blocking filters - what do the numbers mean?

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blocking filters - what do the numbers mean?

Post by EricF »

Hi folks,

I have a couple of questions about the various blocking filters.

Coronado has BF5, BF10, BF15, BF30, Lunt has B600, B1200, B1800, B3400.

Q1: Is the number directly describing the diameter of the filter element (eg 5mm, 6mm, 10mm...), or is it describing the maximum focal length (500mm...)?

Q2: where is this limit physically in the system? Most of these are available in a1.25" 'star' diagonal format. Is the limiting aperture of the filter on the eyepiece side or on the telescope side?

Q3: Neglecting the 'sweet spot' related bandwidth, there is vignetting due to the diameter of the blocking filter. Is that correct?

Eric.


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Re: blocking filters - what do the numbers mean?

Post by Merlin66 »

Eric,
The numbers relate to the diameter of the blocking filters. With the Coronado they are in mm. The BF5 being 5mm diameter. As the diameter of the solar disk is close to 1/100 the focal length, so the BF5 would suit fl up to 500mm.
The Lunt system is similar, the B1200 is 12mm diameter etc. Again the suitable fl would be 1200mm.
Hope this helps.


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Re: blocking filters - what do the numbers mean?

Post by EricF »

Got it. And there is also a limit due to the size of the entrance pupil - probably couldn't use a 90mm etalon with a BF 5 and still get all the light to the image.

Where is that limiting aperture within the blocking filter?


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Re: blocking filters - what do the numbers mean?

Post by GreatAttractor »

EricF wrote: Sun Jul 11, 2021 5:29 pm Got it. And there is also a limit due to the size of the entrance pupil - probably couldn't use a 90mm etalon with a BF 5 and still get all the light to the image.
The entrance pupil size doesn't matter here; it's just that a too-small blocking filter will limit your field of view. The central part of the FOV still receives all the light. It's just like with the field stop in an eyepiece.
EricF wrote: Sun Jul 11, 2021 5:29 pm Where is that limiting aperture within the blocking filter?
It's the last glass element (the one facing the eyepiece); here's a photo of Lunt B600 and Coronado BF15 (which was converted to a straight-through):

Image
EricF wrote: Sun Jul 11, 2021 5:29 pm Q3: Neglecting the 'sweet spot' related bandwidth, there is vignetting due to the diameter of the blocking filter. Is that correct?
Yes; this needs to be taken into account when choosing the camera. I have a 1/3" sensor (ca. 4.9x3.6 mm) and B600 works fine with minimal vignetting; a B400 would be too small (when not using a Barlow; with a Barlow the non-vignetted part of the FOV is enlarged, as seen by the camera).


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Re: blocking filters - what do the numbers mean?

Post by EricF »

Thanks. That makes it clearer where everything is.

I did some simple ray tracing:
I want the light from the upper edge of the objective to reach the upper edge of the solar disc. If it doesn't, I'm losing light in that part of the image.
I measured a diagonal I have and estimated the position of the BF at 33mm from the focal plane.

So I plotted a graph - minimum BF diameter agains focal length, for different front etalon sizes.
blocking filter vs focal length
blocking filter vs focal length
BF Size.jpg (24.62 KiB) Viewed 953 times
It looks like the 5mm filter in the PST is too small. Must cause vignetting. And a SM60 with BF10 would also have vignetting.

Have I missed something?


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Re: blocking filters - what do the numbers mean?

Post by GreatAttractor »

No, you're right, I forgot to take the focal plane's offset into account. But note that it's not a fixed value, it will depend on the combination of eyepiece holder height + eyepiece field stop location, or camera adapter+camera sensor position. E.g., for my low-profile T2/C-mount adapter, I think this offset is closer to 18-20 mm.


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Re: blocking filters - what do the numbers mean?

Post by EricF »

Right, closer to the camera is better - so you need the right camera and focuser assembly to do this.

With a stock system, like SM60, you move the diagonal forward to get the camera to focus, and the situation gets worse because you cut off more of the light cone? So we fix this by using a Barlow, and we end up 'hiding' the vignetting outside the FOV of the camera (as you wrote earlier).

Starting to make sense now. thanks.


related: would the vignetting change the bandwidth of the filter in that part of the field?


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