IR CUT filter issue?

[ebdons]

Hi, looking at the Astronomic website it states that their IR CUT filter it is advised to use scopes with refracting elements which means newts are a no-no? and for cams having a low sensitivity webcams etc, at the moment I'm using the QHY10 with clear window mod on a F4 newt and are having issues with sensitivity  when using an ir-cut and uhc combo, if I use a coma corrector will that in turn make the ir cut filter usable on a newt, I don't find much of a use for a separate ir cut but before I sell it is there something I should know if that is the case? Tony

[Tim]

As I understand it, the only reason for specifying refracting lenses would be to alleviate blurring in the IR range which may give a fuzzy look to your stars. Normally, imaging with an IR Cut filter is best practice anyway, no matter the scope as what IR signal does make it through tbe atmosphere is pretty garbled and likely to mush your images a bit by reason of producing the effects of really poor seeing. With OSC cameras, an IR cut filter can increase colour saturation a little, especially in red ranges, which may be welcome.

As I say, that's just how I understand it, happy to be shot down

[vlaiv]

IR (and UV) filtering is important for refractive optics. Reflective optics don't need it.

Most refractors bring two or three wavelengths to common focus (doublets / triplets) , and are optimized for visual range. Both UV and IR wavelengths are far enough from visual range that de focus becomes significant, even with apo triplet scopes.

This diagram explains it nicely:

Only "superachromat" has suitable correction over whole sensor sensitivity range. Purple halo from achromat can be identified in this diagram by very large focal point deviation below ~450 and above ~680nm. That halo is unfocused light. Similar thing happens for ED doublets (although on much smaller scale). Even triplets don't have common focus in frequencies below 400nm and above 800 and this is the reason why we need filtering and if we don't put IR/UV cut filter result is blurring and bloated stars (unfocused IR and UV light).

Reflective optics does not need this because mirrors work the same regardless of wavelength.

On topic of IR and atmosphere, IR is actually much less susceptible to seeing, many planetary imagers use IR pass filter for lum on the Moon for example. This is because index of refraction varies with wavelength. Blue light is affected the most, while red the least (well, IR even less than this). Therefore any star blur in IR is due to refraction of scope and not atmospheric seeing.

In LRGB imaging with reflector, L channel can benefit from not having UV/IR cut filter. Galaxies give off quite a bit of light in IR and this is additional signal boosting SNR for given exposure that we would otherwise cut off by using filter. R, G and B filters should not pass anything outside designated range so color saturation is preserved.

One can shoot with OSC and reflectors without IR/UV cut, but color balance will be somewhat more complicated.

[ebdons]

Hi, thank you for the replies and the great explanations, I have the L2 filter which I will also use with the F4 newt, and I tried to image ngc2175 with the UHC and ir cut but it was really faint even after 3 hours of subs so after using the CLS and ir cut it was much better even with a 1/3 of the subs taken, this has more to do with the way the QHY10 has some low sensitivity with some nebulas I think, although the UHC is perfect for M42 etc? I could always use the L2 filter in place of the ir cut but the wavelengths are very similar? and if you're doing near IR imaging than you could do without the ir cut or maybe have an H-beta in it's place if looking for nebulas with a strong beta transmission? clear skies! Tony

[vlaiv]

I'm a bit confused with what you are trying to do.

If you use CLS and UHC filter, then there is really no need for IR cut filter (that really depends on manufacturer of UHC and CLS filters, but for "standard" ones). Look at response curves for these three filters:

UHC:

UHC filter it self cuts anything below 450 and above 700nm ... (it does pass some after 1100, but camera is simply not sensitive in that region and will not detect such light).

CLS:

Essentially the same "type" of filter, except two peaks centered on "interesting" wavelengths (OIII on one side, and Ha and SII on the other) being wider. CLS is better suited for general imaging

UV/IR cut filter:

As you see above, both CLS and UHC already cut off range below ~440nm and above ~700nm, so no additional UV/IR cut filter is necessary with them.

Using UHC will result in less bright image than CLS (regardless of IR cut used) because UHC blocks more light. Background will be darker, and often targets as well, even emission type targets (and you should use UHC with emission type targets only). But this does not mean that image is of less quality - it is not the brightness that counts, it is SNR or signal to noise ratio. If you loose some signal, but loose quite a bit of noise your SNR will in fact improve. This is what these types of filters do, improve SNR in certain cases.

It is best if you limit your filter usage like this:

- UHC for narrow band targets - emission nebulae rich in Ha, OIII and SII. Star colors will be way off with this filter (mostly red or blue or combination of the two colors)

- CLS for general targets in light pollution, and emission nebulae as well. This is cheap option for LP suppression, there are better options for LPS filters out there.

- UV/IR cut filter for "color" control when using color cameras

- No filter if you can properly color balance your images and use reflector optics

 

[ebdons]

Hi yes I should have mentioned that my astronomic UHC and CLS are specified for visual but can be used for imaging if you use a separate ir cut? so I have the non CCD versions, as I was using them for some visual as well but  I think I might try the qhy10 without any filter on the F4 newt as it may produce the result that I want? although I will probably move into narrowband esp with the weather lately, h-beta objects are an interest but not many to image? I would reckon on the UHC being better for my Trius 825M than the OSC just need some clear skies to test? Thanks, Clear skies! Tony