Modifying Nikon DSLR; IR questions

[Shibby]

I'm considering removing the filter from the D70 I use, but have a few questions...

- How important is it to filter out IR with a replacement filter? Will the image be noisier without one?

- Does the Skywatcher LPR filter cut out IR at all?

- If not, will I end up having to stack filters?

- As far as I can tell, there aren't any clear glass replacements (to go directly on the sensor). Is that right, or am I just not googling very well?

- If not, then autofocus will never work, correct?

Many Thanks!

[pete_l]

I'm considering removing the filter from the D70 I use, but have a few questions...

- How important is it to filter out IR with a replacement filter? Will the image be noisier without one?

- Does the Skywatcher LPR filter cut out IR at all?

- If not, will I end up having to stack filters?

- As far as I can tell, there aren't any clear glass replacements (to go directly on the sensor). Is that right, or am I just not googling very well?

- If not, then autofocus will never work, correct?

Many Thanks!

I can't answer your questions about the LPR filter. However I can tell you what I did when I modded my Canon DSLR.

First, I found a website that walked me through the modding process. That made it clear that without an IR filter somewhere in the optical train, my images would be slightly out of focus. The reason is that lenses bring different wavelengths to focus at different distances (which causes chromatic aberration). Camera lenses work on the basis that the camera filters out IR, so don't correct for these long wavelengths. So you'll need to filter the IR somewhere.

Second. The focal plane of the camera (where the sensor is) is set at a point which requires an optical element with a defined refractive index, between itself and the lens. If this is not present, the lens may not be able to focus at infinity (or on stars/planets, which are pretty close to an infinite distance away, wavefront-wise).

If you don't intend to use your camera with lenses (just with a telescope) this probably won't be an issue, and you can get away without doing this step.

The instructions I followed called for a Hot Mirror to be substituted for the camera's internal filter. This provided almost enough correction to retain focussing ability AND filtered out all that nasty IR.

I got mine from Edmund Optics, for about £40. It was a smallish rectangle which I had to cut down in size (diamond saw in a Dremel) to the same dimensions as the filter I was replacing. I also had to place some shims behind the CCD sensor to move it slightly towards the lens, to account for the slight difference in RI.

[davew]

Hi Lewis,

The only place I know of that at least used to sell the replacement filter was here -

Life Pixel Infrared Shop :: Nikon infrared hot mirror replacement filter

If they still do them you won't like the price. Most other places stopped because there was no call for them.

Some people tried cutting up galss filters and with some success. Auto focus does work but in the wrong place !! You need the correct thickness of glass ( 1.33mm )

Try running without the glass. this gives full spectrum but you can add IR cut externaly.

Dave.

[Shibby]

Thank you both very much for the advice!

It sounds as though (if modifying) I'll pretty much have to give up on normal photography with this camera, which I can probably live with, but it would be nice to still be able to do widefield stuff - I do have a lens that focuses beyond infinity... But I'm guessing that finding an IR filter to fit the lens won't be too easy? (since IR filter is already built in).

filtered out all that nasty IR.

Does IR create a load of unwanted signal then? If so, does this mean I'll definitely need a new filter (even when prime focus), which I would have to stack along with the LPR? I would guess that heat in the atmosphere might create extra IR signal?

url=http://lifepixel.com/shop/cart.php?m=product_detail&p=2]Life Pixel Infrared Shop :: Nikon infrared hot mirror replacement filter

If they still do them you won't like the price. Most other places stopped because there was no call for them.

Pfff! "Impressively" priced for a rectangular piece of glass!

[pete_l]

Does IR create a load of unwanted signal then?

It produces out of focus, bloated images. If you leave the IR filter off your camera and then use either a telescope that doesn't filter IR, or a camera lens your camera will record the visible and the IR components of the image. The visible light spectrum (from short wavelength violets through to long wavelength reds) will be pretty much in focus - since those are the colours the optics have been designed to work with. However it's very hard (read: expensive) to have a lens, either in a telescope or a camera, that will focus *everything* including IR all down tot he same point. There's also not much call for it, since cameras filter out the IR. What that means is that the visible light will all come to a nice, sharp point so your stars will be small and well-defined. However the IR light won't be in quite as sharp focus and will overlay your nice sharp stars with slightly out-of-focus, fuzzy images of them. The net effect is that the stars appear larger than they should - and you'll be unable to focus them down to sharp images. Same with deep-sky objects.

If you don't want to replace the IR filter in front of your camera's sensor, Hutech make an IR cut filter for the Nikon (UIBAR-FF-N1 ) in the same format as stadard filters, but it'll cost ...