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F-Ratio Calculation Using Astronomy.net

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#1 biomedchad

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Posted 18 September 2019 - 08:11 AM

hello,

 

i am trying to calculate the fratio i am getting through my current setup to see how close i am.  i have uploaded the image to astronomy.net and have the data but as this is the first time ive done this, i am struggling with the conversions and math.  any experts out there have simple way ot example?  the only online google search gave me a decent article but he is leaving out a key conversion and does not explain what calculation from the image he uses to get this number.  i can remote in and give the data if needed or just point me in a direction if possible.



#2 saguaro

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Posted 18 September 2019 - 08:18 AM

This is the procedure I use. Works great!

http://www.astropix....nd-focal-ratio/



#3 mclewis1

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Posted 18 September 2019 - 08:59 AM

I don't like doing any math either. I submit an image to astrometry.net and simply note the long axis of the fov measurement. I take this number and run CCDCalc (choosing my scope and camera/sensor combination that I've previously setup) and just change the focal reducer factor so the CCDCalc sensor size data matches taken that number from astrometry.net. This keeps my head from hurting. grin.gif



#4 Rickster

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Posted 18 September 2019 - 09:00 AM

Another no math way to do it is to use https://astronomy.to.../field_of_view/.

 

Select Imaging Mode

Choose an object (it doesn't matter which one)

Choose your camera

Don't bother with the other settings.

Now, take a guess at your Focal Length and enter it. 

Compare the Resolution to what Astrometry.net says the Pixel Scale is.

Adjust the Focal Length until the Resolution and the Pixel Scale match.

The Focal Ratio and Field of View will be shown at the bottom.


Edited by Rickster, 18 September 2019 - 09:01 AM.


#5 nic35

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Posted 18 September 2019 - 09:01 AM

I use the following  http://www.wilmslowa...tm#ARCSEC_PIXEL  makes life simple

 

j



#6 biomedchad

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Posted 18 September 2019 - 09:31 AM

This is the procedure I use. Works great!

http://www.astropix....nd-focal-ratio/

thats the article i was referring to.. i cant figure out the calculation where he gets the 57.3 in the focal length equation



#7 saguaro

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Posted 18 September 2019 - 09:51 AM

thats the article i was referring to.. i cant figure out the calculation where he gets the 57.3 in the focal length equation

Here's an example. Just substitute your own numbers:

 

1. Plate solve an image at nova.astrometry.net
2. Note the field of view “Size”, for example 21.4 x 12 arcmin. Use the first value (21.4). If in arc min, convert to degrees online.
3. Note the sensor size of your camera, for example, 5.6mm  x 3.2mm. Use the first value (5.6). In this example, sensor size for 290mm is 5.6mm  x 3.2mm.

Focal Length = 57.3/ (fov in degrees/sensor size in mm)
For example = 57.3/ (.356 degrees/5.6), which equals 901.3mm
Focal Ratio = Focal Length/Scope aperture (for 8-inch SCT it's 203.2)
For example, 901.3/203.2, resulting FR is F/4.4


Edited by saguaro, 18 September 2019 - 09:54 AM.


#8 gul1337

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Posted 18 September 2019 - 10:03 AM

thats the article i was referring to.. i cant figure out the calculation where he gets the 57.3 in the focal length equation

since FOV is in degrees, 1 radian = 57.2957795 degrees.



#9 biomedchad

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Posted 18 September 2019 - 10:40 AM

For example.  I’m trying to get the reduction of this image to see if I might be back focused too much or too little for the image I am getting. C6 and 295 and .63 reducer. The system as eaa is just fantastic and I’ve seen dozens of objects in the past few days but I would like to try to figure this out and how to clean it up a little. I have a few ideas but would like some feedback. 

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#10 saguaro

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Posted 18 September 2019 - 04:45 PM

For example.  I’m trying to get the reduction of this image to see if I might be back focused too much or too little for the image I am getting. C6 and 295 and .63 reducer. The system as eaa is just fantastic and I’ve seen dozens of objects in the past few days but I would like to try to figure this out and how to clean it up a little. I have a few ideas but would like some feedback. 

I platesolved your image at nova.astrometry.net and then used the instructions I provided in my post #2 and got a focal ratio of F/7 for your image. I used the 294 sensor size of 19.1mm x 13mm, and 6-inch SCT scope aperture of 150mm.


Edited by saguaro, 18 September 2019 - 04:46 PM.


#11 biomedchad

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Posted 19 September 2019 - 05:15 AM

I platesolved your image at nova.astrometry.net and then used the instructions I provided in my post #2 and got a focal ratio of F/7 for your image. I used the 294 sensor size of 19.1mm x 13mm, and 6-inch SCT scope aperture of 150mm.


Awesome. Thanks. I take it then 57.3 is a constant. That is what was throwing me off. I need to do math after some more coffee and try it out.


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