiece?

if I use a DSLR how do I estimate the max magnification of the barlow that makes sense?

I know that that the scope has a max usable magnification of x225, and I know how to calculate the magnification using the focal length of the scope and eyepiece and eventually the magnification of the barlow included.

But I don't know how to get an idea of the max magnification of the barlow that makes sense

thanks!



#2 c2m2t

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Posted 19 January 2021 - 02:26 PM

Hi Dave!

I think the most critical consideration is to minimize any loss of the light cone...which is the passage of light through the scope. The 2 " barlow will minimize the potential for pinching the light at the viewing/imaging end. Also, over time you may acquire 2" eyepieces with which the 2" barlow can be used.

 

Cheers, Chris.



#3 c2m2t

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Posted 19 January 2021 - 02:44 PM

Hi Dave!

I forgot to respond to the other part of your question. I have the same telescope and I have spent the last 10 years imaging double stars. I have been imaging with a setup called "eyepiece projection imaging". The eyepiece acts like a magnifier just as it occurs with observing through an eyepiece. With any magnification, there is a significant loss of available light to expose an image. Just as you increase the magnification at the eyepiece and find a general darkening of the image (more contrast), the same process is happening when you image through an eyepiece or a barlow. I began imaging with a 13mm Hyperion eyepiece but it caused the stars on the outside perimeter of the image to be distorted, a phenomenon resulting from field curvature...a topic for a later date possibly. I purchased a 2.5x, 2" diameter barlow, Celestron Luminox, and it provided the same magnification/fov as the 13mm Hyperion but with a minimum of distortion. 

 

To address your question specifically, if you are going to image the moon and the bright planets, magnification with a barlow will work. If you want to image galaxies and other DSO's, Deep Space Objects, magnification will not work, primarily because of the reduction of available light to excite the imaging chip. You can certainly image with just the camera and using the telescope like a large telephoto lens and there are many DSO's that can be imaged with your telescope. I have done it. They call this A-Focal or prime focus imaging. 

 

Hopefully that answers your question. I think the quickest way to learn the limitations of your equipment is to get out and image. Make sure you make or purchase a Bahtinov focusing mask. It is indispensable in acquiring sharp focus.

 

Cheers, Chris.



#4 nfotis

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Posted 22 January 2021 - 08:13 PM

A 2" barlow will provide a light disk covering up to a full frame sensor.

 

I am pretty certain that an APS camera sensor or larger will get vignetting (not a problem with planets, but quite a problem if you shoot the Moon disk).

 

The use of a Barlow will affect the magnification if you shoot with your camera on prime focus. You can check the resulting size by a field of view calculator like this one (select "Imaging Mode" first, then your solar system targets, possible Barlow ratios and various cameras/sensors, to get a feel about the view:

https://astronomy.to.../field_of_view/

 

N.F.




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