IR Cut filter for ASI120MC?

[Droogie 2001]

Having finally had a chance to image the moon for what seems like an age I have come back across a familiar question which I have never really managed to get to the bottom of.

I own a ZWO ASI120MC (USB3 version) which is the colour version of two model types.

I believe that this model comes with a built in IR filter as it is colour camera? However as I predominantly want to image the moon I understand that I can improve the effects of glare and seeing with an IR bypass filter?

There now appears to be two types currently available.

The first link looks like this maybe better suited partly due to the description that it is indeed ideal for colour cameras.

http://www.365astronomy.com/ZWO-850nm-IR-Band-pass-Filter-31.7mm-1.25-for-IR-Sensitive-Cameras.html

There is a second one which I have looked at before, this is also an IR cut off filter but the description is a bit ambiguous and it all honestly I don’t know what it is telling me. Is it suitable for colour cameras? If so what are the differences between the two? Which one is more suited?

http://www.365astronomy.com/ZWO-Infrared-IR-Cut-Off-Filter-31.7mm-1.25.html

As I mention I did look at all of this about a year ago but came out none the wiser. There was a suggestion I would have to remove the existing IR filter or else the filters would offer no improvement and at the time I did not want to start pulling apart my new camera. Would this still be the case regardless of what filter I chose, I would suspect not?

Does anyone have an experience with either filters with the ASI120MC or any other ZWO colour camera?

In hindsight I should have gone with the Mono camera but you live and learn.

Anyone help would be appreciated.

Thanks

[Vox45]

Reading the description I had to read twice this affirmation

"These filters will cut off ultraviolet UV as well.To make you well informed I tell you a secret about the latest: there is nothing surprising in it, as this is a usual property of most of the glass materials; that's why we never get tanned through a window"

Anyone can expend on this ? Is this explanation over-simplistic ?

[DanL]

From what I can gather, glass will block UVB but will pass UVA. UVB is mostly absorbed by surface layers of skin and triggers tanning as it damages the cells. UVA contributes but generally is absorbed by deeper skin layers due to its wavelength and contributes largely to premature skin ageing and damage but blocking the UVB will largely eliminate the tanning aspect.

But going back to your camera... There is a glass plate protecting the sensor. In your colour camera this glass will also block IR. So you really do need to remove that if you want to allow IR in and use the other band pass filter. Not sure why you would need the cut off filter as you are already rejecting IR with the existing glass.

To replace, it's very easily done - literally unscrew the inner part of the camera to access the glass plate, unscrew the washer and take it out! You can then replace with this:

http://www.365astronomy.com/Replacement-CLEAR-Filter-without-Cell-for-ZWO-ASI120MM-Cameras.html

[DanL]

Actually, you might well need the rejection filter to take balanced colour images - but only if you have removed the current IR blocking glass and replaced with the plain glass for a mono camera.

[Droogie 2001]

Thanks DanL.

Okay so I understand the need to remove the existing glass plate and replace it with a non IR blocking clear glass filter. Now I have had the camera for a year I feel brave enough to have a go.

After this which filter should I go for, the IR Cut off filter or the 850nm IR Band-Pass?

If I go with the 2nd one what does balanced colour images mean and you don't need to debayer? I understand debayering in regards to taking colour images of Planets but on the Moon this is not required.

It says you can use it as a Colour or Mono but don't forget the White Balance. I know about White Balance on my Canon DSLR but not sure where I would control this with the ASI120MC?

Thanks

[DanL]

Looking at the transmission curve, the band pass will only allow IR above 850nm through so you will only capture the infrared.

The rejection filter blocks the IR so using that should give you a fairly normal looking image. If you've ever seen a pic taken with an IR modded dslr you'll see that the IR has a rather extreme effect.

I think you should be able to set the white balance somewhere in your capture software.

Debayering bit - probably best left to someone who knows [emoji4] I think it is related to a colour Bayer matrix being optimised for sensitivity to visible light. IR light through the filter won't contain visible light RGB components, just IR luminance, so the Bayer matrix doesn't process it as colour a colour image. But I'm not sure about that.

Please someone else chime in to correct me if I got that one totally wrong! [emoji37]

[Droogie 2001]

Thanks again Dan L

Phew that's a lot of information to digest!

I have a modded DSLR with a Baader IR filter inserted. This lets in more certain wavelengths of IR as opposed to less. As for WB I use Firecapture so will have to look at this...

I also use the ASI120MC as a guide camera so I am wondering whether modding the ASI will affect its ability to do this. There are stars but if I start messing with the Bayer matrix performance etc it may have uncertain side effects. Starting to get the same feeling as last year

Maybe I should just get a Mono as well and be done with it! Not very cost effective but at least it covers all of my options.

[DanL]

I think it would be fine as a guide cam without the IR filter, but if that's the justification you need to buy new kit... [emoji4]