Exposure Advice For Tonight Please!

[CheapAsAstronomer]

Hi all.

Tonight might be the last time, and only time this year for me to capture M42 Orion nebula. 

I'll probably only have two hours imaging time and I'm looking for advice on how to image this without blowing out the core!

I'll be using an Altair Astro 183c CMOS camera with an IDAS NBZ (Duo Band) filter.

I've seen Chucks Astrophotography video where he used only 5 second exposures for just 21 minutes and it worked well, but with the IDAS NBZ I'll probably need longer exposures.

I don't want to waste my imaging session!

Any ideas?

Cheers

[scotty38]

Don't forget he was using his RASA at f2 so you'll need a bit longer anyway all other things being equal

[CheapAsAstronomer]

7 minutes ago, scotty38 said:

Don't forget he was using his RASA at f2 so you'll need a bit longer anyway all other things being equal

Fair point. I'm at f5.8 so I presume I'll need at least 3 times the exposure time...plus an allowance for the filter.

[scotty38]

Yeah probably but lots of other factors too as you know but he makes a good point of not blowing out the core so I'd try a few experiments and see what it looks like. At least with short exposures it won't take long to try a few different times to see what you get.

[michael.h.f.wilkinson]

I have been using the Optolong L-eXtreme filter at F/4.8 and used 30s subs on my Canon EOS 550D on M42 without issue. I did find the neighbouring reflection nebulae and reflection components in M42 itself were severely attenuated, so stopped after a few minutes of imaging, and switched to another target. Here is the resulting stack of 6 subs:

The core isn't blown out, so 30s works, but you could use shorter subs if you like.

[Clarkey]

28 minutes ago, CheapAsAstronomer said:

Fair point. I'm at f5.8 so I presume I'll need at least 3 times the exposure time..

Don't forget F ratios are not linear so it will be much more than 3 times. Probably nearer 8. However this value will have so many other variable that you need to take it will a large pinch of salt.

FWIW here is an image I took whilst I was waiting to start a 'proper' imaging session and I had 30 minutes to spare. This was 15 and 30 second exposures with no calibration frames. Just a bit of fun really. This was at just over F6 with a mono camera (ASI1600MM) but it shows what you can get quite quickly.

[Clarkey]

Just to add to above. Given how bright M42 is you can probably shoot without a filter and get away with the full moon. Just use short exposures.

[CheapAsAstronomer]

5 minutes ago, michael.h.f.wilkinson said:

I have been using the Optolong L-eXtreme filter at F/4.8 and used 30s subs on my Canon EOS 550D on M42 without issue. I did find the neighbouring reflection nebulae and reflection components in M42 itself were severely attenuated, so stopped after a few minutes of imaging, and switched to another target. Here is the resulting stack of 6 subs:

The core isn't blown out, so 30s works, but you could use shorter subs if you like.

Ah, the L-eXtreme is much like my NBZ so that's useful!

[CheapAsAstronomer]

2 minutes ago, Clarkey said:

Just to add to above. Given how bright M42 is you can probably shoot without a filter and get away with the full moon. Just use short exposures.

Unfortunately I'm in a bottle 8 area so filters are necessary!

[michael.h.f.wilkinson]

Just now, CheapAsAstronomer said:

Ah, the L-eXtreme is much like my NBZ so that's useful!

I much prefer the result without filters, as reflection components and dust are filtered out too much, although with a full moon it may be needed. On the same camera, with loads more exposure (7 h 11.5 min) I get this

or with unsharp masking

[CheapAsAstronomer]

8 minutes ago, michael.h.f.wilkinson said:

I much prefer the result without filters, as reflection components and dust are filtered out too much, although with a full moon it may be needed. On the same camera, with loads more exposure (7 h 11.5 min) I get this

or with unsharp masking

Nice!!!!

[Clarkey]

13 minutes ago, CheapAsAstronomer said:

Unfortunately I'm in a bottle 8 area so filters are necessary

You are probably right but personally I would still try for M42 due to it being so bright. I live in B6 which is not ideal but unless imaging NB I would rather remove the light pollution and gradients than cut out most of the detail with aggressive filters. As you are using BYEOS you only need to try one 30 second image and see what you get. You can do this before full darkness to get an indication of how good / bad it would be without eating into your imaging time.

Either way - good luck with it. I look forward to seeing the result on the forum.

[Budgie1]

It may be worth going for a couple of different exposure times with 183c, something like 90s for the main cloud and darker dust and 30 or 45s to cut down on the brightness of the core and then stack them all together. You'll have to use two Master Dark calibration frames when you stack them (one for each exposure length) but it normally works fine in DSS.

[CheapAsAstronomer]

59 minutes ago, Clarkey said:

You are probably right but personally I would still try for M42 due to it being so bright. I live in B6 which is not ideal but unless imaging NB I would rather remove the light pollution and gradients than cut out most of the detail with aggressive filters. As you are using BYEOS you only need to try one 30 second image and see what you get. You can do this before full darkness to get an indication of how good / bad it would be without eating into your imaging time.

Either way - good luck with it. I look forward to seeing the result on the forum.

Hmmm... In that case would I be better off using the Optolong L-pro?

Not very good at editing out gradients

[CheapAsAstronomer]

47 minutes ago, Budgie1 said:

It may be worth going for a couple of different exposure times with 183c, something like 90s for the main cloud and darker dust and 30 or 45s to cut down on the brightness of the core and then stack them all together. You'll have to use two Master Dark calibration frames when you stack them (one for each exposure length) but it normally works fine in DSS.

There's a thought.

Would I need to do two separate images then blend them together in PS (not got) or GIMP to get the core right? Or would it all work out stacking the lot in DSS?

[Budgie1]

1 minute ago, CheapAsAstronomer said:

There's a thought.

Would I need to do two separate images then blend them together in PS (not got) or GIMP to get the core right? Or would it all work out stacking the lot in DSS?

I just stack them together in DSS. Put all the subs in the list + flats & dark flats, then add in the master darks, one for say 30s and another for the 90s and run the calibration & stacking.

If that doesn't work for you then you can still stack them separately and combine the two results, either in DSS or PhotoShop.

If you don't have the master darks already then you can take them on the night and just add the two sets of darks to the stack and DSS will calibrate them to give you the masters for each exposure time.

[Jamgood]

I've only had one go at Orion before and that was with a DSLR.

I took 12x300s - 12x30s - 12x10 and stacked them all together and got a pretty decent result.

Longer exposures got the nice clouds and the shorter ones got the core. 

[CheapAsAstronomer]

8 minutes ago, Jamgood said:

I've only had one go at Orion before and that was with a DSLR.

I took 12x300s - 12x30s - 12x10 and stacked them all together and got a pretty decent result.

Longer exposures got the nice clouds and the shorter ones got the core. 

Awesome image.

So stacking the different exposures together takes out the core issue?

[Stuf1978]

I wouldn't bother with the duo band filter either as like others have said I don't like the look they give on this target and they kill the more reflective parts of the nebula. I haven't really imaged M42 in anger this year but I did manage to get this unfiltered, from a bortle 8 zone 18x150 second subs (45 minutes) wth a stock dslr. I processed the stack twice, once for the core and once for the rest of it and then blended the two together

 

 

[Budgie1]

13 minutes ago, CheapAsAstronomer said:

Awesome image.

So stacking the different exposures together takes out the core issue?

It should do. The core may still seem bright but you can reduce this using the Curves tool on GIMP or PS.

[Clarkey]

1 hour ago, CheapAsAstronomer said:

Hmmm... In that case would I be better off using the Optolong L-pro?

Not very good at editing out gradients

Possibly. You might still lose some of the detail but less so than the other filter option. It also depends what sort of lighting you have locally. LED is a bit of pain due to the broad spectrum but standard mercury vapour and sodium will get removed by the L-pro

The wipe module in Star Tools should help remove gradients, but you might need to 'play' with the settings to get the best out of it. I'm not a ST expert as I used APP and Affinity for most things.

[Mr Thingy]

Some lovely examples of M42 shared here 😁

On my first attempt imaging M42 I stuck to 1 exposure time (45s iirc). To solve the issue with a burned out core I stitched in the core from my unstretched file (which I stretched a little to get the right luminosity). Allowed me to shoot all the data at 1 exposure time. 

Image below with about 45 min of data.

Edited March 29, 2021 by Mr Thingy