Atik 383 bias and dark masters

[Uranium235]

As a user of a 383 for about three years, I'd say there is nothing wrong with your darks. In fact, I never use them! A fair few of my hot pixels turn out the same way, but thats only noticeable when you severely stretch the image - otherwise they would look like points. I have no technical explanation as to why, but if you take enough subs you wont need darks becuase sigma clipping or SD mask stacking will eradicate them for you. Another useful tool (if you dont have enough subs for sigma) is the kernel filter, or bad pixel map in Maxim DL does a really good job.

A good place to start is to cool your camera as far as conditions allow, during the winter months I cool to -25 or -30c which does a good job of reducing the hot pixel count.

As for bias, take those at the same temp as your lights - same goes for flats (in cooled mode). However, the ideal 383 minimum flat length for me is 10s... as long as its enough to saturate to a maximum (not average) level of 20,000. I've recently found that going for an average value will lead to overcorrection, so best to ignore the Artemis histogram (and its white/black point levels) and assess the flat in another software package.

[gnomus]

Listen to Ironman. There ain't nothin' wrong with your darks IMHO.

[Freddie]

That's interesting then as it looks like the Kodak chip tends to have a slightly less bright pixel to the right of most hot pixels. Don't ever look to be on any other side. This then shows up as the slight trail off to the right. I wonder what causes that? A characteristic of the Kodak chip I guess as the sony tends to just have individual pixels with no "trailing".

[alcol620]

Thanks everyone for your inputs, much appreciated. It would be interesting to hear from other Atik 383 users to see if this issue crops up regularly.

Thanks for the reassurance Rob & gnomus. I am using APT to control the camera and I haven't got round to the issue of flats yet. I have quite a bright home made light box and I think I will have to turn the brightness down a bit bearing the mind the requirement to have relatively long exposures to cater for the 383 shutter!

Regards

Alec

[alcol620]

Just one thing I would like feedback on. The four images I posted suggest to me that the "noisier" two are those where the bias has not been subtracted??? Or am I misreading what the images are saying?

[FLO]

Oh dear, does that mean there is a problem?

Yes, unfortunately your camera is faulty. Is it one of ours? 

Steve

[gnomus]

Steve

I assume that must mean that mine is faulty too (see a few posts above). And, yes, it is one of yours. Please advise me what I should do.

Steve

[FLO]

Please advise me what I should do.

I can't tell who you are from your username but if you email us we will arrange a DHL collection then ask Atik to check it over

It is not a common fault (I think it was about four years ago when I last saw it). Perhaps a voltage thing or part of the board is failing. Best that we have it back, just to be sure. 

HTH, 

Steve

[Freddie]

Ah, an interesting development !!!!

[alcol620]

Unfortunately I recently bought it second hand and have just started to use it taking darks and bias frames. What are the options Steve, apart from getting back to the vendor?

Alec

[FLO]

Unfortunately I recently bought it second hand and have just started to use it taking darks and bias frames. What are the options Steve, apart from getting back to the vendor?

If the vendor won't help then you will need to contact Atik Cameras to arrange a service/repair. Fortunately their service centre is based here in the UK so it should be quite painless. 

HTH, 

Steve

[ollypenrice]

A side issue but why subtract bias from darks?

Olly

[alcol620]

Hi Olly

So confusing all this. Having read numerous articles which suggest the master bias gets subtracted from individual darks to calibrate them before integrating (stacking) them!!!!! Is this not correct?

Alec

[gnomus]

I can't tell who you are from your username but if you email us we will arrange a DHL collection then ask Atik to check it over

It is not a common fault (I think it was about four years ago when I last saw it). Perhaps a voltage thing or part of the board is failing. Best that we have it back, just to be sure. 

HTH, 

Steve

Thanks Steve. I have sent an e-mail. My camera was bought nearer 4 months ago than years!

Good service bringing this to our attention, by the way. Thanks very much.

Steve

[gnomus]

A side issue but why subtract bias from darks?

Olly

I hesitate to weigh in here, but if you subtract bias from the light frame and then subtract a dark (which also has bias signal in it) from the light frame, haven't you subtracted bias twice?

I anticipate being put firmly in my place by our Sensei.... ;-)

[ollypenrice]

The dark does contain the bias and it's important not to double subtract it. However, the way to avoid this is not to subtract a 'bias-only' frame at all. Some stacking routines offer you the chance to put a bias into the calibration mix in order to scale the darks. This is intended to compensate for temperature variation but set-point CCDs don't need this.

If using classic darks (which I no longer do) my only use for a master bias is as a dark for flats.

I calibrate in AstroArt 5. One thing I like about it is that I can control exactly which caibration files go where and do what. I don't have to worry about the inadvertent double subtraction of the bias. I'm sure you can read up on what the different softwares do with your calibration files but in AA5 it's all there laid out neatly in front of you.

I've never seen this 'creeping hot pixel' phenomenon before.

Olly

[gnomus]

The dark does contain the bias and it's important not to double subtract it. However, the way to avoid this is not to subtract a 'bias-only' frame at all. Some stacking routines offer you the chance to put a bias into the calibration mix in order to scale the darks. This is intended to compensate for temperature variation but set-point CCDs don't need this.

If using classic darks (which I no longer do) my only use for a master bias is as a dark for flats.

I calibrate in AstroArt 5. One thing I like about it is that I can control exactly which caibration files go where and do what. I don't have to worry about the inadvertent double subtraction of the bias. I'm sure you can read up on what the different softwares do with your calibration files but in AA5 it's all there laid out neatly in front of you.

I've never seen this 'creeping hot pixel' phenomenon before.

Olly

Yes. I'm not having much luck with my equipment at the moment....

I'm pretty sure that PixInsight wants a 'Master Bias' (or SuperBias), a Master Dark and a Master Flat. The dark has the bias subtracted and the flat has both bias and an 'optimized' dark subtracted. All three masters are then used to calibrate the lights.

I had understood, from here and elsewhere, that there was an advantage to doing 100 or even 200 biases. Typically, I only take 20-25 darks.

I could be wrong about this, of course. I do the preprocessing manually in PI following a tutorial on the Light Vortex site. It doesn't take very long. If there is a better way, of course, I'd be very interested to hear about it. I probably haven't spent enough money this year....

[alcol620]

 Hi gnomus

That's the same procedure that I use in Pixinsight.

on a slightly separate issue. I managed to get some LRGB subs last night and following analysis they have a number of dust motes or whatever on them. See attached. As they are similar on all the filters. I assume that they are on the camera sensor rather than anywhere else. Would this be correct? They didn't appear when I used my DSLR with the same optic train.

Thanks for feedback

Alec

[perfrej]

Why don't you all try without darks? I haven't used a single dark frame for years. Instead, I use 50 BIAS-frames sigma-stacked into a master BIAS and flats. That's it.

/per

[alcol620]

perfrej

good point, will give it a try. As I am dithering using APT and PHD software this probably gets rid of most of the problems of hot pixels?

Alec

[ollypenrice]

 Hi gnomus

That's the same procedure that I use in Pixinsight.

on a slightly separate issue. I managed to get some LRGB subs last night and following analysis they have a number of dust motes or whatever on them. See attached. As they are similar on all the filters. I assume that they are on the camera sensor rather than anywhere else. Would this be correct? They didn't appear when I used my DSLR with the same optic train.

Thanks for feedback

Alec

Ouch, those are not nice. They must be very close to the sensor to be so dark and hard-edged. Which camera was this? Modded DSLRs, in particular, seem highly susceptible to this, having been opened up and messed about with. (I'll probably get shot at for this remark but I'll stick with it because I think it's true.)

Olly

[alcol620]

Olly

The camera is a Atik 383. I have sent the image to Atik support for their comments.

[Uranium235]

Its a surprise development in that the camera may be faulty. I've been looking back through a few random projects, and while I have a few smeared hot pixels (pretty sure I'd seen them before, usually when binning), admittedly its nowhere near as many as you have (seems to be on every one). Wait and see what Atik say about it, theyre pretty good.

As for the dark spots on the sensor, that could either be dust, or condensation spots. Both fixable by 1) cleaning the camera window, or 2) removing the desiccant tablets and baking them at 180c for 90-120min - pop them back in and wait 48 hours before you use the camera.

A handy tip with the 383, if you have a condensation spot that is refusing to die quickly - set the camera to 1x1bin and set artemis to loop with a 1second sub. The action of reading the chip out repeatedly will warm it slightly and get rid of the spot more quickly.

[alcol620]

Hi Rob

When you say on the camera window, is this the glass that protects the chip or the chip itself?

Regards

Alec

[gnomus]

Those black dots look pretty horrible. They are so dark and well defined that I wonder if they might be on the sensor itself. As the camera is going back to Atik, I'd definately mention them and get them to sort it for you.

Steve