Kenko 1.4x Teleconverter for Astrophotography

[megauravk]

Hello,

I am using Canon 600D with Canon EF 200mm f/2.8L lens for astrophotography and it is very good lens. 

Now I am looking to extend focal length with a teleconverter but Canon 1.4x is much costly so I am considering Kenko 1.4x DGX tele plus 300 pro. I know it will reduce f ratio to 1 stop down. Now question is that is it worth to invest in it for my astrophotography purpose? 

Also, what is role of crop factor (for Canon 600D crop factor is 1.6x) with this teleconverter? Will effective focal length be become with this teleconverter, 200x1.6x1.4=448mm or FOV will be same as 280mm (200x1.4)? I am really interested in arc-sec/pixel resolution. Currently I am getting a resolution of 4.44" arc-sec/pixel with 200mm lens without teleconverter. Now as per calculation with 1.4x teleconverter at 280 it will be 3.17" arc-sec/pixel, however if crop factor (1.6) is really in role here then I should get fov of 448mm lens with a improved resolution of 1.98" arc-sec/pixel. Please correct my understanding if I am wrong?

Thanks,

[wxsatuser]

All the crop factor tells you is the focal length of the lens you would need to use to get the same area on a full frame camera.

"A given lens casts the same image no matter what camera it is attached to."

[StuartJPP]

Forget about the 1.6x crop.

To calculate the FOV and hence arcseconds per pixel you need the focal length, the size of the pixels (usually in μm) and the number of pixels on the sensor (number of pixels is only required for FOV). In the case of the Canon 600D (as you have listed above):

5184x3456 @ 4.3 μm @ 200mm =  6°22'56'' x 4°16'1'', this equates to ~4.44 arcseconds per pixel.
5184x3456 @ 4.3 μm @ 280mm =  4°33'40'' x 3°2'54'', this equates to ~3.17 arcseconds per pixel.

A 1.4xTC will add one extra stop meaning it will halve the light gathering ability, it will also make the corners more fuzzy as it will make any imperfections in the glass more pronounced.

If you have got one handy, no harm in giving it a go, but not sure how well it will work...if you do get one, let us know...

[Alien 13]

I have the Kenko MC7 x2 TC and have used it quite successfully for Lunar images with a Canon 70-200 F4 L but I dont know how good it would be on stars as Stuart has said the outer edges will be soft.

Alan

[Stub Mandrel]

3 hours ago, Alien 13 said:

I have the Kenko MC7 x2 TC and have used it quite successfully for Lunar images with a Canon 70-200 F4 L but I dont know how good it would be on stars as Stuart has said the outer edges will be soft.

Alan

Surely a teleconverter is just a Barlow in a fat tube... I assumed they would perform better as they usually appear better made with more glass & coatings (and fatter pricetags)?