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Removal of IR Filter in Canon 20D SLR

By Chris James - 2nd August 2009

 

 

Overview

 

This document is an attempt to give the reader a rough guide on how to remove the internal IR Blocking filter from a Canon 20D digital SLR in order to improve the Red Spectral Response for astrophotography. It should give the reader an idea what to expect and how to dismantle the camera to access the IR filter in front of the CMOS Sensor.

 

This document may help you to decide on whether to proceed or not with these modifications depending on your desired outcomes and personal skill levels.

 

It assumes the reader has the basic skills such as soldering techniques and knows how to take the required anti static procedures to perform this work.

 

Note: On completion of this modification it is highly unlikely that you can reverse this modification. The moment you begin this modification you have voided any warranty that may apply to your camera. I have assumed at this stage in your 20D life cycle that it is well and truly out of warranty and its possibly a second SLR that you are intending to use solely for astrophotography work. In any case you would only consider doing this modification yourself if you are prepared to lose the camera as there is always a chance that the camera will not work after attempting this.

 

Having said that, if you take your time you should have a pleasant outcome.

 

Depending on some choices you make, the camera will have different characteristics in the areas of focus and white balance which we will quickly cover now.

 

 

Focus

 

When you remove the existing Infrared blocking filter from in front of the sensor as shown in figure 1 below, you have two choices:

 

1.       Simply remove it without replacing it.

This results in the optical path between the reflex mirror to the sensor no longer being matched to the optical path between the reflex mirror and the focusing screen. As a result when you manually focus the camera looking through the eyepiece or the camera automatically focuses, the image will not be focused on the sensor when the mirror operates. In other words you can no longer auto focus or manually focus the camera as you cannot simultaneously focus on the focusing screen and the sensor as the two optical paths are now not matched.

 

Under these conditions focus is done using the image download from the camera and can be assisted with software programs such as DSLR focus.

 

The desired L-Filter (UV-IR Block) is inserted externally to the camera (or at least on the outside of the reflex mirror) in order to prevent purple fringes around bright stars.

 

2.       Replace it with the desired L-Filter (UV-IR Block).

If you elect for this option, you will need to have the replacement filter on hand before you pull the camera apart. Replacing the filter maintains the balance of the optical path from the reflex mirror to the focusing screen and the sensor. The result is when focus is achieved automatically or manually on the focus screen it will also be achieved on the sensor when the mirror operates.

 

The risk however is, if you get dust between the sensor and the new UV-IR blocking filter during installation, you will not know until you re-assemble the camera. Then it can only be removed by pulling the camera apart again. You would require very good work practices and a suitable work environment to ensure this does not happen.

 

 

Figure No 1

Cross-Section View Of DSLR System

 

 

 

1 - Lens

2 - Reflex mirror

3 - Focal plane shutter

4 - Sensor

5 - Focusing Screen

6 - Condenser lens

7 - Pentaprism

8 - Eyepiece

9 - Infrared blocking filter

 

 

 

 

 

The path I took - For obvious reasons it is more desirable to have the option to be able to manually focus the camera looking through the eyepiece to achieve focus at the same time as sensor focus. Option 2 above would be used given the choice. However as I did not have a replacement L-Filter for the existing internal IR blocking filter at the time and I did not want to risk dust between the sensor and filter due to my skill level and work environment, I did this mod using option 1 as most people doing this work themselves do.

 

As a result I will need to check the final focus on the sensor by continuing to download the image from the camera to check focus and use focus assist software such as DSLR focus. I can still use the eyepiece to assist getting the target on the CMOS Sensor but not for final focus for the imaging session.

 

 

White Balance

 

With the internal IR filter removed, the white balance when the camera is used for normal terrestrial photography results in a pink colour cast. If the camera must be used for terrestrial photography then a correction filter needs to be applied to the front of the terrestrial lens to correct this colour cast. Alternatively the pink colour cast needs to be correct when processing the images downloaded from the camera. In addition to the colour cast you will need to adjust the exposure compensation to compensate for the extra red light now arriving at the sensor to prevent the reds from saturating and blowing out during terrestrial photography.

 

In practice the camera is now an astrophotography camera as it is no longer practical to use for terrestrial photography once the Canon internal IR filter is removed.

 

 

 

Performing The Modification

 

Now that you have a good understanding of what your precious 20D can and cant do after the removal of its internal IR blocking filter and you are confident to proceed then its time to prepare for the modification.

 

 

 

 

 

 

Tools Required

 

Small flat blade jewellers screwdriver

Small Phillips head jewellers screwdriver

Small temperature controlled soldering iron

Magnifying glasses

Anti Static mat and wrist strap

Solder Sucker

Sensor Brush

Compressed Air

 

 

 

 

 

Chose a well lit area that is free from air borne dust and lay out the anti static mat and attach the wrist strap to your wrist connecting it to the mat.

 

You will need a container with multiple compartments or some sticky tape laid out so you can place the small screws on it that you are about to remove so that they are not lost and you can remember where each of them came from.

 

Turn the camera off and remove the battery.

 

 

Removing The Back Cover

 

 

 

 

 

 

 

 

 

1.      Remove the eyepiece.

 

Place it aside somewhere safe.

 

 

 

 

 

 

 

 

 

2.      Remove three screws

 

Two each side of the eyepiece and one to the left of the menu button.

 

 

 

 

 

 

3. Remove screw behind rubber grip

 

Carefully lift the corner of the rubber grip to expose the fourth screw which is now removed.

 

 

 

 

 

 

 

4. Remove the two screws on the bottom of the camera.

 

Only remove the two screws at the back of the camera in the back case.

 

Be careful that the back of the case does not come off at this stage.

 

 

 

 

 

 

 

 

 

5. Remove the two screws next to the digital and video connectors.

 

 

 

 

 

6. Open the back compartment.

 

Carefully open it to the right as shown. Be careful not to strain the ribbon connector which connects it to the main circuit board.

 

You will need to unlock the ribbon connector on the circuit board by carefully slipping a small flat blade screw driver between the flat cable and the connector lock gently prying it up to unlock the connector. Once unlocked, carefully pull the ribbon cable out of the connector and put the back cover safely aside.

 

 

 

 

 

Removing The LCD

 

 

 

 

 

 

 

7. With soldering iron and solder sucker remove excessive solder from LCD frame in top right hand corner.

 

 

 

 

8. Peel back all three copper noise reduction sheets. The third one with the screw hole in it is underneath the large sheet.

 

These are sticky on the back, so be careful not to tear the thin sheets of copper.

 

 

 

 

 

 

9. Remove the yellow tape and carefully removed the two wire connector with the Red and Black wires. Use some small pointy nose pliers and pull the connector out being careful not to pull on the wires.

 

10. Release the ribbon connector holding the ribbon cable for the LCD and pull the ribbon cable out of the connector.

 

 

 

 

11. Remove the three screws holding the LCD bracket and now remove the LCD putting it aside.

 

 

 

Removing The CMOS Sensor Shield

 

 

 

 

 

 

 

12. Using a small soldering iron and a solder sucker, remove the solder from the three lugs connecting the CMOS Sensor shield to the PC.

 

Once its free, remove the CMOS Sensor shield and put it safely aside.

 

 

 

Removal Of Large PCB On LHS

 

 

 

 

 

13. Unlock the three ribbon connects on the main board on the LHS of the camera and remove the three ribbon cables. You can carefully use a small screwdriver or a toothpick thru the holes of the cables to slide them out of the connectors.

 

14. Unlock and slide out the two ribbon cables that go to the CMOS Sensor. (Top and bottom LHS of this picture insert).

 

 

 

15. Remove yellow tape on LHS bottom corner to reveal another ribbon cable and connector.

 

16. Unlock and remove ribbon cable.

 

Note:This ribbon cable connector unlocks using a different method to all the rest. Put a small flat blade screw driver behind the blue lugs at each end of the connector and pull both of them out towards the back of the camera. Then gently slide the ribbon cable out.

 

 

 

 

 

 

17. Now lift and over the PCB to the left, being careful not to stain the blue and white wires still soldered between the PCBs.

 

 

 

 

Removing the CMOS Sensor Assembly

 

 

 

 

 

 

 

18. Remove the four screws holding the CMOS Sensor assembly to the main chassis.

 

19. Remove the CMOS Sensor assembly.

 

 

 

 

 

 

 

 

Caution: If you need to tip the camera over to recover a dropped screw for example, the six brass shims shown will dislocate. There are three locations for these shims, so there are two shims at each location.

 

 

 

 

Removal of Hot Mirror IR Blocking Filter

 

 

 

 

 

 

20. Remove screws from IR assembly.

 

Up until now everything is reversible. Fromhere on however it is very unlikely that you can reverse the changes.

 

If you have some thin cotton gloves, it would be a good idea to wear them as you donít want to touch the CMOS Sensor by mistake and leave grease on it.

 

 

 

 

21. Remove the IR blocking filter assembly.

 

This is glued in so you will need to work your way around the assembly leverage it with a small flat bladed screw driver.

 

Once you have filter assembly containing the filter, put the sensor assembly aside protecting it from any dust contamination.

 

 

 

 

 

 

22 Cut and remove the glue holding the IR filter in the black plastic mould.

 

Once the filter is removed, I cleaned the black plastic frame to remove the remains of the glue.

 

 

 

Cleaning The CMOS Sensor

 

With the camera now re-assembled and fully functional it should comes as no surprise that you need to clean the CMOS Sensor.

 

First check the CMOS Sensor for dust by taking a test image. To do this fit a normal terrestrial lens to the camera and go outside. Take a shot of some clear blue sky, or white smooth clouds. When exposing the test image ensure your lens is stopped down as far as it can go, f22-32 for example. The shutter is set to give the correct exposure. Dont forget when you focus on the focusing screen, the image will not be sharply focused on the CMOS Sensor.

 

Now download the image and view it @ 100 percent (actual pixels ). This will show if you have dust on the CMOS Sensor and where its located.

 

I use a set of Sensor Brushes to clean the mirror chamber and the CMOS Sensor surface. You need to be extra careful now as there is no longer the IR blocking filter protecting the surface of the CMOS Sensor. You will more than likely need to take more test images, check for dust and clean a few times until you are happy the CMOS Sensor is dust free.

 

 

Fitting The L / UV & IR Filter

 

The final step now is to fit a suitable L filter to suit your set-up. With the extra Red Spectral Response you may get purple/violet fringing around bright stars on long exposures without the addition of a suitable filter.

 

The filter can be fitted externally anywhere past your T-Ring adaptor or by using a EOS Clip Filter as shown in the image below. The added advantage of Clip Filter being it helps keep dust out of the mirror chamber when you are doing this in the dark during an imaging session.

 

 

 

 

 

Astronomik EOS Clip- Filter

 

 

Written by:

Chris James

55 Blackthorn Road

Greenwood WA 6024

 

 

cwjames@bigpond.com

 

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