Choosing 1st OSC Cam...HELP!.

[rnobleeddy]

3 minutes ago, Stuart1971 said:

Why can’t you use darks with a DSLR, I did all the time...the only issue is you need to take on the night of imaging after or before the lights....but it works extremely well....same sub length and will be same temp too, which is the important bit... 👍🏼😀

In fact with the canon DSLR I had it could be set to automatically take a dark of the same sub length after each light frame, this obviously doubled your imaging time if you did after every image, so I just tended to do at the end in one go...20 or so...

Whereas with a dedicated Astro cam you can build a library at any time with different exposure lengths and temps...

Yeah, I get that it's possible, I just can't justify losing half of my imaging time. I'd also never been particularly sure that taking them all at the end works as you could easily have a 5 degree temp swing. 

Perhaps I'm impatient as I'm new to this, but if you add up the cost of kit and compare it to the amount of imaging time I get (between needing to work/sleep, but mainly clouds) then I'd actually argue that a cooled camera is well worth the investment to double one's imaging time! 

On the flip side, I found that once I have more than 20 or 30 subs, dithering works a treat. Again, I've got lot to learn, but this certainly feels like an area where more subs also improves matters. Most people appear to recommend dithering over darks, but clearly, if you have time, darks + dithering would be best.

 

 

[vlaiv]

1 hour ago, rnobleeddy said:

Thanks for the detailed reply! What's more, it makes sense. My mistake was assuming the thing I'm imaging had a signal far above the background sky - of course, that's because of what I see in the final stretched image, but I'd do better to assume it's the same level as the background sky for these purposes.

In most cases - target is far fainter than background sky. For example, outer parts of brighter galaxies are around 26-27 mag / arc second squared. If you are lucky, your sky is 21.8mag to mag22 and in most cases these days it is below mag21. Yes, that is around 5 mags of difference in some cases - or target brightness is only 1% of sky brightness (mag5 = x100)!

I image from mag18.5 skies - very high light pollution, border of red and white zone. I plan to move to mag20.8 skies and that will be major improvement for my imaging.

Even if target brightness is so low - we can still pull enough SNR with long exposure - simply because shot noise grows as square root of signal. Both target and LP signal grow linearly with time - it's just that noise grows as square root of that and image for enough and target signal will overpower LP noise even in worst conditions.

There was, at one time, belief held by many that you can't record faint signal that is below noise level - or that you can't image in heavy LP - you can do both - provided that you spend enough time on the target.

If you want to roughly asses magnitude of the target - use Stellarium. It gives average brightness per arc minute squared as surface brightness. That is only average but good indication.

Oh, look at that - it has been changed - it now shows in per arc seconds squared (it used to be per arc minute squared - but conversion is straight forward, one just subtract a constant - 8.89 or something like that - can be calculated - yep it is that one and value is obtained by 2.5 * log(3600) - ratio of surface of arc second and arc minute squared)

That is like I said only average value - 21.45 as core is much brighter than faint outer arms. I once did rough magnitude map of M51 - let me see if I can find that.

Well - you get the sense - cores are at mag17 and fainter parts around mag25-26

[alacant]

2 hours ago, rnobleeddy said:

Canon 450D

Yeah. You need a recent release. Anything with the 18mp sensor is good.

Cheers.

[vlaiv]

1 hour ago, Stuart1971 said:

Why can’t you use darks with a DSLR, I did all the time...the only issue is you need to take on the night of imaging after or before the lights....but it works extremely well....same sub length and will be same temp too, which is the important bit... 👍🏼😀

In fact with the canon DSLR I had it could be set to automatically take a dark of the same sub length after each light frame, this obviously doubled your imaging time if you did after every image, so I just tended to do at the end in one go...20 or so...

Whereas with a dedicated Astro cam you can build a library at any time with different exposure lengths and temps...

Of course you can and you should.

Unfortunately these are not very reliable as even small temperature difference will cause significant signal change. Sensors have dark current doubling temperature of about 6 degrees. This means that your dark current grows double for every 6C° increase in temperature.

It is power law so we can see how much it raises in for example 2C° change - that is not major change and will happen between start and end of imaging session (for those that shoot darks at the end to try to match temperature) and certainly will happen between two days.

Change is actually 26% in dark current.

You'll be leaving about 1/4 of dark current in if you shoot your darks at 2C° lower ambient temperature.

Why is this important? Because it creates problems with flat calibration later on.

I recommend a few things to try to fix this:

Shoot half of your darks before imaging run and half of them after. If temperature change is fairly linear (and it usually is unless cold front swoops in and temperature suddenly drops) - you have fair chance that things will average out.

If your sensor has stable and usable bias (and most DSLRs have it as far as I understand) - use dark optimization. It is algorithm designed to scale your master dark to compensate for any temperature changes.

 

[paul mc c]

On 19/12/2020 at 09:46, alacant said:

Hi

What are you looking for in the upgrade which your dslr doesn't provide?

I've often been urged to dump my dslr for something 'better', but I've not yet seen anything which comes close. I once tried an asi294 and apart from it's smaller sensor (and you really notice the change even if it's only to 3/4) it was hard to distinguish its frames from the dslr. Anything APS-c is gonna cost you €silly! 

Just my €0,02. Cheers

If any one can give me a reason to keep using the dslr its your self.....your images are brilliant.Just hear so much about dedicated cams it makes you thing you need one.lol

[paul mc c]

Hi all,some great replys to read,but unfortunately most of it is beyond my tiny brain....making me thing that maybe i am not ready for a dedicated camera.

Probably should try harder with the dslr until i can completely understand this AP milarkey..lol

Paul