Help me choose a filter

[Albastars]

This months full moon coincided with a lovely clear night and this got me thinking.  Is there a filter that can help counter the effects of moonlight as well as light pollution?  TBH I'm doubtful because moonlight is broadband, right?  But I've read that the Baader Neodymium filter can help - reports seem to vary.

Here's my set up/situation:

• Modded (not full spectrum) Canon 800D
• WO ZS73 scope with flatteners, both the x1 and x0.8
• Fornax Lightrack mount (3 min unguided subs with the above are OK to my eyes)
• Have Bortle 4/5 skies.  North and east of me I have B4.  To the south and west I have B5.
• I'm targeting emission nebulae and the large galaxies at the moment and I want to preserve star colour as much as possible.

In my limited experience I haven't found light pollution to be a problem so far.  However, my research so far is suggesting the Baader Neodymium, the Optolong L-Enhance and the Optolong L-Pro might help.

I guess that the L-Extreme filter is the best option but I don't think I can achieve long enough subs.

Any suggestions?  

 

 

[vlaiv]

No filter can help with moon glow when imaging galaxies - they can only hurt, even if background seems darker. That is because they block light from target as well.

What you can do is go for UHC / narrowband type of imaging. You say that you are interested in emission nebulae? That should be your target of choice when the moon is out.

L-Pro is worst for this as it passes the most of spectrum besides emission lines. It is good LPS filter when you are in area dominated with non led artificial lighting - like low and high pressure sodium lights.

L-Enhance would be my filter of choice.

L-Extreme is too narrow. It removes completely H-beta line and leaves only Ha and OIII.

Don't worry about sub duration. If you can't go long - then go short. It is better to go short with narrowband filter in adverse conditions than with no filter at all.

In the end - avoid imaging when the target is very near the full moon. Maintain at least 30 degrees of separation between them.

[vlaiv]

Forgot to say: If you want natural looking stars - then you'll have to do separate exposures without filter and use just star color over original exposure with filter or alternatively - do some really clever math to get star color from filtered data.

Stars emit black body spectra and you really only need two points to solve for black body spectrum shape and get star color based on temperature.

[GalaxyGael]

Basically what Vlaiv says, it tackles most points. One possibility to hone your skills and test what needs tweaking for longer subs and still get a target imaged, is to focus on star clusters or various types. right now, the double cluster in perseus (Caldwell 14) is a great option, not to near the moon, high enough and insensitive to the moon glow to some degree. Its direction is toward you Bortle 4 zone. Processing there, is more simlar to galaxies (stars and background anyway), which is always good practice and some of these clusters make great images when they get enough integration time.

Same goes for Caroline's rose cluster and possibly (because it is interesting) the Sailboat cluster which has a nice dark nebula feature there too.

Galaxies are a wash out at the moment, I am in the same situation myself.

On a moonless night, the slightly wider band filters work great with OSC, such as the quad band filters. These keep Ha and SII, while having a 35 nm band pass for H-beta and OIII. It does improve emission nebula signal a lot, but wide enough to retain a lot of RGB star color and only take s small photometric color calibration to remove the small amount of redness form the stars. 

[Albastars]

Thanks vlaiv and Colm.  Great info and tips there.  

I think I'll hold on the filter for now (maybe keep a look out for a bargain on the L-Enhance) and I'll read up on quad band filters.  I will take a look at star clusters - you are right Colm, I could do with more practice, lots of it!

5 hours ago, vlaiv said:

or alternatively - do some really clever math to get star color from filtered data.

Are you meaning Photometric Colour Calibration there vlaiv?

[DaveS]

I wouldn't even think about galaxies in moonlight, and I'm in a Bortle 3/4 location. About the only thing that is remotely moon-proof is H-alpha, and as narrow as you can afford.

Even the brighter open / globular clusters might be marginal.

[vlaiv]

4 minutes ago, Albastars said:

Are you meaning Photometric Colour Calibration there vlaiv?

No. I personally don't understand what Photometric Colour Calibration does - and according to my understanding it does not produce correct color, but that is besides the point.

Planck's law is given by this formula:

And it only uses one variable - T or temperature (other is frequency and we are interested in range ~ between 400 and 700 nm) - all other things are constants - e : base of natural logarithm, h : Planck's constant, Kb : Boltzmann's constant and c : speed of light.

It produces distinct spectrum for each temperature. Other unknown is integration time (multiplicative constant).

In principle you need only to measure graph in two points to be able to solve it for temperature.

Once you have temperature - you have color of the star:

[Albastars]

3 hours ago, vlaiv said:

Planck's law is given by this formula:

And it only uses one variable - T or temperature (other is frequency and we are interested in range ~ between 400 and 700 nm) - all other things are constants - e : base of natural logarithm, h : Planck's constant, Kb : Boltzmann's constant and c : speed of light.

It produces distinct spectrum for each temperature. Other unknown is integration time (multiplicative constant).

In principle you need only to measure graph in two points to be able to solve it for temperature.

Once you have temperature - you have color of the star:

 

😱 Well, I did ask.  More years ago than I care to remember I scraped a Grade C Pass in maths. That is the stuff of nightmares for me! 

Thanks for explaining though vlaiv.