Should I use an IR/UV Cut Filter with an Astro-modded DSLR?

[Rickvanman]

After publishing my latest YouTube video showing me bumbling my way through trying to photograph stars from my shed, someone commented I should use a Baader semi-APO filter on my refractor telescope ( explore scientific AR152 ) along with a IR/UV Cut Filter. 

Using a semi-APO filter makes absolute sense to me,  and reading up on them, they look like a really worthy investment. However, as I'm using a basic Astro-modded DSLR camera ( Canon Eos 1300D - I just had the basic mod: single filter removal), the idea of adding a IR/UV Cut Filter is not making sense to me.

Wasn't the filter that was removed from the modded camera, a form of IR/UV Filter? If so, wouldn't I just be nullifying the modification by using one?

Appreciate any clarity on this - much thanks ??

 

PS Heres a link the video in case you were curious:  https://youtu.be/iNUevy47fAc

 

 

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[bobro]

In recent years Canon DSLRs have been fitted with 2 filters : UV/IR and Red Balance filter. The Red Balance filter is removed to increase the red sensitivity of the camera, with the UV/IR filter remaining in the camera, unless a full spectrum mod is carried out where both filters are removed.

So your camera still has a UV/IR filter in place. 

HTH

[steppenwolf]

An IR cut filter is still required with an astro-modded DSLR camera (or indeed an astro CCD camera) as these sensors are very sensitive to IR and failing to fit a cut filter will lead to bloated stars. Do you know exactly what was done to your camera during the modification.

[spillage]

Basically if you remove the ir filter from you dslr you allow all ir through. A standard camera will cut (i think) all ir out. 

In astrophotography we do not want all the ir only a selection of the ir wavelenght so a filter does this for us.

 

cheers

Spill.

 

[Rickvanman]

7 minutes ago, steppenwolf said:

An IR cut filter is still required with an astro-modded DSLR camera (or indeed an astro CCD camera) as these sensors are very sensitive to IR and failing to fit a cut filter will lead to bloated stars. Do you know exactly what was done to your camera during the modification.

Hi Steve,  it was the basic mod,  where only one of the two filters is removed from the camera. I believe only the red balance filter was removed, and the original IR/UV filter remains. 

[Adam J]

1 hour ago, Rickvanman said:

After publishing my latest YouTube video showing me bumbling my way through trying to photograph stars from my shed, someone commented I should use a Baader semi-APO filter on my refractor telescope ( explore scientific AR152 ) along with a IR/UV Cut Filter. 

Using a semi-APO filter makes absolute sense to me,  and reading up on them, they look like a really worthy investment. However, as I'm using a basic Astro-modded DSLR camera ( Canon Eos 1300D - I just had the basic mod: single filter removal), the idea of adding a IR/UV Cut Filter is not making sense to me.

Wasn't the filter that was removed from the modded camera, a form of IR/UV Filter? If so, wouldn't I just be nullifying the modification by using one?

Appreciate any clarity on this - much thanks ??

 

PS Heres a link the video in case you were curious:  https://youtu.be/iNUevy47fAc

 

 

.

 

 

 

 

 

This one again. 

There are two filters as standard in a DSLR.

In an astro modification the rear filter is removed to improve sensitivity to H-a. It may either be replaced with a Baader UV/IR cut filter (in which case you already have effective IR/UV cut with IR passed out to about 700nm) or, in some cases, it is removed and not replaced. 

If not replaced then the second (front) filter that is not removed as a part of the IR modification acts as a mild IR cut passing IR out to about 750nm, so 50nm or so more than the Baader replacement. However this still cuts out the majority of the unwanted IR and so can still be effective dependent on how well corrected your optics are.

Optics with better chromatic correction or a Newtonian using a decent coma corrector will notice little difference between the baader replacement and the no-replacement modifications. However, less well corrected optics (typically ED Doublets or lower performance) will suffer from increased bloat and will benefit from using an additional UV/IR cut.

In a full spectrum modification all filters are removed and hence a external UV / IR cut filter or other filter which incorporates a UV/IR cut is always required. 

Bottom line...are you getting bloated stars or stars with a purple halo in your images? If you are then a semi-apo filter will be of benefit. If not then you don't need one. I don't think there is any point in using a external UV/IR cut and the semi-apo filter at the same time as the semi-apo will do both jobs as I understand it. 

 

[Merlin66]

I have uploaded a few times the transmission curves for both the colour correction filter (filter#2) - the one usually removed to improve the red response and the other front anti-alias/ dust shake filter (filter#1) which acts as an effective UV/IR filter.

 

[Adam J]

5 hours ago, Merlin66 said:

I have uploaded a few times the transmission curves for both the colour correction filter (filter#2) - the one usually removed to improve the red response and the other front anti-alias/ dust shake filter (filter#1) which acts as an effective UV/IR filter.

 

Interesting but what this shows is that it still has over 20% transmission out to 700nm and does not show what happens past 700nm 20% not really a cut. It's 750nm before it reaches 10% and about 780nm before it hits 2% transmission. For the baader it levels out at 6% at about 720nm. So however you look at it the front Canon filter is weaker than a conventional IR cut and that extra ~50nm of transmission can have a limited effect dependent on your optics. It's not horrible at all just not optimal. 

[Merlin66]

Is it really a problem?

The Bayer Matrix filter and the inherent response curve of the sensor is causing the response to tend towards zero in these extreme wavelengths.

The total "system" I think needs to be considered. I have confidence in the Canon designers that they ensured that the best balance and response was being achieved.

I don't remember seeing any blue bloat or red bloat with the Canon camera...

Has anyone got some good before after shots showng an UV/IR problem??

[Adam J]

 

11 hours ago, Merlin66 said:

Is it really a problem?

The Bayer Matrix filter and the inherent response curve of the sensor is causing the response to tend towards zero in these extreme wavelengths.

 

The brayer matrix allows plenty of IR through especially via the red filters and blue also at near IR wavelengths, hence you get the purple (red and blue). The IR block is there for a reason and as for the inherent response curve of a CMOS sensor, well that stretches out to 950nm with no filtering before it dips below 10% QE, at 750nm the QE is almost still 2/3 what it is at 530nm.  Like I said using only the front canon filter may not be an issue is the scope is decently corrected, but for semi-apo scopes you may benefit from an additional filter depending on how picky you are. So not a huge problem just not optimal...would likely show only on very bright stars or very poor quality optics.  

[Merlin66]

My point was that the UV/IR filtering of the modded camera is no different to that in the original camera.

The results should therefore be similar.

If an additional Uv/IR filter is advocated then it should also be used with the "basic" unmodified camera.

 

 

[Adam J]

8 hours ago, Merlin66 said:

My point was that the UV/IR filtering of the modded camera is no different to that in the original camera.

The results should therefore be similar.

If an additional Uv/IR filter is advocated then it should also be used with the "basic" unmodified camera.

 

 

Ah perhapse this is where the misunderstanding is then.

Both the filters in a unmodified Canon are in effect IR filters the rear filter (the one you remove in modification) also blocks IR and as you know results in 80% of the H-a signal being blocked. This block extends further into the IR than just H-a though and so it stacks with the front filter (the one you leave in place).

So at 700nm in a unmodified camera you have two filters each blocking over 80% of the incident light, this means that the amount of light that is transmitted at 700nm is 20% of 20% of the original incident light  and so the two filters combined only let 4% of the 700nm IR light through as opposed to 20% when you have removed the rear filter.

So no, with an unmodified camera you dont need an additional filter.

However, as previously stated I am only saying that you will need an additional IR block with a basic filter removal mod in some situations, for example when chromatic correction is marginal to begin with. I am not saying that you will always need it. Essentially you are letting additional IR though out to 750nm as opposed to 700nm if you dont use an additional IR block though and that has a small effect on image quality.

[rigradio]

15 hours ago, Merlin66 said:

My point was that the UV/IR filtering of the modded camera is no different to that in the original camera.

The results should therefore be similar.

If an additional Uv/IR filter is advocated then it should also be used with the "basic" unmodified camera.

 

 

No, they are not similar. Baader/Astronomik has wider band pass than the original camera filter.

Adding UV/IR filter if -> you use a full spectrum mod dslr and you have glass in your imaging train (refractor, lens, focal reducer)

Ketut

 

[Merlin66]

Ketut,

Unfortunately the D350D data is based on an older single filter element design and not truly representative of the current two filter design.

The curves I presented were based on one example tested, others show that the IR drop off is quite significant.

Is there any feedback from current users?????

(I only use my modded 450D- full spectral mod, both filters removed, for spectroscopy)

[alacant]

Hi. My findings and explanation from the engineer of my 700d.

A Wratten 8 will cut most of the violet and uv recording as visible on the sensor. At the other end of the spectrum, the uv-ir filter will block the long wavelength ir recording on the sensor which the hot mirror modification used to handle. They're only cheap items so, no harm done in seeing which combination works best for you. HTH.

[Mered]

Reading this thread with interest as I'm considering buying a modified 600D from cheapastrophotography to replace my 350D.

 

Am I correct in thinking that, based on all the above, I can order the basic modified 600D and get improved response on emission nebula without any other filters? I've no need to use the camera for daylight use, so the full Baader mod isn't necessary for me.

[Merlin66]

The answer is basically yes.

If the "basic" mod you are considering is just the removal of the colour balance filter then note that you will not be able to focus standard EF lenses (say for wide angle shots) without adding at least a clear filter element (I use an Astronomik clip in clear screen). No issue when used with a telescope.

 

[Mered]

That's great, yeah I will only be using with my ED80, so looks like I'm good to go! 

[Adam J]

32 minutes ago, Mered said:

That's great, yeah I will only be using with my ED80, so looks like I'm good to go! 

As I have said above, unless its a full APO then you will end up with larger halos with a removal only. 

Look at it this way, if you already have coloured halos around bright stars before the mod, after the mod they will be worse as you will be letting more IR light through by the nature of the mod.  As such if you either get a replacement filter or use an external IR filter you will reduce the impact of the mod. I would assess this after the mod and consider if you think an external cut filter will help. 

Focusing is only effected for shots taken at or near infinity as without the additional glass path length some lenses don't have sufficient adjustment to reach infinite focus. As you say with your scope it will be fine.

[Mered]

4 hours ago, Adam J said:

As I have said above, unless its a full APO then you will end up with larger halos with a removal only. 

Look at it this way, if you already have coloured halos around bright stars before the mod, after the mod they will be worse as you will be letting more IR light through by the nature of the mod.  As such if you either get a replacement filter or use an external IR filter you will reduce the impact of the mod. I would assess this after the mod and consider if you think an external cut filter will help. 

Focusing is only effected for shots taken at or near infinity as without the additional glass path length some lenses don't have sufficient adjustment to reach infinite focus. As you say with your scope it will be fine.

Does the Baader mod that cheapastrophotography offer also block the unwanted IR, or is it only necessary if you want to retain the use of regular lenses with the camera?

[Adam J]

2 hours ago, Mered said:

Does the Baader mod that cheapastrophotography offer also block the unwanted IR, or is it only necessary if you want to retain the use of regular lenses with the camera?

Its also an IR block. I use an external filter with mine though not the internal baader mod.