Cookbook CCD camera Modifications
Since The CCD Camera Cookbook was published, many builders have
suggested a wide variety of modifications. Some of the suggestions are
excellent. Unfortunately, a few widely circulated "upgrades"
may harm your camera's performance. This list contains modifications that
1) help the original camera, 2) do not harm the original camera, or 3)
adapt the Cookbook CCD camera to the demands of field operation. We have included
items on this list that are, in fact, already described in the book but
have been overlooked by some builders. Beyond those recommended, modifications
listed here are for your information only, and are understood to be undertaken
at your own risk.
To date, over 3,000 builders have constructed Cookbook CCD cameras and we
can state with assurance that the original published design functions as
well as we could have hoped. Before you consider any modification
of the original design, we feel that you should know that opening the camera
and changing it carries a finite risk of converting a working camera into
a non-working camera. If you lack sufficient experience in electronics
and circuit design to troubleshoot problems that may result, we advise
you to change nothing, but instead put your Cookbook CCD camera to use by shooting
great astronomical images.
Recommended for all
builders:
- 1000uF capacitors:
Add two 1000uF electrolytic capacitors to interface board at entry of +15,
ground, and -15 VDC leads. See Cookbook Appendix C.3. Pros: Eliminates
possible oscillation of voltage regulators on the interface board and prevents
noise pickup from power cable. Cons: None. Assessment: Just
do it.
- Jones plug:
Replace six power supply banana jacks with polarized 8-pin plug, such as
a Jones plug, as described in Cookbook Appendix C.1 on page 160. Use two
pins for plugged-in indicator light. Pros: Eliminates possibility
of incorrect hookup of power supplies. Cons: None. Assessment:
Excellent safety feature.
- Intimate contact between thermal
surfaces. Smooth the cold finger surfaces to a high shine;
use small amount of heat sink grease; apply pressure to squeeze grease
to very thin layer. Pros: CCD runs colder; reduced noise and better
images. CCD should cool to -30C from +20C ambient. Cons: Potentially
very messy if you have to rework surfaces and reinstall the cold finger
and CCD. Assessment: Best to do it right initially. If after you
place it in service your CCD isn't getting cold, poor heat transfer may
be the culprit.
- Center the CCD:
In your hurry to reach First Light, don't forget to center the active area
of CCD in window by moving the cold finger on the Peltier device. It may
be necessary to groove the base of the cold finger by 1 or 2 millimeters
to provide sufficient lateral offset. Pros: Reduces vignetting.
Cons: May requires cut-and-try experimentation and additional handling
of the CCD chip. Assessment: Center the CCD if it would be more
than ~0.8 mm off center. Important: Mount the CCD on the cold finger as
described in the book. Do not attempt to center the CCD by moving the chip
along the top of the cold finger. Instead, loosen the screws that hold
down the cold finger and slide it and the Peltier until the CCD is centered
in the window.
- Heatsink VR3:
Attach heat sink to VR3 in interface box. This procedure is described in
Cookbook section 6.3.6 on page 69 but some builders have missed it. VR3
regulates the voltage for the clock signals. Note that the heat sink must
not touch the case. Pros: Prevents VR3 device from overheating.
Cons: None. But note that the heatsink must be insulated from the
box. Assessment: Do not overlook placing the heatsink on VR3.
- Eliminate Reset drift:
Stabilize temperature of transistor Q7. To accomplish this, seal the gaps
around the edges of the preamplifier cover with masking tape. Pros:
Greatly reduces drift in ref/reset levels. In conjunction with 245PLUS
drift subtract, drift problem 100% eliminated. Cons: Minor inconvenience
to make the change; the tape looks ugly. Assessment: This is an
extremely simple fix that is well worth doing.
- Perform the LDC modification.
Note that this modification can be switched on and off. Use accompanying
245PLUS software. Pros: Drops dark current by a factor of 10 or
more. Software eliminates bias drift. Improved chip clearing routine. Autoscaling
a big convenience. Integration times down to 1 millisecond. Significantly
improves camera's performance on deep-sky objects. Cons: LDC reduces
full-well capacity of CCD so blooming begins with less overexposure. Assessment:
Use LDC ON for low-light subjects; use LDC off for bright subjects and
shooting flat-fields.
Simple and safe modifications:
- Use a 12VDC submersible bilge pump in place the windshield washer pump.
Pros: Bilge pump is designed for continuous operation, and may be
quieter. Cons: Pump may require modification of the power supply.
Assessment: Good idea for portable and battery operated systems.
- Replace copper coil heat exchanger with open sump. Use container with
handle, such as a 5-gallon plastic Jerry can. Pros: Simpler system
and easier to maintain. Cons: May require more powerful pump to
prime open fluid loop. Assessment: Good idea for portable systems.
- Use large-capacity sump, such as 30-gallon plastic garbage can. Pros:
Improved temperature stability. Cons: Not portable. Assessment:
Good idea for CCD used in observatory setting.
- Use a 120VAC submersible pump such as Little Giant P-AAA to circulate
coolant. Pros: Silent operation, guaranteed strong flow of water,
highly reliable. Cons: Not portable. Assessment: Excellent
option for permanent setups.
- Use non-toxic automotive antifreeze in coolant loop. Pros: Necessary
to prevent coolant freezing in cold climates. Cons: None. Assessment:
Use isopropyl alcohol and water in mild climates; use non-toxic automotive
antifreeze in cold climates.
- Place quick disconnects in water lines. Pros: Easier setup and
takedown. Cons: Minor obstacle to flow. Assessment: Added
convenience is nice.
- Place opaque baffle just outside CCD window to reduce light scattered
from shiny aluminum surface. Thin black paper works well. Paint unthreaded
parts of camera body cap black. Pros: These measures reduce or eliminate
reflections into image area. Cons: If the baffle is too small, danger
of vignetting. Assessment: May offer a significant contrast gain
for very little effort.
- Obtain adapter for 35mm camera lenses, such as the one that University
Optics sells for $59.95. Available for most makes of cameras. Pros:
Greatly increases the versatility of the camera; allows wide-angle imaging
and daytime pictures of people, etc. Cons: Cost of adapter. Assessment:
Added versatility repays its cost many times over.
- Use silicone sealant instead of hot-melt glue to seal Peltier leads.
Pros: Tighter and longer-lasting seal. Cons: May be difficult
to remove old hot-melt glue; if so, leave it. Assessment: Reseal
Peltier leads with silicone if hot-melt seal becomes lose or fails.
For those with sufficient technical
know-how:
Important: Many of the modifcations that follow are intended to lower
the operating tempeature of the CCD chip. However, shot noise rather than
thermal noise is the limiting factor in the standard Cookbook CCD camera with
the LDC modification for most types of astronomical imaging. Cooling below
the nominal -30C operating point obtainable with a single-stage Peltier
and water cooling produces little improvement in performance.
- Construct power supply in smaller box, or use a compact commercial
supply, with specs per Cookbook Appendix C.7. Place all plugs and switches
on front of unit. Pros: Makes camera more compact and portable.
Cons: Undertake this only if you have adequate electronics background.
Assessment: Nice but not necessary.
- Substitute larger Peltier, such as Melcor CP 1.4-71-045L suggested
in Cookbook Appendix C.5 on page 162. Note that two-stage devices are also
available. Pros: Lower thermal noise from operating CCD at a lower
temperature. Cons: Larger power supply required. Supply must be
free of AC ripple. Assessment: Worthwhile providing you can build
the larger power supply. Elaborating a simple system may invoke law of
diminishing returns.
- In conjunction with the LDC modification, increase feedback resistor
R43 to 1.5 to 2.5x original value. This compensates for reduced dynamic
range when LDC is switched on. Pros: Makes more effective use of
12-bit data sampling, improves deep-sky imaging. Cons: Slightly
impairs camera performance on bright objects. With the Sipex ADC, large
values of resistors may cause excessive undershoot. This is not a problem
with the Analog Devices ADC. Assessment: An option for those who
image deep-sky objects as their primary observational activity. With the
Sipex, do not exceed 1.5x the original R43 value.
- Lengthen cable between interface box and computer. Pros: greater
flexibility in setup, placement of parts, observer comfort. Cons:
Possible signal attenuation or reflection. Assessment: Determine
if length desired works by trial and error. A few builders have reported
using cables up to 75 feet, but no guarantees. Most cameras seem to work
well with ~30 to 40 feet of ribbon cable.
- Lengthen cables between preamp and interface boxes. Pros: Greater
flexibility in mounting camera on telescope. Cons: Improper clocking
or noise pickup if cables are too long. Assessment: Limit these
cables to 5 feet as recommended in the Cookbook.
- Install connectors so that preamp and interface boxes can be separated.
Pros: Easier setup, take-down, and storage. Noise should not be
a problem. Cons: Possible electrical noise; opportunities for connector
failure. Potential for hot insertion problems. Assessment: Do only
if convenience is an important factor. Use high quality polarized and locking
connectors.
- Drill small hole in the cold finger and install temperature sensor,
or (less accurate alternative) glue temperature sensor to the side of the
cold finger as described in the Cookbook section 3.4.3 on page 39. Pros:
Allows user to monitor temperature of CCD. Allows later addition of temperature
control circuit. Cons: Additional leads to pass through camera body.
Assessment: Potentially useful but not necessary.
- Use longer, finer copper wire to hook up CCD inside camera body in
place of #30 wirewrap wire. Alternatively, use fine platinum, nichrome,
or constantan wire. Pros: Wires are principle conductive heat leak
to CCD; CCD will operate colder. Cons: Hard to find ultra-fine insulated
wire. Installing fine wire is difficult. Assessment: Potential gain
is small, but if the wire is available to you, should allow CCD to run
a bit cooler.
- Insulate the inside of camera body with closed-cell foam (Armalite).
Pros: Reduces convective heat load; CCD will operate colder. Cons:
Potential dust and volatile contamination inside camera body. Assessment:
Gain is small, but if foam is readily available to you, another step toward
running CCD colder.
- Fill camera body with non-condensing gas with low thermal conductivity,
such as argon, carbon dioxide, or sulphur hexafluoride. Pros: Reduces
convective heat load; allows colder CCD. Cons: Requires access to
gas and gas handling equipment. Assessment: Potential gain is too
small to be worth the effort.
- Evacuate camera body to 0.1 atmosphere or less. Pros: Reduces
convective heat load on CCD; allows colder CCD. Cons: Camera body
is not designed for vacuum; window and seals may fail. Body must be heavily
modified to hold vacumm. Assessment: Significant effort for relatively
small gain.
- Replace liquid coolant system with air-cooled fins. Ventilate with
12VDC fan. Pros: Camera system more compact, easier to transport
and operate in the field. Cons: Air cooling is less effective than
water cooling; lowest temperature is typically 10 degrees higher than water
cooling; lower thermal stability. Assessment: An option useful in
portable camera systems. Use with LDC ON to compensate for higher- temperature
CCD operation. The camera will be less stable with air cooling than it
is with a liquid coolant system.
- Substitute LT1037 opamp for LF356 apamp. Pros: LT1037 has better
specifications than LF356 and is inexpensive. Cons: Noise dominated
by CCD readout noise; LF356 contribution is negligable. Assessment:
This easy and inexpensive modification does not improve camera performance.
- Optimize charge transfer per Cookbook Appendix C.4 on page 161. Pros:
CCD operates better. Cons: Requires knowledge of CCD electronics.
Assessment: Do this if you have reason to suspect poor charge transfer
efficiency in your CCD.
- Add shutter to front of camera. Pros: Necessary for imaging
very bright objects such as planets or daytime scenics at low focal ratios.
Cons: No reliable source for inexpensive shutters; current software
does not support shutter without hardware mods. Assessment: Useful
but required for specialized observing projects only.
- Upgrade to TC245-30 or TC245-20 instead of standard TC245-40. Pros:
TI rates -30 and -20 as having fewer defects of smaller amplitude. Cons:
Standard reduction procedures in astronomy already remove image defects
from -40 rated CCDs. Doubles or triples the cost of the CCD. Assessment:
TC245-20 and -30 grade chips have not been shown to produce better astronomical
images.
- If you live in a humid climate, add a heater to the window in the CCD
housing. The heater can be made from a loop of nichrome wired that is glued
around the window on the inside of the housing and powered by two "D"
cells. Pros: Heats window slightly and eliminates dew forming on
outside of window. Cons: Adds to complexity of camera body. Assessment:
Useful for observer in exceptionally humid climate areas.
- Replace P1 with precision 10-turn potentiometer. Optionally locate
it in the interface box. (Do this when installing LDC option; use 6-conductor
ribbon cable to carry additional three lines.) Pros: Pot is handy
and easy to adjust. Cons: It is not normally necessary to adjust
P1 constantly. Increased opportunity for noise pickup. Assessment:
Gain in camera functionality is small.
- Replace thermal grease with silver-filled epoxy. Pros: High
thermal conductivity allows CCD to reach as much as 8C lower temperature.
Cons: Epoxy bond is permanent; CCD may become difficult to service;
danger of short circuits. Assessment: Attractive option for builders
familiar with electronic sensor design. Requires good surface prep on the
CCD and the cold finger. The adhesive bond line should be around 0.002
inches.
This list Cookbook CCD camera modifcations is maintained actively. If useful
new modifications come to our attention, we will add them to this list.
We welcome suggestions for modifications from Cookbook CCD camera builders.
If at all possible, document the performance of your camera before and
after any modification so the potential improvement can be quantified.
Richard Berry operates The Cookbook CCD camera Home Page in support
of amateur astronomers who build their own CCD cameras.
Return to the Text DataBase Page
Return to the Cookbook Home Page