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Andover 656FS02-25 as a blocker

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#1 markthais

markthais

    Mariner 2

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Posted 19 April 2024 - 04:13 PM

Hi Group,
There was a post early using this filter for a blocker. This is some information that was missed,

 

I see your talking about the Andover 656.3+.2/0 nm  25mm diameter filter as a blocker . They are usually around 656.4nm to 656.5nm at 23C

First it is a standard design. This one is a 2 cavity 1nm +.2/0nm HW. The bandpass is cemented to an ITF with a  RG630 window on the back. This design is really for measuring energy and not really an imaging design. I say this because you have two mirrors cemented back to back. Unless there is no angle between them, which is rarely the case you will have internal reflections inside the filter. What you will see is usually a dimmer image of the edge of the sun on one side of the image. This is from the wedge when it was cemented together.

Now the specs are blocked from X ray to FIR. I'm not sure how they came up with this spec. But I wouldn't worry about the blue side not being blocked. The RG630 will block any of the blue light that could get through the air. The FIR is blocked with the ITF to at least 2400nm. So there is no reason to add any KG glass for the IR blocking.

At the beginning of the post. There was a drawing showing how to isolate the one peak. This is where another problem comes in. Most air spaced etalons have a FSR that is less than 15 Ang. The 1nm two cavity filter if it was centered on the Ha line.  Then you need to be concerned about the two peaks on either side of the one on Ha. So from Andover’s catalog a 2 cavity 1nm  filter at 10% point of the curve the BW would be 20 Ang wide. So most likely either or both peaks would be leaking.  Or one would leak at a higher %.  This is a signal/ noise problem, the leaks will down grades your contrast. 

How could you use a 1nm bandpass with and air spaced system. Now if you are using a tandem system. Then there is a good chance that the two etalon peaks would block each other and you would only have the two peaks that you’re concerned about for your Ha signal. Outside of the bandpass we don’t care. The blocker will take care of them,

There was also a post that said that there Andover filter had 55% T. This would be only true if it was only blocked to 1200nm and not the FIR. This is fine if you only using it for imaging. But don’t use it for visual. There is still a lot of IR light coming from the sun at that point.

Mark W,


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