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Filters and DSLR


Guest Mr T
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I image with an unmodified Canon DSLR and I want to know what type of filters would be worth using. I know CCD imagers use filters of many different wavelengths to really capture some great detail, but would any of these work with my DSLR?

Thanks

Mark

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My understanding is that the built in filter on your DSLR cuts out most of the infrared light, but in doing so, cuts out lots of the hydrogen alpha light too which is the stuff that shows a lot of detail from the distant structures:


 


http://www.astrosurf.com/~buil/baader/baader1.png


 


You can get filters which also cut out light pollution wavelengths like the Astronimik ones, which filter out much of the LP and preferentially let through wavelengths which are 'good', though if you have an unmodified camera (like me) then your existing inbuilt Canon filter will filter out some of these good wavelengths too:


 


http://www.zodiaclight.com/images/GraphofBaaderUHCSversusAstronomikCLSFilter.jpg


 


It's important to think about the filters in terms of 'blocking' or 'let through'; if you get a hydrogen alpha 'let through' filter for an unmodified DSLR, then only the hydrogen alpha light will get through the first filter (as it's let through), but most of it will then be blocked by the in built Canon filter.... Hence why people modify their cameras and have the inbuilt filter removed / replaced.


 


I've got the Astronomik CLS filter, and it is good for cutting out light pollution, but generates a blue-hue on the resultant image which then has to be processed out.


 


I'll be interested to hear what the clever guys say about this.


 


James

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Thanks James that's really useful. I live in a rural location and don't suffer from light pollution and guess a light pollution filter would not make much difference for my situation.

Sometimes using an OIII filter improves contrast when observing visually, would this have the same effect when imaging with my unmodified DSLR?

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Mark, don't advertise your light-polluted free skies, or we might all descend on you :) Make the spare room up, I'm packing the car....

 

You need one of the clever guys like Stephen or Ibbo to give you the top-notch info.

 

You can probably find out online exactly what wavelengths your in built Canon filter cuts out, then look at what wavelengths the OIII lets through. If they significantly overlap, then it doesn't sound like it would be of great benefit, other than result in you needing 100x times longer exposures to get even a hint of the OIII wavelengths (unless of course the cut in built cut filter was 100% effective at those wavelengths then you'd get nothing).

 

Sorry I can't help more. But I will be watching with interest.

 

JD
 

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Guest ecopley

It's nowhere near as hard to remove the filters from your camera as is made out. You need the right size screwdriver, a clean surface, a scalpel or small sharp craft knife and a piece of paper to make a note of which screws came from where. Also a pair of tweezers is useful.

I bought a second hand 450d on eBay and did the job in about an hour. I replaced the canon IR filter with one that was recommended on one of the guides. I forget what it was exactly but can check if you want to know. Remember that the auto focus won't work after you have removed the filter but since you'll be focusing with the telescope it's not a big deal.

There are guides on the interweb to help but the most difficult things are:

1) keeping the sensor clean - just be careful. James will tell you that I tend to dive into things with more enthusiasm than sense and mine is fine, no marks or anything (I'm rather proud of it to be honest!).

2) getting all the screws back where they belong. I have a few left over because, though I made notes, I couldn't get some of them to go back. I didn't want to force them but decided that, since the camera won't be treated as roughly as my unmodified daytime 600d, I didn't really mind.

Since doing it the weather has been awful on the nights I've been able to go out. The only picture I have is of m42. Lots of lovely red, very clear and no artifacts other than the noise one would expect.

I'll take some more if there is a break in the weather and post them but, honestly, it wasn't a big deal and was very satisfying to do.

If I can help then I'd be happy to try but there are plenty of guides.

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The filters for imaging are different to the filters for observing - they block light at different wavelengths. So there are in fact two different types of O3 filters depending on purpose. :)


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That's very interesting, I only have one DSLR which I use for all photography. I suppose that gives me an excuse to upgrade, so I can pull the filters out of it, although I do plan on getting a ccd when funds allow.

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Dslr's image in full colour - narrow band filters are only required with mono ccd cameras. You can upgrade the dslr to have the red filter removed in order to capture all the Ha (red) wavelengths given off by emission nebulae (eg M42).


 


Sometimes the filter is replaced with an ultra high contrast (uhc) filter - depending on which camera you're mod'ing. This link gives a good explanation of the mods available (some folks do their own):


 


http://www.astronomiser.co.uk/eosmod.htm


 


Hth :)

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  • 2 weeks later...
Guest AstroOlly

Here is a document I made comparing the same exposures with an unmodified DSLR at night (looking out up my street), with and without the CLS CCD filter clip:

https://dl.dropboxusercontent.com/u/1673286/light%20pollution.pdf

JD

Why would you use a CCD clip in an unmodded camera, surely you only need the normal Clip, as the CCD version cuts IR and that is already being done by your inbuilt camera filter, then when you mod and have this removed, you still have a filter left Infront of the sensor which cut IR so you still do not need the CCD clip, this is only needed after a full spectrum mod, and both filters are removed from the canon camera.

Well that what I thought

Olly

Edited by AstroOlly
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