Altair 26c LCG / HCG

[SkyluxMax]

HI all. I hope someone can help me out?
I have an Altair 26c that I have been using for a while now and been very happy with it. However, I think I could be doing better with regards to its settings and Read Noise levels (in both SharpCap and APT, been using it for a long time for planning/ object DB). Currently, I have been using the 26c by and large at Gain 120, Offset 50 with about 3 Min exposures in Ultra Low noise mode and in LCG mode. I recently ran a Sensor Analysis routine and came up this info (see attached). It would seem that running the Gain level at 300 would be most beneficial?
As I understand, at Gain 300 SharpCap switches over from LCG to HCG? So should I be running APT in HCG or just keep it in LCG at Gain 300? If I do change it to HCG at 300 does that correspond to '300' in both apps as I have seen that these values can change EG 300 in one app means 150 in another?? Confused! I mainly use SharpCap for PA, Focusing, Planetary and Flats.
Also the Offsets in SharpCap started off with 794 should I keep to that?
Hope this makes sense!?

[admin]

Hi,

for the IMX571 sensor in the 26C, the switch from LCG into HCG applies an extra gain of a factor of 3 to the normal camera gain. SharpCap hides this from you by incorporating the LCG/HCG switch automatically into the gain control, whereas other applications may make both controls available.

To give some examples to make it (hopefully) clearer

Code: Select all

Other Application Gain     SharpCap Gain     Total Brightness
100 LCG                    100               1.00x
200 LCG                    200               2.00x
300 LCG                    n/a               3.00x
400 LCG                    n/a               4.00x
600 LCG                    n/a               6.00x
900 LCG                    n/a               9.00x
100 HCG                    300               3.00x
200 HCG                    600               6.00x
300 HCG                    900               9.00x           

In SharpCap once you get to 300 gain (3.00x brighter than minimum gain), HCG is automatically switched on and behind the scenes SharpCap sets the basic camera gain back to 100, so it gets a total gain of 3.00x entirely via the HCG changeover. As gains are increased beyond 300, the camera remains in HCG mode and SharpCap turns up the camera gain to provide the extra brightness boost needed.

The reasoning behind this is that HCG mode provides the benefit of a significantly reduced read noise, with no real downside as far as I am aware, so switching into HCG mode when possible seems to be a win-win situation. SharpCap therefore does this automatically to avoid users needing to learn about LCG and HCG unless they want to.

As a concrete example, to get a total gain of 3.00x minimum, you have two options

1) LCG, 300 Gain, Read noise ~3e
2) HCG, 100 Gain, Read noise ~1.4e

With the HCG approach you can get away with sub exposures that are 4x shorter than with the LCG approach, which might be the difference between getting good subs and having guiding/tracking issues and wondering whether you need a better mount...

chers,

Robin

[SkyluxMax]

Hi Robin
Thank you for the detailed reply. Makes sense.

[8611paulm]

Do these setting's also apply to Zwo 2600mc pro?

[admin]

Hi,

the ZWO camera does the same trick of combining the LCG/HCG switchover into the gain- in fact in the ZWO camera case it is built into the camera control software supplied by ZWO, so *all* software does it that way.

If you look at the product page on the ZWO web site, you will see that the switchover occurs at gain 100 (ZWO cameras measure gain in a different way to Altair ones).

cheers,

Robin

[SkyShed Admin]

Hey Robin!

I've been running my 26C at 100 for years! DSOs. Time to shoot new darks!

I'm in Bortle 4. I've been running successfully at a 10 dark offset. I suppose I should move that up as well then?

Sensor analysis didn't give me settings I liked (years ago when I ran it). So I set things manually.

So what about the Sony IMX410 chip in the Altair 24CFX? The same 300 gain setting to get HG running?

Thanks! Getting ready for another season.

Wayne

[admin]

Hi Wayne,

it just shows what a capable sensor the IMX571 is that even when not being used at its most efficient, it can still produce brilliant results

It is worth noting though that using the higher gain of 300 to get into HCG mode doesn't magically get you higher quality pictures than gain 100. What it does do is let you get optimum quality using shorter sub exposure lengths (about 5 times shorter). That might not matter much for LRGB imaging where the sky background is going to be fairly significant even in Bortle 4, but in narrowband it might be the difference between needing to use a 1500s sub and a 300s sub.

If you use the Smart Histogram 'Brain' function then it will (as well as suggesting gain, exposure, etc) show a graph of minimum recommended exposure at different gain levels for your measured sky brightness (measure with any filters in place so it takes account of the impact of those) - that helps you get an idea of whether a shift to 300 gain is useful or not in your situation.

For the black level setting, you can leave that alone providing that the histogram for dark frames is still separated from the left hand side at the higher gain.

Finally, the data I have for the IMX410 sensor is that you need gain 500 to switch into HCG mode for the Altair model - I haven't actually had one of those in my hands to test/confirm that though.

cheers,

Robin

[SkyShed Admin]

Thanks Robin!

I have not been unhappy with results I must say. I'm doing mostly narrow band so I'll try 300 for darks and subs, maintain my 2 minute sub length and see what happens.

Way back, I tried the Hist. Function. It wanted me to take shorter subs at a higher a gain, I wanted to take longer subs so I reduced the gain to what I thought was the lowest High Gain possible.

Google AI would agree w/ you! Since I'm adding the 24CFX I Googled about the gain setting in SC. Here's what it replied. I will try the Hist Function again as well. To at least see the results, and use them as a guide.

AI Overview
For the Altair Hypercam 24CFX in SharpCap, you should generally use a gain level between 520 and 1500 to activate High Conversion Gain (HCG) mode, as SharpCap automatically switches to HCG at this range. This setting optimizes the camera's performance, especially in low-light conditions.
Detailed Explanation:

HCG Mode:
The Hypercam 24CFX has a High Conversion Gain (HCG) mode that enhances the camera's sensitivity to light.

SharpCap's Automatic HCG Switch:
SharpCap automatically activates HCG when the gain is set between 520 and 1500.

Optimal Gain Range:
Within this range, you can fine-tune the gain based on the specific target and conditions.

Other Applications:
If you are using applications like NINA or the ASCOM driver, which don't automatically switch to HCG, you need to manually enable HCG and then choose a gain level from 100 upwards.

In summary:

Set Gain: In SharpCap, set your gain between 520 and 1500.
HCG Activation: SharpCap will automatically switch to HCG mode within this gain range.
Fine-Tune: Adjust the gain within the 520-1500 range based on your target's brightness and desired exposure time.

[admin]

Hi Wayne,

yes, will definitely be worth a try at gain 300 for narrowband on the 26C

Google is actually right on the IMX410 chip if it is an Altair version - I just found the code that sets the changeover point to 520 for that model. Oddly for the ToupTek and Mallincam equivalents, I have code that uses 500 instead, but I don't think the difference is very significant.

In all of these cameras, sadly, I have to work from the camera name to deduce the sensor type, then from the sensor type I have the parameters for where HCG should start and how much additional brightness it gives. This means that if the manufacturer changes their SDK so that the name of the camera changes in SharpCap, they will suddenly stop getting the right LCG/HCG changeover point. So... when you have one of these cameras, it's worth checking that the image brightness varies fairly smoothly as you change the gain to/fro across the HCG changeover point and shows no big jumps at other positions (in other words, run gain up from 100 to say 2000 slowly and make sure that you get a smooth increase in brightness throughout). If you start seeing big jumps at one point with either the brightness going up a lot or down a lot, the LCG/HCG may be being applied wrongly (let me know!).

cheers,

Robin