Televue Barlow and imaging magnification

[gerardsheldon]

I am thinking about the getting a Televue 1.25 inch 3x Barlow and using it to image with a DSLR.  According to the chart from Televue, Tele Vue Optics: Barlows  the magnification is much more than 3x, and with a DSLR  it would be closer to 5x (the DSLR image sensor would be at least 100mm from the surface of the barlow).    As my telescope has a focal length of 650mm,(f5) the focal length with the 3x barlow would be about 3250mm  (f25) for imaging.    So, in other words ignore the 3x when considering imaging. 

1) Is what I have written above correct?  I ask as the TV chart shows the magnification for 10mm and no camera could get that close, and does not show the magnification greater than 100mm which can happen.  Have I misinterpreted the table?    

2) Do other companies show a table like this?

Thanks

 

[fifeskies]

Other than imaging bright planets , trying to image with a f25 system will be very difficult. This makes targets very dim.

For planets, video capture then stacking works ok.

 

Trying to guide at an effective 3250mm will need a highly tuned mount to get anything like a stable image for still photos.

Edited April 8, 2021 by fifeskies

[gerardsheldon]

Thanks. I would only use the 3x barlow for imaging planets and the moon.  

[fifeskies]

You should be fine with the moon and planets.

The exact multiplication factor will vary by your particular setup.

If you image with it and take a star field snap , platesolving for the stars will tell you the exact focal distance the telescope has been converted to, from this you can find out the factor by dividing the new distance by the scopes native focal distance.

Or you can use Stellarium and tweak the lens settings to match the view you get, so then read of the factor.

 

While a DSLR wont be able to get as close as 10mm , dedicated astronomy cameras have a much smaller face to sensor distance, some are down at 6.5mm , this is why spacers are required often to match the image train to backfocus recommendations.

The charts cover such a big range so they cover all the camera types out there.

[gerardsheldon]

Thanks.  May be I am not reading the chart right.  I thought the distance was between the surface of barlow lens to camera sensor.  If so, then I can't see how that could be 10mm or less.  The length of the Barlow itself is about 80mm and DLSR would be further out, so a total distance of 100mm or more.  Other cameras can't go in very far in the top end of the barlow.  

I would be grateful for any clarifications.

Thanks

  

[gerardsheldon]

One thought I just had. Do I remove the black tube used for eyepieces and connect the barlow to camera.  I have just tried that but don't have anything that will connect the barlow to the DSLR.  Is that what people do and how do I connect to a DSLR?