ASI533MC Pro Gain and Offset settings

[Todo43 (Lachlan)]

Hey everyone,

I have big plans for tonight with new gear and dark skies. I have seen a couple of people in the past few weeks in their image details, they have their camera settings that say their camera and then "set to highest dynamic range". This made me wonder, with my ASI 533MC Pro, and my plans for the horsehead nebula, should I be using "highest dynamic range", "unity gain" or "lowest read noise"?

I plan on imaging the horsehead nebula, so quite a faint target, in about B2/3. It is a pretty slow telescope at f/6.67 and my recent images of the horsehead have been extremely noisy. These previous attempts have only had about 30-60mins of data however. I am aiming for approximately 4-5 hours tonight at 300s exposures.

I have a limited understanding of these such camera specs such as gain and offset which is why I would love some input. Thanks so much everyone!


Attached is an example of the noise I am getting in my images. I take flats after every session, and have a library of darks and biases all at my normal temp of -10c

[Startrek (Martin)]

I have the 2600MC which has similar performance specs to the 533MC ( 2600MC has slightly less read noise at Gain 0 ) I’ve imaged using both Gain 0 ( highest dynamic range) and Gain 100 ( lowest read noise ) and found in most cases Gain 0 HDR works best for me for both Broadband and Narrowband. Gain 100 does have a lower read noise but doesn’t take advantage of close to full well capacity.
Using Gain 0 does require a lot more data and longer subs as your taking advantage of full well capacity ( nearly 50k )
Also with the 2600MC there’s no need to take Darks as the dark current when camera is cooled to -10C is negligible ( I just take Flats and calibrate them with Bias frames ) I don’t know what the dark current is for the 533MC at say -10c ??
In regard to imaging the Horsehead , capture plenty of good data ( at least 3 to 4 hours or more ) , try Gain 0 highest dynamic range, cool your camera to -10 C , take at least 3 to 5 minute subs and most importantly “dither” each sub to eliminate fixed pattern noise
You do have a slower scope which doesn’t help on this object but plenty of data should fix that
Good luck
Cheers
Martin

[Todo43 (Lachlan)]

Thanks Martin,

I have been getting 5min subs which are great so I will be able to do that. Have just received a ZWO EAF so I will be able to get many hours on the horsehead without needing to get up and refocus.

Should I dither every frame? I have found that dithering every 3 frames works pretty well. I haven't had any walking noise at all and my darks seem to subtract any hot pixels extremely well.

I have been looking into a reducer and am thinking of the Bintel 0.8 reducer for short telescopes but am doubtful of the quality. I also have a flattener already installed. If I got a reducer, would I use both the flattener and reducer at the same time or would I only be able to use one of them at the one time? 0.8 reducer would get me down to 480mm focal length and f/5.3 so a decent reduction in f ratio.
Thanks
Lachlan

[Startrek (Martin)]

Originally Posted by Todo43

Thanks Martin,

I have been getting 5min subs which are great so I will be able to do that. Have just received a ZWO EAF so I will be able to get many hours on the horsehead without needing to get up and refocus.

Should I dither every frame? I have found that dithering every 3 frames works pretty well. I haven't had any walking noise at all and my darks seem to subtract any hot pixels extremely well.

I have been looking into a reducer and am thinking of the Bintel 0.8 reducer for short telescopes but am doubtful of the quality. I also have a flattener already installed. If I got a reducer, would I use both the flattener and reducer at the same time or would I only be able to use one of them at the one time? 0.8 reducer would get me down to 480mm focal length and f/5.3 so a decent reduction in f ratio.
Thanks
Lachlan

I dither each sub as it only requires a settle time of 15 sec. Dithering every 3rd or 4th sub is a waist of time IMO
Walking noise occurs when your fixed pattern noise has frame shift over time due to poor polar alignment
If you get your PA under an arc minute and dither each sub , you eliminate both problems
Also sorry can’t help you with flatteners and reducers , I only image with Newts

Cheers
Martin

[Todo43 (Lachlan)]

I will try dithering every frame. It may help?
I always try to get my PA under 20 Arc seconds. I have found that Sharpcap gives me good results. I did need to turn on the atmosphere refraction option. It says that the atmosphere is refracting the light 1.4 minutes of arc. Do you use software for your PA?

No worries about the reducer!
Lachlan

[Startrek (Martin)]

Originally Posted by Todo43

I will try dithering every frame. It may help?
I always try to get my PA under 20 Arc seconds. I have found that Sharpcap gives me good results. I did need to turn on the atmosphere refraction option. It says that the atmosphere is refracting the light 1.4 minutes of arc. Do you use software for your PA?

No worries about the reducer!
Lachlan

I use the Synscan PA routine in the handcontroller, been using it since day one about 5 years
I just use my old Canon 600D in the newt , connect to BYEOS ( use PA alignment window reticle target ) switch on live view , zoom in 5x and select 2 brighter stars east of the south meridian above 30 degrees and below 65 degrees, one star is the PA alignment Star and the other is the second alignment Star , 2 or 3 iterations to centre PA Star by tweaking Az and Alt and 10 minutes later I’m under an arc minute. Take a 60 sec sub on that PA Star , zoom in , check that’s it’s nice and round ( my focal length is 1000mm ) and we’re done !!
I can use any 2 stars in the sky ( don’t need a view of SCP ) but closer to the south meridian the more accurate and quicker the process becomes
It’s as good as a Pole Master
Cheers

[Drac0 (Mark)]

With the 533 you want the gain at 100 (which is also unity gain for the camera) - that's the point you get the biggest drop in read noise (from almost 3.5e to 1.5e), dynamic range restored to ~13.5 stops & maintain a good full well capacity. Higher gain will give minimal improvement to read noise at a huge cost to both dynamic range & full well capacity. And as Martin says, dither often.

[Todo43 (Lachlan)]

Thanks Mark,

If I were to pull the gain down to 0, what effect would it have on the outcome? Would it be under exposed? Or would I just need longer exposure times to make up for the lower gain. Will it give me better colour and dynamic range?

[Drac0 (Mark)]

You would need longer exposures with the higher read noise with the gain set to 0. At 100 you get pretty much the same dynamic range as at 0. It's like an electronic switch in the camera - at 100 it drops the read noise and basically resets the DR. The one thing it does affect is the full well capacity, that point where a pixel has collected as much data as it can & starts to "bleed" into adjoining pixels. But that is compensated for by much shorter exposure times - less time to reach the full well level.

Martin is much more experienced than me, so you might want to try what he suggests first. I haven't really tried 0 gain, so I can't say if it does work well enough - I'm happy to be able to take the shorter exposures to get the same data resulting in more data in the same time frame.

Cheers

[Todo43 (Lachlan)]

From what I can see, you're much more experienced than I am. Just one final question, how much of a difference does a higher dynamic range give me? Is it enough to warrant doing it?

[Drac0 (Mark)]

Originally Posted by Todo43

From what I can see, you're much more experienced than I am. Just one final question, how much of a difference does a higher dynamic range give me? Is it enough to warrant doing it?

The difference between the DR on 0 & 100 is very negligible - it drops from 0 to 99, then is almost reset with the way gain is handled when you get to 100, giving you the pretty much the same range on both.

In the most basic terms, DR is the range from the darkest to brightest in an image above the background read noise, which is why it goes back up when the read noise drops significantly at 100 gain. A high DR is always good and either setting will give it to you.