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Old 21-05-2019, 09:06 AM
Nino (John Peacock)
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Filter distance from camera chip question

Hi everyone, how do you work out how far or close a filter has to be to the camera chip to be useful? Is there some software or a web page, Cheers John.
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Old 21-05-2019, 09:52 AM
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The only thing important here is to avoid or minimise vignetting.. otherwise it doesn't matter
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Old 21-05-2019, 10:07 AM
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multiweb (Marc)
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Quote:
Originally Posted by Nino View Post
Hi everyone, how do you work out how far or close a filter has to be to the camera chip to be useful? Is there some software or a web page, Cheers John.
The further the better if you want to avoid reflections between the filter and sensor glass.
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Old 21-05-2019, 10:21 AM
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Atmos (Colin)
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Quote:
Originally Posted by Not quite Correct, see EDIT
The calculation that I have used is:
SD*(FL/(FL-STF))+(STF/FR)
SD - Sensor Diagonal
FL - Focal Length
STF - Sensor to Filter
FR - Focal Ratio

For a FF DSLR and a 10” F/4 Newtonian.
43.3*(1000/(1000-55))+(55/4)
= 54.67mm
Now this is calculating vignetting. As Bojan has said, it’s about vignetting. What the example above shows is that with a 10” F/4 Newtonian on a FF DSLR, M48 threads are too small and should be at least 55mm to avoid vignetting.

EDIT: Correct equation is:
SD*((FL-STF)/FL)+(STF/FR)

Last edited by Atmos; 21-05-2019 at 11:43 AM. Reason: Fixing equation
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Old 21-05-2019, 11:29 AM
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billdan (Bill)
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I think Colin's math is a little incorrect.

Quote
For a FF DSLR and a 10Ē F/4 Newtonian.

SD * (FL/(FL-STF))+(STF/FR)
43.3*(1000/(1000-55))+(55/4)
= 54.67mm


But 43.3*(1000/(1000-55))+(55/4)
= 43.3*(1000/945) + 13.75
= 43.3 * 1.058 + 13.75
= 59.56mm

If we used this formula instead

SD / (FL/(FL-STF))+(STF/FR)
= 43.3/(1000/945) + 13.75
= 43.3 / 1.058 + 13.75
= 54.67mm

I'm not sure which one is correct, 59.56mm using Colins formula as written, or 54.67mm uing the modified formula.

Either way, 50mm filters will give you vignetting at that 55mm distance from filter to sensor at F4.
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Old 21-05-2019, 11:38 AM
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billdan (Bill)
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Its also true to say that if the filters are too close to the sensor then you run the risk of ugly reflections.

Check out these images, all the Stars with the filter located at 41mm to the sensor look OK, all the Stars with the filter at 31mm to the sensor look ugly.

https://www.flickr.com/photos/109613...h/28921555845/
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Old 21-05-2019, 11:41 AM
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Atmos (Colin)
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Quote:
Originally Posted by billdan View Post
I think Colin's math is a little incorrect.

Quote
For a FF DSLR and a 10Ē F/4 Newtonian.

SD * (FL/(FL-STF))+(STF/FR)
43.3*(1000/(1000-55))+(55/4)
= 54.67mm


But 43.3*(1000/(1000-55))+(55/4)
= 43.3*(1000/945) + 13.75
= 43.3 * 1.058 + 13.75
= 59.56mm

If we used this formula instead

SD / (FL/(FL-STF))+(STF/FR)
= 43.3/(1000/945) + 13.75
= 43.3 / 1.058 + 13.75
= 54.67mm

I'm not sure which one is correct, 59.56mm using Colins formula as written, or 54.67mm uing the modified formula.

Either way, 50mm filters will give you vignetting at that 55mm distance from filter to sensor at F4.
Apologies, wrote it out wrong! Iím used to doing it on my phone calculator and donít use an equation as such.
SD*((FL-STF)/FL)+(STF/FR)
So
43.3*((1000-55)/1000)+(55/4)
=54.67mm
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Old 21-05-2019, 11:53 AM
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Old 21-05-2019, 12:03 PM
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lazjen (Chris)
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Using the "final" formula - SD*((FL-STF)/FL)+(STF/FR) - the result gives the Minimum Thread Size (MTS) to avoid vignetting, right?

If I already know MTS (e.g. 48mm), how do I unwrap that equation to determine STF?
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Old 22-05-2019, 07:47 AM
Nino (John Peacock)
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Thank you for all your replies, I will do the maths later and see how I go tonight, Cheers John
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Old 22-05-2019, 07:49 AM
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Atmos (Colin)
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Quote:
Originally Posted by lazjen View Post
Using the "final" formula - SD*((FL-STF)/FL)+(STF/FR) - the result gives the Minimum Thread Size (MTS) to avoid vignetting, right?

If I already know MTS (e.g. 48mm), how do I unwrap that equation to determine STF?
I have in the past derived an equation but it is telescope specific. I find that itís generally faster to do trial and error.
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Old 22-05-2019, 07:52 AM
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Or do a drawing on piece of paper.. or better still, use LibreCAD (it is excellent CAD package for windows, freeware)
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Old 22-05-2019, 09:29 AM
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lazjen (Chris)
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Theoretically it should be:

STF = FL * FR * (MTS - SD) / (FL - FR * SD)

(I cheated and used an online tool to help since I am lazy)

MTS = 48 mm (this could be the thread size of the filter size, whatever is the smallest)

And using the example values above of
SD = 43.3
FL = 1000
FR = 4

Would give a STF of:
1000 * 4 * (48 - 43.3) / (1000 - 4 * 43.3)
= 18800 / (1000 - 173.2)
= 18800 / 826.8
= 22.74

If you've got your filter more than 22.74 mm away from the sensor when using 48 mm threads, you will get vignetting. A quick reality check shows that the result would be negative for filters of less than 43.3 mm in size, which is what you would expect (and therefore would automatically be vignetting no matter the scope).

Please check the math and numbers.

Last edited by lazjen; 22-05-2019 at 09:30 AM. Reason: Threads or filter size...
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