QHY 5L-IIc dark frame issue/question

[Ant-33]

Hi Folks,

I recently bought a new QHY 5L-IIc, and I've noticed that, even with the black rubber cap covering the sensor end, the exposure looks to be showing a large number of scattered bright pixels. I'm hoping the attached image will show up in the post. This image is as seen through the viewing panel in PHD2, set for 2sec exposures. If I change the exposure or gain, I get the same field of bright pixels, but with varying degrees of noise in the background. I then tried in Sharpcap, and saw a very similar image.

Is this normal? Am I confusing myself about what I should see? I would have expected a substantially black frame with, perhaps, the occasional bright pixel that can be compensated using a dark-frame. What I see here looks more like a star-field - there are loads of bright pixels.

Any thoughts or suggestions would be appreciated.

Many thanks - Tony

[Astroboffin]

That is normal, they are what's called hot pixels, and by taking a dark to start with it will remove them for you, before starting your guiding you click on the take dark button, and the software will automatically subtract this from the subsequent images so removing any artefacts, such as hot pixels.

hot pixels will show on any CCD or CMOS camera, even the most expensive CCD cameras have a few, even when cooled to -30....

its nothing to worry about and completely normal, I had the same camera but the mono version for guiding and my dark looked just the same.

hop that puts your mind at rest

AB

[Ant-33]

8 minutes ago, Astroboffin said:

That is normal, they are what's called hot pixels, and by taking a dark to start with it will remove them for you, before starting your guiding you click on the take dark button, and the software will automatically subtract this from the subsequent images so removing any artefacts, such as hot pixels.

hot pixels will show on any CCD or CMOS camera, even the most expensive CCD cameras have a few, even when cooled to -30....

its nothing to worry about and completely normal, I had the same camera but the mono version for guiding and my dark looked just the same.

hop that puts your mind at rest

AB

Thanks, AB, that's really reassuring. I had expected to see some hot pixels, but I guess I was taken aback by just how many there are.

Cheers

Tony

[Thalestris24]

Hi

PHD2 autostretches the on-screen image. You might find it better to make and use a bad pixel map rather than a dark library. With the mono version I find the bad pixel map works best for me

Louise

[Ant-33]

2 minutes ago, Thalestris24 said:

Hi

PHD2 autostretches the on-screen image. You might find it better to make and use a bad pixel map rather than a dark library. With the mono version I find the bad pixel map works best for me

Louise

Interesting, Louise, thank you. I've just looked at PHD2 and I see the bad-pixel map option there. Do I use darkframes as well, or does this work as an alternative?

Cheers

Tony

[Thalestris24]

25 minutes ago, Ant-33 said:

Interesting, Louise, thank you. I've just looked at PHD2 and I see the bad-pixel map option there. Do I use darkframes as well, or does this work as an alternative?

Cheers

Tony

Hi

It's one or the other. However, you can create both and see which works best for you.

Louise

[Ant-33]

Many thanks, Louise.

So I've created a bad pixel map in PHD2 using the default parameters - 25 15s exposures and aggressiveness of 75 for hot and cold pixels. Once I've clicked the 'Generate' button it says 'New bad pixel map being used'. I've also set up a dark library - also using the default parameters (1s to 6s, five of each).

Sounds good - All done, I guess, and ready to start guiding  so I can go on and compare the performance with using dark-frames and dark library.

So... are the default parameters good enough?

Also, I can't see an option to view the dark pixel map or dark frames - am I missing it or is there not one?

Many thanks for your help

Cheers, Tony

[Thalestris24]

8 hours ago, Ant-33 said:

Many thanks, Louise.

So I've created a bad pixel map in PHD2 using the default parameters - 25 15s exposures and aggressiveness of 75 for hot and cold pixels. Once I've clicked the 'Generate' button it says 'New bad pixel map being used'. I've also set up a dark library - also using the default parameters (1s to 6s, five of each).

Sounds good - All done, I guess, and ready to start guiding  so I can go on and compare the performance with using dark-frames and dark library.

So... are the default parameters good enough?

Also, I can't see an option to view the dark pixel map or dark frames - am I missing it or is there not one?

Many thanks for your help

Cheers, Tony

Hi

I've never tried to view the darks or the pixel map - can't think of any reason to really. The defaults should be fine - it's up to you to judge (see PHD2 guide). They are only to help give a cleaner image and prevent you from trying to guide on a hot pixel.

Louise

[Ant-33]

8 hours ago, Thalestris24 said:

Hi

I've never tried to view the darks or the pixel map - can't think of any reason to really. The defaults should be fine - it's up to you to judge (see PHD2 guide). They are only to help give a cleaner image and prevent you from trying to guide on a hot pixel.

Louise

Thanks, Louise,

I think I'm all set, then. A few things have to come together before I can try this out for real, guiding my scope - not least of all the weather.

Many thanks for all your help

Cheers, Tony