Exposure Settings

Hi recently got a Canon EOS 10D so I could start taking better astrophotograghs. For about a week it went well and I managed to pull some awesome moon and star shots in the jpeg format. After awhile I noticed a strange red spot on my photos I did some research on it and I found out that it was a bad pixel. I took several shots at different isos and shutter duration and the spot either stayed there or was joined by two other spots. I tried cleaning the sensor and it didn't help.Even during day shooting and closeups of my cats the only way to correct the picture was to shoot in RAW and let photoshop fix it.So I shelved the camera since I bought it used and got a still in box 20D that had all the documentation and support disks.

Now on the 20D I'm noticing two small grey spots. I switched lenses between my EF-S 18-55 and Ef 28-90 and the spots where still there.

My question is should I start just shooting in the RAW format or is this a correctable issue?

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It's really hard to tell, as these shots are so severely underexposed. When an image is that underexposed, you're likely to see all sorts of weird things.

Do these show up on properly exposed images as well?


My first guess would be dirt on the sensor.

Take a well exposed photo of a light colored wall, ISO100 F22 Av mode, EC=1. Make it a 2 second exposure, and move the camera around slightly during the exposure so that spots on the walls aren't confused with spots on the sensor.

Hot pixels will show up whether you shoot Raw or jpeg. Typically it's just a few that you can do spot removal on.

But there is an "urban legend" about a way that some people have used and have claimed success in "mapping out" hot pixels. It is to put a lens cap on or to cap the camera without the lens and then turn manual sensor cleaning on (not the vibration cleaning but the actual flipping up of the mirror). Leave it on for a minute or two then turn the camera off to close the sensor cleaning mode. You could repeat it if you wish.

Like I said this is "legend" and may do nothing, but it's fun to try.

tim wrote in post #12355375
Take a well exposed photo of a light colored wall, ISO100 F22 Av mode, EC=1. Make it a 2 second exposure, and move the camera around slightly during the exposure so that spots on the walls aren't confused with spots on the sensor.

Thank the 20D is fine now but I can still spot errors with the 10D.

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Those are hot pixels and will be normal to see a few of them as well as some blue or white cold or dead pixels.

For normal every day terrestrial photography this usually isn't a problems since they just blend in. However, in astrophotography they are easily noticed against the dark sky background.
This is one reason we shoot dark frames. Shooting dark frames not only helps with noise but will also remove any hot cold or dead pixels. If you stack your image files using Deep Sky Stacker you can adjust the Cosmetic settings to detect and remove hot and cold pixels even more in the even you're darks didn't remove them all.

There is a trick that may or may not work. Some people have had success with it and other not so much. If I remember correctly you want to take a high ISO long exposure of at least 30 seconds. Then with the dust cap in place turn on the Manual Sensor Cleaning for the same amount of time.
Like I said however, it may or may not work.

You can try long exposure noise reduction, a function in the camera, not sure if the 10D has it or not. Be warned though that if you take a 30 minute exposure it will take another 30 minute exposure with the shutter closed right after your first exposure.

Thanks everyone for all your assistance. I'll just start shooting in raw from now on so that Photoshop will auto-correct any errors.

tim wrote in post #12355671
You can try long exposure noise reduction, a function in the camera, not sure if the 10D has it or not. Be warned though that if you take a 30 minute exposure it will take another 30 minute exposure with the shutter closed right after your first exposure.

You don't want to use LENR for astrophotography. Not only does it use up valuable time that could otherwise be spent gathering more image data, and time is often critical, but it can also remove data that you actually want such as small stars, or small faint celestial objects.
Shooting Dark frames to be extracted in the calibration and stacking process is the best option for this purpose. Some people might agree that it is the only real option. LENR is out of the question.

Catstar wrote in post #12355867
Thanks everyone for all your assistance. I'll just start shooting in raw from now on so that Photoshop will auto-correct any errors.

Huh???? Arggg!

Shooting Raw is the best and possibly the only right way to shoot astroimages. But shooting Raw with the idea that Photoshop is somehow going to auto-correct errors is just wrong. Sure there are some automated adjustments, but it will not automatically correct sensor errors like cold or hot pixels. For that you will have to manually find them and use the Spot Healing Brush. Or like I already described shoot darks to be subtracted in the image calibration and stacking process. If you've been shooting jpeg, some of those spots could also just be artifacts.

How much experience do you have with Astrophotgraphy? and how much with Photoshop?

tkerr wrote in post #12355923
You don't want to use LENR for astrophotography. Not only does it use up valuable time that could otherwise be spent gathering more image data, and time is often critical, but it can also remove data that you actually want such as small stars, or small faint celestial objects.
Shooting Dark frames to be extracted in the calibration and stacking process is the best option for this purpose. Some people might agree that it is the only real option. LENR is out of the question.

Good to know, I know nothing about the area

How much experience do you have with Astrophotgraphy? and how much with Photoshop?[/QUOTE]


I've been reading E-books on it and I've been only taking photos when there is a full moon. The red dots prevented me from getting a telescope and adapter. The moon and cat photo here were one of the first shots I took the moon is jpeg and the cat was done in raw. To do the cat I followed a Ehow guide.

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Catstar wrote in post #12356467
How much experience do you have with Astrophotgraphy? and how much with Photoshop?


I've been reading E-books on it and I've been only taking photos when there is a full moon. The red dots prevented me from getting a telescope and adapter. The moon and cat photo here were one of the first shots I took the moon is jpeg and the cat was done in raw. To do the cat I followed a Ehow guide.

Ahh, so you're just starting to get your feet wet with astrophotography. There is a lot to learn and it ain't going to happen in a few days. In fact at the rate things keep changing I don't think we can ever learn it all.

You probably have plenty to read if you've been downloading e-books, but if you ever want more I have put some information together in pdf format available for download at Scribd. Mine are just the basics without too much technical mumbo-jumbo. For that I have included references to books and other web links.

Introduction to Astronomy Photography
http://www.scribd.com …y=key-ro5p2f3hwwh4f204syn

Introduction to DSLR Astrophotography
http://www.scribd.com …y=key-gnoflczzrljkxl1qzzd

Thanks for the links I will add them to my reading.