mono camera with light pollution need to understand the filter choices

[Wirral man]

Hello all yet again im just trying to get a understanding of what i can use as this is all new to me i live in ellesmere port which has some nasty light pollution.

I have the asi1600mm pro with both sets of filters the lrgb and narrow band ha7nm oIll sll all are 1.25" now i also have a 2" light pollution filter which i used before this new gear,

i know narrow band is not a problem with light pollution its the lrgb side im not sure about say for instance i want the shoot galaxys under my lovely light polluted sky again sorry if these questions seam daft but i like to be a 100% in what im doing and when i get the chance of a clear night i dont want to waste that opportunity anyway thanks again in advance cheers John

[david_taurus83]

Just give it a go! Next chance you get, set everything up and pick a nice bright target and capture some images in LRGB, maybe 10 x 120s exposures in each and see what you can get! The best teacher is experience! Enjoy the new kit and don't get hung up on getting instant results. It's a long learning curve.

[Tim]

Use your 2" light pollution filter (CLS or IDAS???) in conjunction with and in front of your LRGB filters, it should help cut unwanted gradients a little.

[kens]

Set the camera to cool to at least -10C to cut out as much dark current noise as you can.

Use a gain of 75 and offset 50 (I think offset is fixed at 50 in the Pro version).

Take a test shot with each filter. I currently use just RGB - no L. At this gain and offset you want the median of the histogram to be around 1000 to 1200 on a 16bit scale. At this level the read noise is not dominant. For me, in a orange zone around full moon, I end up with an exposure of 30s. You may well have less if your LP is worse. Use the same exposure for all filters so you don't have to make so many darks.

Take darks at the same exposure, temperature, gain and offset as your lights - you can save them in a library for reuse.

Take flats with an exposure time of at least 0.5 seconds. I aim for a histogram median of around 20000. One set per filter with the same temperature, gain and offset as your lights. Take them as soon as possible after your lights (or before if you know what settings you'll be using).

Match the flats with flat darks with the same exposure, temperature, gain and offset as the flats. These are instead of bias frames which don't work too well with the ASI1600. I usually take a set of flat darks per filter since each one needs a different flat exposure.

By calibrating your lights with darks, flats and flat darks you'll give yourself the best chance to clean up the image in post processing.

Finally, take as many subs as you can. Integration time is all important. 

And as mentioned - experiment and have fun. 

 

 

[Wirral man]

sorry ken but i need to read up on what "offset and median" mean i know gain is like iso the higher the more noise you get but as i said im coming from a dslr background and all this is foreign to me once i get it then no problem!!

[vlaiv]

Simple: narrow band filters go by them selves in front of the sensor.

If shooting LRGB, use L, R, G or B (depending what you are shooting) in front of the sensor and LPR filter before that. I use it at 2" nose piece (actually I'm switching to threaded connection for both RC and APO, so I'm using M63 adapter / rotator that has both male and female 2" thread - male on camera side, female on scope side - this is where I put LPR filter).

I'm in red zone (white not far away), with mag 18-18.5 skies. Hutech IDAS LPS P2 is my filter of choice.

Remember there are two things that you need to know when shooting in LP. First - your SNR will suffer (a lot with broad band) - you need to throw more imaging time at image to get the same results as in dark skies. Second - there will be nasty gradients due to uneven sky illumination. Larger the FOV - it will be more noticeable. There is a way to deal with this - either correct your subs prior to stacking, or do some sort of background removal after stacking. I prefer the first choice - because camera FOV rotates in respect to LP gradient - stacked image will tend to have non linear gradient. If you deal with individual frames - LP gradient (is much harder to notice by eye) will be more linear (although with different orientation in different frames). I use frame normalization that can deal with such cases and orient LP gradient the same on each frame (my algorithm). I usually select the least affected frame to be the reference frame (all others have their background "oriented" like that) - the one in zenith, or just below it to the east (the least LP affected part of sky).

On the matter of exposure, I recommend the following for ASI1600:

- broadband - 1 minute / Gain 139 (unity) / offset - choose one you like and stick with it, and make it over 40 or so (default with latest drivers is 50 - good value).

- narrow band - 4 minutes / other settings same as above.

My flats and flat darks are extremely short exposure because I have very strong flat panel - less than 10ms for broad band (2-3ms for L) and it works fine. I also recommend very large number of calibration frames. I use 256 of each (darks, flats and flat darks - darks I record and use that master for quite a long time, flats and flat darks I do after every session, or filter change - mobile setup, so I tear down after imaging - no guarantee that it will be the same next time, and it almost never is).

 

[swag72]

I can only speak with regards to filters.... I had a 2" LP filter and all my filters are 1.25". To use the 2" filter that would have meant putting it somewhere in the imaging train between the filter wheel and the scope and it would have been in there all of the time even when using narrowband. I didn't want this as I didn't want the LP filter cutting out any of the NB light. Also I didn't want to use another piece of glass in the imaging train potentially giving reflection or image degradation. So I swapped (via SGL) my 2" filter for a 1.25" and then use it in the filter wheel instead of a luminance filter. I don't need it with NB filters, I'm certainly not going to break things down to remove it when using NB filters either. This way there's no LP removal on the red, green and blue filters, but I haven't found that a problem.

Hope that helps

[Wirral man]

13 minutes ago, swag72 said:

I can only speak with regards to filters.... I had a 2" LP filter and all my filters are 1.25". To use the 2" filter that would have meant putting it somewhere in the imaging train between the filter wheel and the scope and it would have been in there all of the time even when using narrowband. I didn't want this as I didn't want the LP filter cutting out any of the NB light. Also I didn't want to use another piece of glass in the imaging train potentially giving reflection or image degradation. So I swapped (via SGL) my 2" filter for a 1.25" and then use it in the filter wheel instead of a luminance filter. I don't need it with NB filters, I'm certainly not going to break things down to remove it when using NB filters either. This way there's no LP removal on the red, green and blue filters, but I haven't found that a problem.

Hope that helps

yes i have thought about that as the efw is a 8 position and i have one space empty so it could be a option 

[ollypenrice]

Even without LP I get gradients in RGB for some reason, and sometimes in luminance, sometimes not. So Sara's point is an interesting one and I can see exactly why she does things this way.

Olly

[wimvb]

+1 for Sara's advice. I have a fair amount of light pollution, but don't use a lp filter with my mono camera. (More glass = more reflections.) If you find that luminance has too much lp, don't use it until you've swapped it for a light pollution filter. Just shoot rgb and nb.

[vlaiv]

I've also tried LP as Lum and just plain R, G and B (without LP filter) and while it works on some targets, with others I have issues.

It depends on position of target over the course of the evening in regards to LP and the level of LP you are dealing with. Result of such shooting is that background is nice and even in illumination, but there is a color cast gradient over the target. Color balancing gets to be the a real problem.

 

[kens]

17 hours ago, Wirral man said:

sorry ken but i need to read up on what "offset and median" mean i know gain is like iso the higher the more noise you get but as i said im coming from a dslr background and all this is foreign to me once i get it then no problem!!

Offset is just one of the settings for the ASI1600 that you might see the setup screen when you connect to the camera, along with gain. 

In the newer models I believe it has been removed and is set to 50 always. If it is still there just set it to 50 and leave it at that.

Median is a kind of average. All you need to know is that it equates more or less to the peak on the histogram, same as you would have seen on your DSLR. Common advice for a DSLR is to expose so the peak is 1/4 to 1/3 of the way across. On the ASI1600 that is way too much and will only make it harder to pull out the fine details in post processing. So use the numbers I gave above as a starting point. Most applications will list out the median so you just need to look for that rather than trying to read off the histogram. Here's an example from FITS Liberator where the median value is 1280.  If Median isn't listed on your application, Mean is usually close enough..

By the way, since you mentioned imaging galaxies under light polluted skies the shot above is one sub of just that (NGC4945 - Gold Coin Galaxy) along with a big dose of moonlight so that's what you can expect to see. It does come out better after doing all the things I listed out earlier then stacking and post processing. That was with the ASI1600 and 1.25" Astronomik  filter (Green in this example). 30 seconds at gain 75, offset 50 at -10C. One of 80 subs per filter so 240 in total.

[Gerry Casa Christiana]

3 hours ago, kens said:

Common advice for a DSLR is to expose so the peak is 1/4 to 1/3 of the way across

Is that stretched or as is? On APT there is a auto stretch. I usually have it on. 

Gerry

[kens]

5 hours ago, Gerry Casa Christiana said:

Is that stretched or as is? On APT there is a auto stretch. I usually have it on. 

Gerry

To be honest, I've never seen it qualified but assumed it to be unstretched. So I blindly followed that advice till I understood the rationale. Now I go by the actual values so stretched or unstretched doesn't matter. So I advise to work out the appropriate value for one's own camera AND the display units used and expose to that amount.