CCD Monochrome imaging in heavy light pollution

[Crowmium]

Hi all,

As an astrophysics student, my department lets me use one of it's telescopes on the roof of the physics building. The rig itself is a monster; a 10' Meade LX200 SCT with a QSI CCD Monochrome camera + filter wheel for H alpha, R, V (Green), B and Infrared.

The only problem with it is that it's situated right in the middle of the city, so the levels of light pollution are incredibly high. What would be the best way to deal with this in post processing (im using Photoshop CSS), and is there a 'limit' on the maximum exposure I should be taking into account?

Finally, I'm able to take darks by closing the telescope's dome (the rig is all controlled remotely). Since I'm using a monochrome CCD camera, do I need to take darks for each filter and stack them separately, or can I just take darks in one filter band (for example the V band) and just use them for all the colour channels?

Here's what I've gotten for M51 with a single 400s exposure in the R, V and B bands, one without processing (I just aligned the colour channels into an RGB image) and one is with processing. When I get more time on the scope, I'll try to get multiple frames and stack them - will this help any with the light pollution?

I wasn't sure if this question would have been better suited for the processing section of the forum. Apologies if it's in the wrong section.

 

Cheers,

Crowmium

[Zakalwe]

Stacking will reduce the noise, but it won't help with the light pollution.

Try using the hydrogen-alpha filter. Although the exposure time will need to go up significantly, a good h-a filter makes a good light pollution filter.

[Crowmium]

Thanks for the reply, how would I go about integrating the h-alpha filter - would it be in post processing? I've heard of people combining it with a colour layer (the red channel I believe) to produce a H-alpha RGB image.

[Waldemar]

Hi Crowmium,

The images are out of focus. You can see that by the donut shape of the stars. The little hole in the middle is the central obstruction of the SCT
Your subs are too long. The cores are washed out. Take a lot more much shorter subs and stack them. That will make a difference in the light pollution also.
H-alpha will greatly enhance your image. You could use it as a luminence channel.

Take dark, bias and flat frames. That will take care of the artifacts in the image, clearly shown in the righthand lower corner of both images.

Waldemar
 

[Pompey Monkey]

Galaxies are broadband targets and generally don't respond well to narrowband imaging, with a few exception. They really need dark skies for good results, although stacking many exposures can increase the SNR between the target and the sky and target. Watch this from the start: * 

In areas with significant light pollution, narrowband targets like emission nebulae are the best to go for.

Also, as Waldemar pointed out, your images are out of focus.

Have fun!

 

*woo - I didn't expect the embedded video - cool.

 

[Crowmium]

Thanks for the replies. I had a feeling the images looked a little off, for some reason I must have forgotten to focus on the galaxy properly - something i'll definitely do next time!  Ill also try stacking many shorter exposures next time, and probably try getting some emission nebulae.

[MartinB]

Shorter exposures won't really get you around your LP problem.  I'm surprised the core is saturating after 400secs.  I'm thinking your image has had some stretching.  If not then your exposure time is too long because of the core being blown out rather than the LP.  It may not be possible for you but a light pollution filter placed at the front of the imaging train (before your LRGB filters) will help a lot in cutting out LP from sodium street lighting.  

As far as darks go, the filters don't come into it because no light should be coming onto the chip!  I don't know how dark your observatory is when closed but it needs to be pitch black, unless your QSI camera has a manual shutter (which it might - 532, 683 full frame chips)

[wuthton]

How is the Meade mounted, is it forks, forks and wedge or an EQ mount and is it guided? 

I ask as narrowband imaging requires long exposures to gain the full benefit and your mounting and guiding arrangements will have a bearing on the best advice on how to proceed. 

[Crowmium]

Sadly it wouldn't be possible for me to put a light pollution filter in the imaging train as i'm not able to actually reach the telescope myself. I think the observatory is more or less pitch black when closed; good to know that I don't need to take darks for each separate filter!

As for the mounting, I'm not exactly sure which mount is used but it's certainly a motorized alt-az mount (this one I believe, minus the tripod). The rig uses a 80mm refractor as a guidescope and can pull off long exposures really well, the image in my original post is made of single 400s exposures and I believe some people have used even longer exposures using the scope.

Thanks again for all the replies, this is all really helpful

[D4N]

The alt-az mount will limit the maximum exposure time as you will start to get field rotation (unless it is on a wedge), it will depend where your target is as to how long you can go.

Watch out for this in the corners of your subs, if you start to get trails you heed to reduce the exposure time.

When you stack you need to be sure that the software knows it needs to correct for rotation, I believe that DSS assumes this in the default settings but not all software does.

 

Remember to check focus for each filter, the focal position can change slightly when you change filters.

 

As for how long to expose you need to pull the histogram away from the left edge whilst ensuring nothing is overexposed.

A bit of trial and error and you should get a good idea of what gives good results pretty quickly.

The Ha will be good for much longer exposure lengths than the RGB filters are.

[Crowmium]

Ah, didn't know/think about field rotation. Ill try some really long exposures and see what happens.

If I'm doing colour images with Ha, do the RGB exposure lengths need to be the same as the Ha exposure lengths?

[MartinB]

I would be very surprised if your scope hasn't been placed on a good quality wedge (making it equatorially mounted)  and carefully polar aligned.  I fact I'm certain, given the roundness of your stars.  

Your Ha exposures can, and should, be much longer than your RGB ones.  Much less light gets through an Ha filter.  The Ha data will only really help with the small Ha regions within the galaxy

[D4N]

Considering your light pollution I would say that a way to get some nice results would be to aim for an emission nebula that is strong in Ha and take some long Ha subs, then combine these with some shorter G and B subs to get star colour.

It is a pity you don't have an OIII filter though as there a lot of nebulae that would be good for that setup.

[Crowmium]

8 hours ago, D4N said:

 

Considering your light pollution I would say that a way to get some nice results would be to aim for an emission nebula that is strong in Ha and take some long Ha subs, then combine these with some shorter G and B subs to get star colour.

It is a pity you don't have an OIII filter though as there a lot of nebulae that would be good for that setup.

 

Sounds like emission nebulae are going to be the best targets for this rig. Would I need to also R subs along with Ha, B and G? - I'm still fairly new to monochrome imaging.

Since the scope has a fairly narrow FOV I unfortunately can't try imaging some of the larger nebulae. My current target the next time I get on the scope is NGC 7380 (The wizard nebula).

[D4N]

Yes unless you mosaic you will be limited to smaller nebulae, cocoon or crescent may also be good targets for you.

 

Whether you need R depends on how you plan to process the data.

Personally I would just use the Ha as the red channel, you should get pretty good star colour that way whilst still having strong nebulosity.

You could also do HaRGB where you use the Ha as a luminance layer but this will suppress any nebulosity that isn't Ha.

[Crowmium]

Great, ill give those 2 nebulae a try. Ill also try using Ha as the red channel and see how it goes. I might even see if I can get a mosaic going, the telescope uses Maxim DL and I'm fairly certain i've seen a mosaic option in there before.

Thanks again for all the replies everyone, much appreciated

[MartinB]

The problem with Ha as the red channel is that your Ha stars are likely to be much smaller and the brightness much less even if you have x10 then length of exposure.  It can work but the processing will be pretty messy.  If you have a red filter in the set up then better to go for a separate red channel

[ollypenrice]

2 hours ago, D4N said:

Yes unless you mosaic you will be limited to smaller nebulae, cocoon or crescent may also be good targets for you.

 

Whether you need R depends on how you plan to process the data.

Personally I would just use the Ha as the red channel, you should get pretty good star colour that way whilst still having strong nebulosity.

You could also do HaRGB where you use the Ha as a luminance layer but this will suppress any nebulosity that isn't Ha.

 

No, the last two points are not correct. Firstly you can't just replace Ha with red because the background sky value in Ha will be much lower, causing a colour imbalance, and the stars will be much smaller. This would mean that the outer parts of the stars, which contain all the colour information, would be green and blue with little or no red.

Nor should Ha be used as luminance other than very sparingly. It turns everything pink and throws up big blue star haloes. Ha should, in my view, be applied accurately where it belongs - in the red channel. Photoshop allows it to be added to red in a blend mode called Lighten. So where the Ha is brighter than the red it is applied. In Ha the stars are not as bright as in red so no Ha is added, leaving the proper stellar colour balance.

 

Darks must be taken in absolute darkness. If any light whatever gets onto the chip they will do more harm than good. Unless you camera has a shutter you should do them with the camera removed and the chip window covered with its dedicated cover. I have tried using the scope's metal lens cover and, even with sealed electric filterwheels, light was getting in. Bad darks are worse than no darks.

Olly

[Astrosurf]

If the camera is cooled, does it need darks at all?

[Toxophilus]

1 hour ago, Astrosurf said:

If the camera is cooled, does it need darks at all?

Even a cooled camera will produce noise that needs to be removed, it's just reduced. However a set-point cooled camera does give you the advantage that the noise at a given temperature should be consistent, thus allowing you to run your camera at a 'known state' for the whole imaging session. It also allows you to keep a library of pre-produced darks to use rather than having to do them for every session. 

[ollypenrice]

A master bias used as a dark and an aggressive hot pixel filter when stacking will probably do quite a good job in the absence of good darks.

Olly