Society for Popular Astronomy

What focal point ?
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Author:  bob_clarke [ Wed Jan 14, 2015 6:04 pm ]
Post subject:  What focal point ?

Hi All,
I understand the process of attaching my DSLR to my Nexstar 130 using T-adapter however, What is my is my f stop ? . Now I have taken off my lens I have no control over my focal length .
So, how do I find my f/ length ? It says on the data sheet that the reflector / mirror is 650mm and I have to use a x2 barlow to focus .. F = ?

I have taken lots of images of the moon by altering the ISO and/ or the time. Then using "live view" to check the exposure.
This method would be very hard if trying to photograph a star or nebula .

All help very welcome :o

Author:  gfletcher [ Thu Jan 15, 2015 7:04 pm ]
Post subject:  Re: What focal point ?

Your focal length would effectively be 1300mm by using the 2X barlow.
You say that you cannot achieve focus without the barlow ?
What scope are you using?

Author:  gfletcher [ Thu Jan 15, 2015 7:06 pm ]
Post subject:  Re: What focal point ?

With the 2X barlow it is effectively an f/10 focal ratio.
Just seen in your thread that you are using a 130mm objective/mirror

Author:  Brian [ Thu Jan 15, 2015 8:37 pm ]
Post subject:  Re: What focal point ?

In an ideal world your telescope would have exactly 650mm fl and the barlow would amplify that by exactly x2 :) The telescope focal length is probably close to that stated, but some barlows are known to amplify more than the stated factor (maybe 2.5x rather than x2) and in any case the actual amplification is affected by the spacing between the barlow's lens and the imaging chip (CCD). The greater this distance, the greater the amplification within reason. So for your telescope, f10 (fl=1300mm) with the barlow is a good estimate, but might be out a little (maybe closer to f12.5, fl=1625mm) - possibly.

A simple way I've used to get a better estimate of focal length for my imaging setup is to take an image of say the Moon, or Jupiter with the camera at prime focus (assume f10, 2500mm in my case), then insert the barlow (Celestron Ultima x2) and take another image. I then measured the two image diameters in pixels using PaintshopPro (other image manipulation programs will also do this) and compared the numbers. So, for example, the f10 image gave me a diameter of 100pixels and the barlowed image gave 250pixels, a ratio of 2.5:1. So I got f25 (fl 6250mm) as a better estimate for that Ultima barlow :D

If you know the angular size of your target (Moon say) in arcsec, the pixel size of your camera in microns and the number of pixels across your image then you can calculate a focal length :
focal length = (object[pix] * pix_size * 206.3) / object[arcsec]

Some planetary imaging softwares do this calculation automatically for you :)


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