Need help with choosing cooled modded DSLR

[jtamir]

After reading several astrophotography books and reading a lot on several forums I finally decided to get a cooled, modified DSLR as my first imaging camera. This may combine the relative ease of a DSLR with large sensor, cooled low noise camera and rational price. This camera is intended to be used with a 102 triplet refractor.

My questions are:

1) Which camera to buy? I was considering the canon 1100D or 600D?

2) The camera will be used for astrophotography only! After removing the IR filter, should I choose the Baader IR/UV filter or maybe a clear glass would

be better? (I understand not all CCD’s have filters?!) Will my triplet APO have IR color focusing issues with the clear filter only?

3) Does anyone have experience with the JTW ultimate modification or the CentraLDS modification? The JTW sounds more professional, does it justify the extra cost?

[Gina]

After reading several astrophotography books and reading a lot on several forums I finally decided to get a cooled, modified DSLR as my first imaging camera. This may combine the relative ease of a DSLR with large sensor, cooled low noise camera and rational price. This camera is intended to be used with a 102 triplet refractor.

My questions are:

1) Which camera to buy? I was considering the canon 1100D or 600D?

From my investigations the 1100D gives best value for money.

2) The camera will be used for astrophotography only! After removing the IR filter, should I choose the Baader IR/UV filter or maybe a clear glass would

be better? (I understand not all CCD’s have filters?!) Will my triplet APO have IR color focusing issues with the clear filter only?

The 1100D has two filters one is a light blue colour and reduces red sensitivity - this should be removed. The other is an IR/UV blocking filter - we want this for normal AP (not specialised IR) and after taking this one out (very carefully) it should be put back in to retain IR/UV blocking. Unfortunately, the IR/UV filter needs removing to get at the colour filter. Replacing the Canon filter with another make is not needed as the Canon filter is considered to be at least as good as the Baader one. Only if you break the IR/UV filter in getting it out will you need to get a Baader replacement. NB Gary Honis actually calls these filters by their wrong names in his web site instructions but otherwise his explanation is excellent.

3) Does anyone have experience with the JTW ultimate modification or the CentraLDS modification? The JTW sounds more professional, does it justify the extra cost?

I don't have any information about these - I carried out all modifications including filter removal and Peltier cooling myself.

[ghonis]

Gina: You wrote: "Gary Honis actually calls these filters by their wrong names in his web site instructions but otherwise his explanation is excellent."

The filter labels on my web site are correct. They are the labels used by Canon and other reputable web sites. Your misunderstanding may be caused by a Canon diagram that has been altered by Brent Oliver and is widely available on the net. His diagram uses an original Canon diagram that was re-labeled by him showing the front filter as the UV and IR blocking filter and the second filter as a color limiting filter. Brent Oliver fails to inform others that his diagram is not from Canon but instead his own creation. You can see his incorrect diagram here:

The original diagram with labeling provided by Canon can be found on reputable websites such as:

Dpreview:

http://www.dpreview.com/reviews/CanonEOS550D/images/whatsnew/cmos2.jpg

That is the diagram Brent used for the filters, but he changed the labels. Note that the second filter is labeled by Canon as the "Infrared and Ultraviolet absorption glass". Brent relabeled it to read: "Low-pass filter #2 Color Limiting Filter". On his web page he says: "LPF-2 is only a Color Limiting Filter......"

and Canon's own website:

http://www.usa.canon.com/cusa/support/consumer/eos_slr_camera_systems/eos_digital_slr_cameras/eos_rebel_xs_18_55is_kit?selectedName=Features&fileName=0901e0248003e54c_feature6.html

and Imaging-Resource:

http://www.imaging-resource.com/PRODS/E60D/E60DA.HTM

I have the correctly labeled diagram on my web site here:

http://garyhonis.com/600Dreinstallsteps1a.html

I don't do modifications that only remove the stock IR filter (LPF#2). That is because the front filter (LPF#1) does not fully block UV and IR and can result in star bloat. The problem with having only the front filter (LPF#1) in place is that it is a weak IR blocker and does not block all infrared light. If you planned to image with a reflector telescope only at prime, that would be okay. Once you add a focal reducer, corrector or barlow to the imaging train, you would need to include a UV/IR cut filter in the imaging train to avoid star bloat. If you plan to image with a refractor telescope, you would also need to add a UV/IR cut filter in the imaging train. That is why the "Baader" UV/IR cut filter modification is so popular. I have an animation posted here that shows star bloat using LPF#1 only as the filter:

http://garyhonis.com/Tests/LPF2starbloat.html

We have had discussions of this on my support group DSLRmodifications:

http://tech.groups.yahoo.com/group/DSLRmodifications/message/3797?threaded=1&l=1'>http://tech.groups.yahoo.com/group/DSLRmodifications/message/3797?threaded=1&l=1

Please consider joining my support group if you are not already a member:

http://tech.groups.yahoo.com/group/DSLRmodifications/

Jeremy contacted me by private email, so I have replied to his questions by email.

Gary Honis

[Gina]

Hi Gary

I apologise for any offence, I didn't intend any, but your naming seemed in contradiction to what I found when modifying my own 1100Ds (and Rebel T3s). Maybe Canon changed the order of the filters in later production because the clear IR/UV blocking filter is definitely the larger (front) one in my cameras and needed removing first.

This is a copy of the Canon diagram

And this quote is from my own filter removal which clearly shows the clear IR/UV cut filter and the light blue "low pass filter" :-

Continuing the stripping down...

The screw sheet.

Filter removal.

[ghonis]

Hi Gina,

No offense taken and no need to apologize. Canon has not changed the purpose of its two filters in recent models including your 1100D model. Both filters block infrared to some degree. You can't tell the infrared blocking capability of the filters by just looking at them. The frontmost filter (LPF#1) has a dichroic thin film coating on its back side that reflects infrared light. It alone will not block all infrared wavelenghts above 700nm to avoid star bloat with refractor scopes or camera lenses. The second filter (LPF#2) is also an infrared blocking filter. It is an infrared absorbing filter. Attached is a diagram from Canon. Canon labels the first filter as having a coating on its back side as a "Dichroic mirror (Reflects infrared rays)" and labels the second filter as being an "Infrared-absorbing glass".

Gary Honis

[Gina]

Thank you Gary That is very interesting. So the clear glass plate bonded to the sensor surround (that I found impossible to remove when I tried to get inside to remove the Bayer array) is actually LPF #2 and the light blue glass is both a phase changing element and a red blocking filter.

[Gina]

Picking up on another point that you mentioned in your earlier post - that the front IR block filter is not fully effective... I have read in several places and heard it from an apparent expert in person that the front filter IS sufficient and that no extra IR cut filter is required. Also that this Canon filter is at least as good as the Baader replacement. Your results show that neither of these is the case

I have a wide field DSO imaging rig with 2 1100Ds mounted side by side one for Ha and the other for OIII with virtually identical 200mm telephoto lenses. I have a non-CCD Astronomik OIII clip filter and I do get some star bloat in images taken through that. The Ha clip filter is a CCD version and shows no star bloat in images. Now I know why the OIII filter seemed faulty. So I guess I should use an unmodded camera for the OIII side - don't need red sensitivity anyway.

[ghonis]

Hi Gina,

Ha filter images usually provide very tight star images because they are so narrowband.

One option to add infrared blocking with your 200mm telephoto lens is to use a UV/IR Cut Filter that is a lens thread-on filter. One that has worked well for me is the Rocolax brand. I don't know if they have the size needed for your lens, but I ordered a 58mm size on ebay and it was only $30 including shipping here to the US.

Gary Honis