The new breed of digital SLR cameras from Canon are highly regarded for use in the somewhat demanding task of astronomical imaging.  While they cannot go as deep as most of the dedicated, CCD-based astronomical imaging systems and are limited to 12-bit signal resolution, they can produce amazing results nontheless, especially considering they are not really intended for this application, are not cooled, and use "inferior" CMOS imaging devices!  A quick "Googling" of the web for images produced with Canon DSLR's will turn up many stunning images that prove the capability of these cameras.

Canon DSLR models such as the Digital Rebel XT (a.k.a. the 350D) come from the factory with a pesky internal fixed "low pass" filter that blocks almost all of the astronomically interesting reddish hydrogen alpha (Ha) band, where many nebulae glow brightly.  These filters are intended to keep colors true and to block the infrared portion of the spectrum in order to keep the visible portion sharply focused, and in that task perform admirably.  However, the response of these filters slopes gradually into the visible band, intruding so much that the filter permits only like 10% of the Ha band to pass through.  To solve this problem, the camera can be disassembled to remove the low pass (or IR blocking) filter, permitting the CMOS imaging device to receive and register all available radiation.

But there is downside: once the internal low pass filter has been removed, the camera is useless for normal daytime usage, for two reasons.  Firstly, the camera electronics and firmware have been "tuned" to expect the response curve of the factory filter and without it, colors are not recorded properly.  Secondly, the lack of the glass material (that the filter is constructed from) in the light path means the distance between lens and imaging device is now incorrect (due to the index of refraction of light through air is different than through glass), and the auto-focus electronics will not focus properly.

To correct these flaws and permit the camera to be used for both astronomical imaging and normal daylight usage, the two flaws can be addressed separately.  To record proper color values, a external lens-mounted filter with nearly the same response as the original internal filter can be fitted to the lens, or a custom white balance can be programmed into the camera.  To address the incorrect lens-to-imaging device distance, a piece of clear glass can be installed, which matches the index of refraction of the original filter.  The thickness of the replacement glass is critical and ideally should match the thickness of the original filter glass.  If not, slight adjustments to the distance between the lens and imaging device can be achieved by adding or removing shims to the imaging device support posts (shims are added if the replacement glass is thicker that the original filter, or removed if thinner).

In this article, I describe the removal of the original IR blocking filter and addition of a piece of high-quality, clear, anti-reflection-coated replacement glass into the Canon 350D.  After having previously performed this same kind of modification on my Canon Digital Rebel (a.k.a.Canon 300D), I decided to subject my brand new, unsuspecting Rebel XT to the same indignity ;)  In fact, unlike with the 300D where I sweated and thought long and hard for months before attempting the mod, I cracked open my new 350D and completed the mod the same day it arrived in the mail, as I am now aware of the striking increase in sensitivity that this mod provides.  In addition, I know that the camera will be useful for normal daylight usage as well (as proof, all the images in this article were produced with my modified 300D + X-Nite CC1 filter, providing complete auto-focus capability and correct color rendering!)

DISCLAIMER: On this page I describe how I replaced the factory-installed internal low pass filter with a clear piece of AR-coated glass, for enhanced sensitivity to astronomical subjects.  You can try the same thing yourself, but TAKE HEED - IT IS ENTIRELY POSSIBLE TO ACCIDENTALLY RUIN YOUR CAMERA!!!  And, it should go without saying that doing something nutty like this to your innocent little camera WILL most likely VOID your Canon warranty, so tread lightly and PROCEED ONLY AT YOUR OWN RISK!  Consider carefully whether you really want to attempt this yourself.  If you do and you end up ruining your fancy DSLR, don't blame me!  I cannot and will not be held responsible for a botched mod, nor for any (inadvertently) inaccurate or incorrect information presented herein.  You have been warned!!

Still reading?  Good, I didn't mean to freak you out that bad ;)  First off, the proper tools need to be gathered, as shown in Figure 1.  Probably the most important item is a groundable anti-static mat with wrist strap, for dealing with the extremely sensitive CMOS devices (such as the main imaging chip!)  This is cheap insurance against destroying a not-inexpensive camera with an otherwise inconsequential static charge gathered by merely moving your feet around on the carpet! The other necessary items pictured are:

• Grounded (3 wire) soldering iron, solder, solder wick, and solder flux
• Glue of some sort (I used Weld Bond, a multi-purpose white glue that dries clear)
• Compressed air for blowing dust off imaging chip, etc. before re-assembly
• Hobby knife set - the flat-ended blade is all that's necessary for this mod
• Very tiny Philips head screw driver (that's the black pen object pictured - very tiny!)
• Very fine-tipped probe or long-tipped glass etcher for dealing with board connectors and interconnects
• Small forceps for dealing with board connectors and interconnects

Figure 1

A very useful item for tasks such as this is a compartmentalized box such as the one picured.  The extremely tiny screws and other parts can be placed in each compartment, grouped by modification step.  This makes re-assembly a relatively painless task, and is also handy if you're interrupted - simply close the top and there's no worries about the cat coming along while you're gone and flipping those tiny screws to the floor!  Little zip-lock baggies are another option.

Not pictured, but quite handy, are a few of those anti-static bags that hard-drives and other electronic parts are delivered in.  These are perfect temporary homes for the circuit board and imaging device that will be removed, as well as for keeping the dust off of them while the modification is being performed.

OK, time to get started!  First, make sure you've removed the battery and CF card.  The first few steps are quite easy, starting with the removal of three screws that hold the jack panel (Figure 2).  The third screw is approached from the top of the camera, behind the strap bracket.

Figure 2

Now five more screws need to be removed from the bottom of the camera (Figure 3).  Note that the 5th screw is a bit longer than the other four.

Figure 3

Now open the CF compartment to find three screws (Figure 4).  Note that the 2nd screw is silver colored and the others are black.  Remove them.

Figure 4

One more simple task: remove the eye cup by sliding it towards the top of the camera and off (Figure 5).

Figure 5

Now let's get a little tricky!  With the removal of screws in previous steps, the back of the camera can now be lifted (Figure 6).  Start by wiggling and lifting from the bottom of the back, gradually working the seperation up.  The top of the back is hooked to a couple of catches along the top edge, so pulling the back towards the bottom will finally free the back.  BE CAREFUL - there are two of those very fragile, paper-thin ribbon interconnects that connect the back with the camera - do not lift/pull the back completely off or else you'll damage them.

Figure 6

Once the back is free, lift the bottom of the back slightly to note (at least on my camera) a piece of tape holding a ribbon cable (Figure 7).  Use a probe or other tool to gently lift the tape, avoiding damage to the ribbon.

Figure 7

At this point, the back and the camera can be flipped open like a book, with the bottom of the camera/back acting as the spine.  The ribbons are long enough, but be carefull - avoid strains and tears!  You might note like I did that two funny-looking little rubber parts are loose (Figure 8).  Keep track of these, as they make up the two-button assembly located at the top right corner of the back, and will need to be replaced before re-assembling the back to the camera.

Figure 8

Time to get really delicate now, by disconnecting the first (of many - horrors!) of the paper-thin ribbon interconnects from their equally fragile-looking connectors.  The connector on the right bottom corner of the camera circuit board needs to be opened, and the ribbon slid out (Figure 9).  Note the little black bar laying on the end of the ribbon.  This needs to be flipped upward to unlock the ribbon, so use a probe or other very finely tipped tool to carefully lift the black bar, prying from the ribbon side.  It should "click" up with very gentle upward pressure.  Once flipped, the ribbon can be slid out.  Note that the black bar is hinged to the white part of the connector, so it doesn't actually lift off, but rather is swung up, like opening a door.

Figure 9

Now the second of the two ribbons connecting the back to the circuit board needs to be disconnected (Figure 10).  This connector is on the lower left of the board, and is rather odd - the black bar is not on the ribbon side like most others I've seen.  It must still be lifted, but this time pry it up from the other side, then slide the ribbon out.

Figure 10

At this point, the back is now separated from the camera (Figure 11).  Set the back aside for re-assembly later, as it's not involved in the mod anymore.

Figure 11

The three red wires connected to the board via a connector needs to be disconnected (Figure 12).  Although it appears the connector would slide out towards the CF port, it actually lifts straight up off the board, so gently pry upwards on the connector, under the red wires, until it pops off.

Figure 12

The port cover on the left side can now be removed (Figure 13).  Start lifting on the bottom of the cover, then unhook at the top.

Figure 13

Get that soldering iron heated up!  The shield in the center of the circuit board now needs to be removed (Figure 14).  It has no less than 7 "feet" that need to be de-soldered.  Using solder wicking braid, it is a fairly easy operation, but do keep track of where the soldering iron is held - it's too easy to become distracted and have the iron tip touch something it's not supposed to and ruin it.

Figure 14 indicates the 7 locations.  I would suggest first to remove as much solder as possible using solder wick from each location, keeping the heat applied to a minimum (do not keep heat applied for more than a few seconds at a time, and allow cooling time between applications to prevent burning anything up).  Then, proceeding in the order indicated, go around to each location, heat it and pry upwards where the arrows point, and remove the iron while keeping the upwards prying pressure.  Once the remaining solder cools, release the upwards pressure.  This will result in a completely disconnected foot.  The last two feet, numbers 6 and 7, can be removed using sideward pressure rather than upwards, to slightly swing the shield aside.

Figure 14

[A quick lesson on desoldering with solder wick: lay a clean end of wick on the solder lump, then place the iron on top of the wick end.  In a few seconds, the wick will heat up enough to melt the solder underneath, resulting in a little puff of smoke, and another second or two later the solder will have been absorbed by the wick.  Remove iron and wick at the same time, and observe the noticable lack of solder remaining!  Snip and discard the solder-impregnated end of wick and you're ready to go again.]

Figure 15 shows the shield removed from the board.  Note there could be ever-so-slight bends here and there due to the prying pressure - it's no problem as long as the shield is not completely mangled!

Figure 15

One more job for the soldering iron!  There is a screw that needs to be removed that is inconveniently located underneath an edge of plastic on the port side of the camera (Figure 16). 

Figure 16

Use the iron to make a hole in the plastic right above the screw head (Figure 17).  Then clean up the piercing with the hobby knife to make an access hole, and remove the screw.  Don't worry, this hole will not be visible once the camera has been re-assembled.  Just make sure any plastic melt is removed so the surface of the plastic panel is flat.

Figure 17

Extreme delicacy time again!  There's now a whole load of those little ribbon connectors to unlock (Figure 18).  Nine of them, to be exact, and some of the ribbons hide other ribbons/connectors underneath - ugh!  [You may be asking yourself (like I did), will I ever be able to get this back together again?  Yes, re-assembly is not that bad - see re-assembly comments later in the article.]

Before starting this nerve-wracking task, carefully remove the little square of black cloth tape on the upper right corner (taped on connector #6), taking care to not damage the red wires or pull up any of the utterly microscopic surface mount components in the process.  Also remove the four corner screws that hold attach the circuit board to the camera (the green squares indicate their locations).

I would suggest going around releasing the ribbons in the order indicated.  Note that some of the connectors may have little brown bars (rather than black), and a few might be of the odd type where the prying up occurs on the end opposite of where the ribbon enters.  A magnifying glass is handy here to get a good look at each connector before starting the unlocking.  And, how about that #9 ribbon and connector - positively petite huh?

Figure 18

Whew - that was fun ;)  Now slide off a piece of the CF card cage in the direction indicated (Figure 19).

Figure 19

The circuit board can now be worked apart from the camera.  It's easiest to lift up on the bottom edge of the board first, then the board can be pulled slightly bottomward to free it from the camera.  I found the top edge of the board required a bit of gentle wiggling to free it from the camera.

BE CAREFUL - There is one more bunch of wires connecting the board to the camera near the upper left corner, so don't pull these pieces completely apart just yet; rather, flip the board about the upper left corner to reveal the connector (Figure 20).  This connector slides out in a direction of the wires, so use a probe to gently work it out.  Once out, notice the "polarity" of the connection: one side of the male connector features long dark strips where the wires are exposed - these strips face the circuit board.

The circuit board is now free from the camera, so set it aside (preferably in an anti-static bag) as it's no longer involved.

Figure 20

OK then - we're finally getting somewhere!  The back of the CMOS imaging device is now visible (Figure 21), and five more screws must be removed.

Figure 21

Now the imaging device assembly can be lifted off, but BE CAREFUL HERE: There are three small stacks of shims (Figure 22) that are underneath screws #'s 3, 4 and 5.  It is very easy to hastily lift the assembly and scatter the shims, so go slow, peek underneath while lifting, and make sure you don't misplace them.  If they get knocked- off their piers or are stuck to the back of the assembly, restack them, making sure their little tails nestle properly in their "tail channels".

Figure 22

Now carefully set the camera aside (perhaps placing it in a big ziplock baggie to keep the dust away), as the imaging device assembly is now the focus.  As you can see in Figure 23, I've got all the little screws and parts sitting safely and in an organized fashion in the compartmentalized case, and the circuit board is safely wrapped in an anti-static bag.

Figure 23

The shielding on the imaging assembly is now removed by removing two screws (Figure 24).

Figure 24

A final two more screws should be removed to permit the filter assembly to be removed from the imaging device (Figure 25).

Figure 25

Before the filter assembly can be separated from the imaging device, it must be pried apart (Figure 26) due to the use of a rectangular gasket lighly gluing them together.  Using the flat-bladed hobby knife, gently work around the perimeter of the filter assembly to pry the assemblies apart, taking care not to damage the gasket or touch the surface of the imaging chip.

Figure 26

Once apart, the gasket will probably be sticking to the filter assembly (Figure 27).  The surface of the imaging device is now exposed, so immediately confine it to the safety of an anti-static bag to keep the nasty dust and electrons at bay.

Figure 27

Using the hobby knife, gently go around and pry off the sticky gasket, taking care not to damage it or the filter(Figure 28), then set it aside.

Figure 28

The other side of the filter assembly also has a rectangular gasket, with "sticky" only on one side.  Gently work around the perimeter to pry it loose, taking care not to damage it or the filter (Figure 29), then set it aside.

Figure 29

Now we're down to just the filter holder, and the IR block filter.  The filter is glued-in with a bead of still-flexible black goop, so use the hobby knife to scrape the goop out (Figure 30).  If you wish to save the filter for possible restoration of the camera to stock, spend some time here carefully digging-out the goop, avoiding scraping the filter with the knife.  Once most of the goop is gone, run the hobby knife down near the edge of the filter to score any remaining goop, and begin working around the perimeter, prying gently (be careful though - the filter glass chips quite easily!).  Eventually one of the edges will lift.

Figure 30

The filter is removed! (Figure 31).  You can see that even after being extremely careful, I managed to remove a few chips from the glass - no biggie though, because the chips do not extend into the "working" middle portion of the filter, and would in fact be hidden by the filter holder.  This step completes the disassembly!

Figure 31

The clear glass I used came from Edmund Optics.  It is an anti-reflection coated float glass, with very high transmission and very low reflectivity.  I lucked-out this time, having a piece leftover from a previous 300D modification, that is almost exactly the right size for this mod.  No extra glass scoring/breaking was necessary :)  Excellent instructions on how to score and break glass are given on Gary Honis' 300D modification pages, so I will not repeat them here.

In order to achieve the goal of having this camera continue to work for normal daytime usage (albeit with either an extra lens-mounted IR block filter, or a custom white balance to compensate for the loss of the internal filter), the thickness of the replacement glass has to be considered when shimming the imaging assembly back into the camera.  With proper shimming, the auto-focus camera feature will continue to work. 

Using a micrometer, the measured dimensions of the Canon IR block filter are:

• Thickness: 0.097"
• Width: 0.869"
• Height: 1.103"

...and for the replacement clear glass:

• Thickness: 0.118"
• Width (cut to match Canon filter)
• Height: (cut to match Canon filter)

The difference in thickness between the Canon IR filter and the replacement glass is 0.021". Plugging this into the equation (found and explained on Gary Honis' web pages):

OutOfFocusDistance = (n - 1) / n * ThicknessDifference
  where:
  n is the refractive index (1.517 for glass)
  ThicknessDifference is 0.021" in my case
  OutOfFocusDistance is width of extra shims necessary

I lucked-out again and found some tiny washers in my junk box of exactly the correct thickness!  These, and the piece of clear glass I used are shown in Figure 32.  Note the glass is still clad in the protective blue film in place, which will be removed at the last minute.  If suitable thickness washers cannot be found, other materials such as plastic film or tape, metal foils, etc. can be measured, stacked, and cut to shape.

Figure 32

Installing the clear glass replacement into the filter holder simply involves orienting it correctly, and applying a bead of glue (Figure 33).  Right before placing the glass, I removed the protective blue film from the face-down side, and made sure not to touch it thereafter (I hate trying to clean it!)  After the wait for the glue to dry clear, I carefully scraped down a few high spots in the glue with the hobby knife, removed the remaining piece of blue film, and the glass replacement is complete!

Figure 33

The re-assembly procedure is basically the reverse of disassembly.  Once at the task of reintroducing the imaging assembly back into the camera, the shims gathered or manufactured to adjust for the glass/filter thickness difference are placed atop or within the existing shim stacks (Figure 34), and the assembly carefully placed back on top before being secured with the three screws.

Figure 34

Instead of a long series of images and text documenting the re-assembly, here is a brief checklist of tasks to be accomplished to bring the camera back to life:

1. Position and glue the single-sidedly-glued black gasket back on the filter assembly (Figure 29).  Note that this is the side on the filter assembly where the new glue bead was applied to cement the glass in place.  In my case, my replacement glass was a bit thicker that the original filter, so a slight dome effect of the gasket resulted - not a problem in my case, but if much thicker clear glass were used, the extra thickness might be enough to interfere with the fragile shutter blades - be careful!
    
2. Position and glue the double-sidedly-glued black gasket back on the other side of the filter assembly (Figure 28).  Note this is the side opposite where the new glue bead was applied.  If enough of the original "sticky" is left, no extra glue may be required.
    
3. Position and secure the filter assembly back onto the imaging assembly (Figure 27), using the two little black pins on the filter assembly to locate the pin holes on the imaging assembly.  You may find enough of the "sticky" remains on the double-sidedly-glued black gasket that more glue is not required. HINT: Before joining, make sure each surface is as clean as possible, and perhaps use the compressed air to blow any and all dust off - it will be impossible to clean later without repeating a complete disassembly!
    
4. Secure the filter assembly to the imaging assembly with two screws (Figure 25).
    
5. Secure the shield to the imaging assembly with two screws (Figure 24).
    
6. Make sure your additional shims are placed correctly (Figure 34), and carefully position the imaging assembly back into place - do not knock any shims out of place!
    
7. Re-secure the imaging assembly with 5 screws (Figure 21), remembering where the 2 short and 3 long screws go.
    
8. Retrieve the circuit board from its anti-static haven, and reattach the loose wire connector to the connector on the corner of the circuit board (Figure 20).  Remember that the longish dark strips on the male piece face towards the board.
    
9. Roughly position the circuit board back into place, and prepare for the delicate and arduous task of reattaching all nine of the ribbons to their connectors (Figure 18).  I suggest going around in the reverse order indicated to restore the ribbons (i.e. start with #9, then do #8, etc.)  For the first several ribbons, it helps to have the circuit board almost but not quite back into place.  Extremely gentle grabbing of the ribbons with forceps is useful, as well as the use of a probe to snag and position the ribbons using the small pin hole near the end of each.  Make sure in each case that the ribbon is fully and squarely inserted into each connector before snapping closed its black or brown "gate".  Patience is key here - no need to rush and rip or mangle a ribbon at this stage!
    
10. Reposition the CF cage piece back into place (Figure 19).  The two screw tabs on the piece are positioned on top of the circuit board screw holes.
     
11. Secure the circuit board to the camera with four screws (green squares in Figure 18).
     
12. Re-lay the square of black cloth tape to secure the three red wires (over connector #6 in Figure 18).
     
13. Reattach the single screw through the new hole created in the port side of the camera (Figure 17).
     
14. Prepare to resolder the middle shield back into place on the circuit board (Figure 15).  Dab a tiny amount of solder flux on the bottom of each of the seven shield feet and on the seven foot pads on the circuit board.
     
15. Position the shield back into place (Figure 14).  In addition to the feet on the shield, there are a few (or was it just one?) tabs pointing down from the shield that must be located in corresponding tab holes on the circuit board - a satisfying "click" will let you know its been seated.
     
16. Going in reverse order of disassembly (i.e. starting with foot #7), go around and resolder the feet to the circuit board (Figure 14), making sure to gently press down while the solder is melted to insure the feet are squarely seated on their pads.
     
17. Reattach the port cover by hooking it at the top and gently pressing it back on (Figure 13).
     
18. Replug the three-red-wire connector back onto the circuit board (Figure 12).
     
19. Prepare to replace the camera back to the camera by laying them as shown (Figure 11).
     
20. Reattach the remaining two ribbons to restore camera-to-back connections (Figures 10 and 9).
     
21. Reposition the camera back to the camera, making sure to return the two funny-looking little rubber button pieces in place (Figure 8), and retaping the ribbon to the shield (Figure 7).
     
22. Gently work the camera back back into place by sliding the top edge of the back topwards so the little hooks catch their latches (Figure 6).  Then work down the sides then bottom edge of the camera back to snap it back into place.
     
23. Slide the eye cup back into place (Figure 5).
     
24. Reattach the three CF bay screws (Figure 4), noting the silver-colored one goes in the middle.
     
25. Reattach the five bottom screws (Figure 3), noting that screw #5 is the longer one.
     
26. Reattach the three port-side screws (Figure 2).
     
27. GO DOWN A BREWSKI OR THREE!  You're done!  Well, actually, you're done if the camera works again when you install the battery and CF card ;)  If it doesn't work, then yikes, time to back up, re-disassemble the camera and check all work, and pray you didn't accidentally zap a CMOS device, tear a ribbon, fling a few microscopic surface mount thingies up off their moorings, or perform some other equally horrid bungle... (if you're somewhat lucky, it just might be a ribbon has not been seated properly in its connector; if not, well, let's just not go there shall we?).

Assuming all went well, you now hold in your grubby little hands a very capable and sensitive astronomical imaging instrument!  And, because the filter has been replaced with clear glass and the imaging assembly was properly shimmed, one that can serve double-duty.  One duty as an astrocam, and the other by being used as intended, for normal daylight photography, with the addition of a lens-mounted IR filter (such as the X-Nite CC1), or through the use of a custom white balance to correct for the altered colors.

• Canon Digital Rebel (300D) Modification by Gary Honis
  (http://ghonis2.ho8.com/rebelmodnew.html)
  
  Excellent presentation and instructions on various options on how to modify a Canon Digital Rebel (300D) for astronomical imaging.  This is a must visit site (please go read there first before attempting this 350D mod!), and is the model for my article.
   
• Canon 350D Filter Removal by Javier Laina
  (http://personales.ya.com/javier_laina/ARTICULOS/FILTRO/filtro350deng.html)
  
  Illustrates and briefly describes the steps taken to remove the internal IR blocking filter from the Canon 350D.  Used as reference for the actual steps necessary to disassemble the camera.
   
• digital_astro, an email list hosted by Yahoo!
  (http://groups.yahoo.com/group/digital_astro/)
  
  This is The group where discussions about the use of digital cameras used for astronomical imaging are exchanged.  This is where I learned of the highly regarded Canon DSLR's for this purpose, of the possibility of removing the internal IR block filters for imporoved sensitivity, and of Gary Honis' web page in particular.
   
• Hap Griffin's DSLR Cables
  (http://www.hapg.org/astrocables.htm)
  
  Source of inexpensive cables used to permit a PC to control exposures beyond the standard 30 seconds max provided by the 350D itself.  Hap also offers to remove the internal IR block filter from Canon DSLR's for a modest price, in case you are too afraid to do it yourself ;)
   
• LDP Net (maxmax.com)
  (http://www.maxmax.com/)
  
  Source of the X-Nite CC1 lens-mounted color-correcting filter, which can be used with a glass-for-filter-modified 350D to restore normal colors for regular daytime use.  Also features many other types of specialty filters, such as the X-Nite IR pass filters, which could be used with a modified 350D for IR photography!
   
• DSLRFocus by Chris Venter
  (http://www.dslrfocus.com/)
  
  Inexpensive PC software that permits a PC to control the Canon 350D via serial or parallel port, for beyond-30-second exposures, as well as to automate the sequencing and collection of astronomical images.  Also features an auto-focusing section where, with the addition of a PC-controllable focuser, your PC can be used for precise and automated focusing.
   
• Astrophotography Guide for EOS Digital by Canon
  (http://web.canon.jp/Imaging/astro/index-e.html)
  
  Canon's website describing how to use their DLSR's for astronomical imaging - pretty cool of Canon to know and care about us astro-geeks!