IR/UV blocking filter for OSC

[alan4908]

I image from a low light pollution area with a refractor (SW ED80) and a OSC (SX26C). Currently, I do not use any filters. I get reasonable images of broadband objects (eg galaxies) and not so good images of emission nebula. Examples:

OK image of M31:

Not so good image of heart nebula (IC1805):

I think quality of my images would be improved by putting a IR/UV blocking filter in front of my OSC. This is because I presume UV and IR light will brought to focus at a different points than visible blue/red light. If I look at the spectral response of my camera, it seems particularly sensitive to near IR, so I presume an IR/UV filter will produce much tighter redish stars and slightly tighter blueish stars.

So, is this theory correct ? and if so, what type IR/UV blocking filter would you guys recommended ?

I see a popular choice is the Hutech IDAS LPS but would a light pollution rejection capability actually be a net benefit to me given that I image from a low light polluted area ?

Alan

[alan4908]

Any chance of a reply from an experienced imager ?

Alan

[Oddsocks]

On 21 January 2016 at 14:32, alan4908 said:

 

So, is this theory correct ? and if so, what type IR/UV blocking filter would you guys recommended ?

I see a popular choice is the Hutech IDAS LPS but would a light pollution rejection capability actually be a net benefit to me given that I image from a low light polluted area ?

Alan

Not so easy to answer Alan as there are so many variables...

The IR that the telescope focuses on the CCD does not only come from the stars and nebulae, a large proportion comes from the atmosphere, during the night IR radiated from the earth surface is absorbed and re-radiated back to ground from atmospheric CO_2, the main driving force behind global warming, it also contributes to an overall sky-fog that robs OSC CCD images of contrast.

It is normally accepted that for OSC cameras an IR/UV blocking filter is a must, whether for use with a reflector or refractor, it will reduce star bloat, by how much and which colour stars are reduced depends on the quality and materials used in the optics and the optical configuration. Background sky will be darkened by filtering out overall IR re-radiated from the atmosphere. If you use robotic focussing then an IR filter is also important since you want the software, working with the CCD image, to correctly measure the FWHM value of the wanted optical component and not the IR component.

As to which type of filter to use, if you have truly dark skies with little or no light pollution then a specific LP filter is not necessary and will make processing the final image more difficult as it introduces colour imbalance, the full colour continuum is not present so you do throw a little something away using one unnecessarily.

Only you are able to assess your local light pollution levels to make a judgement call on that one, it may be helpful if you could assess your NELM or Bortle Index, but simply taking a single exposure of a sparsely populated region of the night sky, and without post processing other than stretch, look at the overall colour balance of the debayered image, if the sky background is neutral grey then you don't need a LP filter and would be better off with a simple and inexpensive IR/UV blocking "Luminance" filter, if there is a hint of pink then some Sodium LP is present, blue-green might indicate Mercury LP (but is difficult to differentiate between the normal overdone green from OSC cameras due to the higher ratio of green pixels in the Bayer mask) in this case you would benefit by using a combined LP and IR/UV blocking filter.

I will leave it to others to suggest a particular manufacturer and type of LP filter since I only image with monochrome cameras and LRGB plus Narrow Band filters that incorporate by design a IR/UV blocking function therefore can not suggest one type over another through lack of direct experience, but there is a small consideration here in that if you do have LP free skies and do not need a LP specific filter then there are a range of special luminance filters from Astronomic that give you a choice over how wide the pass band is, allowing a little more of the wanted Ha region through at the expense of blocking a little less IR, unfortunately nothing is published to tell you how you are supposed to determine which of the pass band choices would suit your particular optical setup, probably just added to the confusion but the details are to be found here if interested:

http://www.astronomik.com/en/photographic-filters/luminance-filter-l-1-l-2-l-3.html

 

[alan4908]

Oddsocks - thanks for the reply.

OK - when the next clear sky appears   I shall attempt to determine if I need a light pollution filter using the methodology you've described.  I think I'll also investigate if I can quantify the light pollution of my location via a dark sky map.   

Alan

 

[alan4908]

I found this link (www.lightpollutionmap.info) to a detailed dark sky map (you can just input your postcode). Taking the result of the dark sky map output and combining it with the image result I obtained following Oddsocks above methodology, I've come to the conclusion that I don't need a light pollution filter, so all I need is a suitable IR/UV filter for my OSC CCD. 

Alan