Digital Thermometers for Darks?

[dugnsuz]

Hello all...
Looking to sort out a darks library in near future.
I need a digital thermometer to read air temp to get accuracy in sorting out a matched set of darks.
Found this (very cheap - $2.71AU)) unit on ebay...
http://cgi.ebay.com.au/Digital-LCD-Thermometer-Refrigerator-Freezer-Fridge-/250521851149?cmd=ViewItem&pt=AU_Gadgets&hash=item3a5444150d

Would that be OK to get accurate air temp readings?
Or can anyone recommend something else - I can go up to $3:lol:
Cheers
Doug:thumbsup:

[Tandum]

3 Dollars :)

Jaycar (http://www.jaycar.com.au/productResults.asp?whichpage=1&pagesize=10&keywords=thermometer&form=KEYWORD) have a few different ones. I was looking at one that showed air temp plus temp from an attached probe that you could stick on a tube and they had them in large or small LCD sizes. It was more than 3 dollars :)

[dugnsuz]

Cheers Robin, I'll check out the Jaycar site too
Doug

[Tandum]

I added a link.

[dugnsuz]

Ta Robin - the "Mini Indoor Thermometer" looks like the same piece of c#@p as the eBay unit (roughly), but $12 cheaper!:P

[kinetic]

Doug,

I have one of these Jaycar ones...it has dual inputs so you can read
both ambient and sensor temps if you want. (meter on left)

I use it in my home cooled DSI II here:
http://www.iceinspace.com.au/forum/showthread.php?t=52855 (http://www.iceinspace.com.au/forum/showthread.php?t=52855&highlight=cooling+dsi)

[dugnsuz]

Cheers Steve - don't need anything too sophisticated.
Sensor in the DSLR will have to be assumed to be roughly close to air temp for purposes of acquiring a darks library. So, air temp is all that's needed.
Cheers
Doug

[rally]

Doug,

Wouldn't the CCD temperature be higher than ambient air temperature depending on the length of exposures and the frequency of images taken ?

On my DSLR there is a noticeable increase in temperature if I take lots of images and long exposures.
Also the temperature effects are much worse in the corner where all the readouts occur.
Whereas with a cooled CCD the excess heat is being removed constantly and you dont get the same variation across a CCD.

Thus I would have thought that a laser thermometer would work better - more awkward but at least its reading the temperature off the front surface of the CCD (with shutter open)
Not sure how you manage an imaging session though.

Cheers

Rally

[pmrid]

This one will blow your $3 budget but it does have some plus's - http://www.dealextreme.com/details.dx/sku.7003

Peter

[bmitchell82]

rally your dead right with that one, though i wouldn't want to be shining a laser onto my Cmos sensor!!!! As for taking shots, i dunno about most people but i take mine continuously and generally at 10 min exposures all controlled by PC, as for the Cmos being hotter than ambient yes thats right, but its a function of ambient temp alone. as the amb goes up so does the sensor, so as long as your shooting methods don't change drastically, ambient is just fine.!

[Octane]

Doug,

Just my opinion, but, building dark libraries for DSLR images is a no go.

DSLR sensors aren't as "reliable" as dedicated cooled CCDs. In that they're a lot more susceptible to random artifacting and phenomena such as stuck, hot and/or cold pixels developing over time.

Up to you in the end, but, I'm a bit anal about this and would prefer to do my darks at the time of shooting (before, during and after and median combine the lot for a very close average of what your sensor experienced on that night).

No two nights are going to exhibit exactly the same conditions as far as climate/weather goes.

I guess it just depends on how accurate you want to be. Bear in mind, I know how much of a pain in the rear doing darks is.

H

[dugnsuz]

How many darks do you take during a session?

I imaged last night and took 16x5min shots all with ICNR.
Obviously without ICNR I could have doubled my number.

So, how many darks would one need to take over the imaging session above (for example) to effectively achieve similar noise reduction to the ICNR method?

Doug:thumbsup:

[allan gould]

Have to agree with H on this one. When I used a DSLR, I never used ICNR but prefered to take my darks while I was packing up. Just left the camera with the timer set for 10 x N min and let it do its thing. After packing everything away, most of the darks had been accumulated.
But thats what worked for me.

[Exfso]

Deals extreme looks like a Hong Kong company as well, so you would have freight issues. I noticed the 200mw red laser for less than $30, imagine trying to get that into Australia.

[Phil Hart]

i've found a lot of thermometers to be pretty unreliable under typical astro conditions.. once the dew/frost kicks in they often show temperatures that are clearly way lower than real ambient temp.

i'd mount your thermometer under your observing table or something similar so that it stays dry (less dew) but still gives you a good reading of 'ambient'.

there's quite an art to getting an accurate measure even of something that seems as simple as ambient temp.. a stevenson screen from a weather station would be a good start ;)

[Moon]

Peter,

I've never had any problems buying stuff from DX. Shipping has always been free. In fact I already have one of those TEMPer things from them. It's also a good source for Laptop PC cards (e.g. Serial and USB), HDMI cables and Flashlights etc.

James

[Phil Hart]

if you get it right, you should only need one dark frame to match the ICNR result.

this calculator is designed for CCDs, but with a bit of thinking you should be able to work out some DSLR numbers as well:

http://www.ccdware.com/resources/subexposure.cfm

once you start racking up a half decent number of dark frames, the % noise you are adding to the image will get low pretty quick. if you're chasing perfection, then you'll need a lot to get down to less than 0.5% but it will get very hard to notice the difference.

but we're losing sight of the forest for the trees.. doubling your real exposure time by not using ICNR is the biggest benefit!

[Phil Hart]

talking about forests and trees..

dark frames can only ever reduce the pattern noise in an image. all the random noise cannot be helped.. the reason you need a lot of dark frames is to subtract the noise that has some pattern/repeatability to it without making the random noise significantly worse. hope that makes sense..

[mswhin63]

Totally agree with this, it is all relative, CMOS temp will always follow the outside temp. If you try specialise cooling methods then you will only need to measure sensor temp and not the ambient.

[Peter Ward]

Please excuse my inquiring mind but....

How does ambient air temperature correlate to:

CMOS chip temperature
DSLR body temperature
Exposure time
changes in all of the above

Is the correlation linear or non-linear?
If non linear...how will you correct for this?

[dugnsuz]

Not too sure of the accuracy of ambient compared to CMOS/DSLR body, but stopping short of pulling the camera apart to install a temp sensor I thought that this would be the closest approximation I could get for a starting point for a darks library.
An idea shelved now anyhoo - moving on!:lol:
Doug

[mswhin63]

It would be difficult to know whether it is linear or non but it will be proportional. The main condition is that if the ambient temp for example is 20deg the CMOS sensor maybe 25deg or whatever. You can create library of dark at ambient. The same condition would apply at any given day or time if the ambient temp was 20deg the sensor would be the same as the last time so you would use the same darks in your library.

[TrevorW]

Dougie in all seriousness I wouldn't bother if you read Jim Solomons Cookbook re DSLR imaging it'll give a good insight into dark etc's required when using a DSLR