About to mod a Canon 1100D - Clarification on LP filter?

[StuartJPP]

Hello all,

I am about to modify a Canon 1100D, I have decided to take out the "rear" IR cut filter and leave the front low pass filter in place. I am not bothered about keeping auto-focus as it will be purely for astro use.

From what I have read, by leaving the front filter in place it is not necessary to add another filter as this filter filters out UV and far IR. I have read it on a few forums and on Astronomiser (http://www.astronomi...k/eosfilter.htm)

Is this true of the Canon 1100D? It isn't exactly the same as the 1000D as it doesn't have the piezo dust remover, but I assume that the filter itself is the same in both the 1000D and 1100D?.

Any other tips would be greatly appreciated!

[Merlin66]

Certainly the case on the 1000D. I'd assume the same filters were used on the 1100................

The "rear" filter is actually a colour correction filter....

[Gina]

There seems to be some dispute about this. The UK suppliers of filters etc. and many others reckon the front filter is all that's needed to block IR but some US people say that some IR gets through and causes bloated stars and that an additional IR block filter is needed. Sometime I want to do my own testing, with and without an extra IR filter but I haven't got round to it yet. Of course that trouble is, there's so few clear nights that imaging takes priority.

[StuartJPP]

Thanks for the replies, Gary Honis is adamant that a replacement UV/IR filter is needed:

http://ghonis2.ho8.c...derVrsLPF1.html

There was a discussion here as well, though the source of the graphs is unknown:

http://www.cloudynig...t/1#Post5564112

Gina, you are right, almost all of the modders here in the UK say that it is fine to just do a filter removal whereas US modders seem to insist on replacement.

What I am going to do is remove the 2nd filter and then if I find that there is IR "bloat" then I will probably just get an Astronomik L (UV/IR) EOS clip filter (which is almost the same price as the Baader filter to begin with).

[Gina]

Well, I've been getting CCD filters. That's CLS CCD for One Shot Colour and Ha CCD, I wasn't able to get an OIII CCD and Bern at Modern Astronomy assured me that with the front filter retained, the CCD version wasn't needed. I'm not sure as I seem to get some star bloat but I'm by no means sure that this is down to the filter. There are many causes of star bloat and halos.

[Merlin66]

Those graphs are based on Standard filter data and info on Christian Buil's site.

They clearly show the cut-off of the front filter at 700nm.

I did a similar test with a front filter removed from a Canon 1000D and found almost identical results.

I rest my case!

http://stargazerslounge.com/topic/169285-spectroscopic-testing-of-the-canon-front-filter-element/

[StuartJPP]

Those graphs are based on Standard filter data and info on Christian Buil's site.

They clearly show the cut-off of the front filter at 700nm.

I did a similar test with a front filter removed from a Canon 1000D and found almost identical results.

I rest my case!

http://stargazerslou...filter-element/

Thanks Ken - The problem with the 'net and forums in particular is that there are generally more people talking about doing things than people actually doing things. Trying to search/wade through 100's of posts that seem to contradict each other gets a bit soul destroying after a while.

As mentioned, I will only be removing the filter and not replacing it, and take it from there. Surely Gary could actually do a definitive test as he 1) modifies many cameras, 2) Has clearer skies than us. Personally I don't believe that the IR LED test he has got there is representative, surely it will be a lot more intense than even a very long exposure of the night sky? Of course I am no expert at this. It would have been more relevant to show 3 comparisons, one with no filter, one with just the front filter and one with the Baader filter.

Thanks again.

[Merlin66]

I've already been in dialogue with Gary Honis and the others...I offered to supply as much spectroscopic data as they needed..

I got the impression " I've already got my business and opinions....don't need any facts"

[Gina]

This is very tricky. I am of a scientific background and believe in absolute measurements with proper comparative figures. OTOH I'm also a practical type and recognise that scientific tests have to be very carefully controlled if they are to represent actual field conditions. I think what most astrophotographers want is a practical set of tests on real stars. The problem is getting conditions the same for all the tests. Or in practice, as near as required - absolutely the same is impossible.

The next consideration IMO is to decide which factors are most variable and to reduce those. As I see it these are :-

1. Atmospheric conditions - very variable and virtually no control
   
2. Optics - can be very similar
   
3. Camera - the object to be tested
   

We have no control of number 1. But if tests are conducted at the same time these will be the same.

I'm thinking that with the number of DSLRs I have that I might be a good candidate for carrying out these tests and I would be happy to do so to put this controversy "to bed".

I could set up three cameras with 200mm telephoto lenses - one unmodded, one modded and the third modded with an IR cut filter and all pointed at a star field. I have the three cameras, all Rebel T3 the US badged equivalent of the1100D and I have three 200mm lenses but not the same make - 2 Vivitar and one Asahi SMC Takumar. I wonder if this would be a suitable test in practice. There are some problems. The focus will not be exactly the same - I'm not perfect at focussing. The optics will not be exactly the same - no two lenses will be exactly the same due to manufacturing inaccuracies. The latter could be corrected for by repeating the test with lenses swapped.

I already have two cameras with lenses set up for widefield and I think I have an IR cut filter somewhere but if not I could buy one. Actually, I'm wondering if I need to include the unmodded camera in the tests. Without that I already have the mechanics all ready. To add the unmodded camera I would need to sort out another mounting.

[Gina]

I'm thinking now that I should have bought a CLS clip filter rather than a CLS CDD for my second LP filter. Then I could have used CLS CCD in one camera and CLS in the other. I don't think I have any requirement for a third LP clip filter

[StuartJPP]

Gina, I am sure that if the conditions are right and not variable the following can be done: set up one mount and one lens and by only changing the body take some long(ish) exposures, do this a few times with each body alternating every shot (will probably need to tweak the focus each time). After enough samples check to see what the difference is between the images. Refractors will be most affected so best to use one of them.

I agree with a scientific approach, Ken has done more of this than merely holding up an IR LED torch to the filter and seeing what shines through. If we were imaging for science we wouldn't be modifying a "cheap" £200 camera body would we? So as long as it captures more Ha than it does with the filter, without any discernible star "bloat" it is fine for me and probably 80% of people on here.