LRGB ordering

[MattJenko]

I am taking the plunge into mono CCD imaging soon, and have a couple of questions if you don't mind.

I am making some assumptions which may or may not be correct:

- I am assuming that the filters will not be parfocal, and I don't have an auto focuser (that last point is more a fact )

- I assume I am going to have to split imaging runs into X x 10 mins L followed by X x 10 mins Red etc as opposed to 10 min L, 10 min R, 10 min G etc as I have no intention of sitting by the mount all night switching and refocusing every 10 minutes.

Questions:

- Do people do colour or Lum first or do you try and get the Lum during the time near zenith to get the best images for that aspect of it?

- Does a little out of focus for RGB actually matter if the Lum is spot on? If this is the case, I could focus for Lum and then do L,R,G,B,L,R,G,B etc which would ensure that all images are spread over the night sky and over the same time of night/LP and in the extremely unlikely scenario of clouds making an appearance after an hour or so, I would have enough data to make myself an image!

Cheers

Matt

[Earl]

There is no right way to gather data really, focus is critical for all filters and will ruin hours of work if not right.

Personally I dedicate a night per filter, might squeeze two filters in if its in the right place for me to get the time.

[peroni]

I have just started with CCD mono and do the same as Earl. One night per filter.

I have certainly found the focus point is a little different between filters. It may of course be that I have disturbed focus by switching filters using my manual filter wheel.

[gnomus]

I have just started with CCD mono and do the same as Earl. One night per filter.

I have certainly found the focus point is a little different between filters. It may of course be that I have disturbed focus by switching filters using my manual filter wheel.

If you are only doing one filter per night, why bother with a filter wheel?

[gnomus]

I am taking the plunge into mono CCD imaging soon, and have a couple of questions if you don't mind.

I am making some assumptions which may or may not be correct:

- I am assuming that the filters will not be parfocal, and I don't have an auto focuser (that last point is more a fact )

- I assume I am going to have to split imaging runs into X x 10 mins L followed by X x 10 mins Red etc as opposed to 10 min L, 10 min R, 10 min G etc as I have no intention of sitting by the mount all night switching and refocusing every 10 minutes.

Questions:

- Do people do colour or Lum first or do you try and get the Lum during the time near zenith to get the best images for that aspect of it?

- Does a little out of focus for RGB actually matter if the Lum is spot on? If this is the case, I could focus for Lum and then do L,R,G,B,L,R,G,B etc which would ensure that all images are spread over the night sky and over the same time of night/LP and in the extremely unlikely scenario of clouds making an appearance after an hour or so, I would have enough data to make myself an image!

Cheers

Matt

I'm looking forward to reading the responses you get Matt. Incidentally, have you decided which camera you are going to get?

[steppenwolf]

I always recommend taking Luminance first - this way as an absolute minimum, you get a nice mono image of your chosen subject and you can paint the colour overcoat on top as and when you get the opportunity. It does make sense to get the data when the object is as high in the sky as possible but this is particularly important with the blue data as this suffers the most from atmospheric refraction.

I would recommend that you do your best to focus all your RGB channels accurately because although you most certainly can use a blurred RGB image* with a sharp Luminance channel to get a good image, each of the RGB channels must be blurred by the same amount or you will get colour fringing! In short, nail your focus for each channel, go on, you know you want to!

* A cunning trick with a one shot colour camera in which you have both sharp and slightly duff data is to stack the really good frames and convert them to mono. Then stack all the frames as a full colour image and use the good mono image as an artificial Luminance channel - this works surprisingly well!

[Earl]

If you are only doing one filter per night, why bother with a filter wheel?

because to change filter otherwise would mean more than pressing buttons from my comfy office chair.

[gnomus]

because to change filter otherwise would mean more than pressing buttons from my comfy office chair.

I suppose that is quite a good answer.  

[MattJenko]

I am going for the Atik 414ex. I have lots of plans with it, including a Star Analyzer and some photometry, but the main focus for now will be imaging, as there is vast amounts to learn and practice/set up! I have to admit to be extremely excited by it all. Starting with LRGB until I get the hang of it and then will get myself some narrowband ones as well for moon nights if I am not imaging it.

It does make sense to get the data when the object is as high in the sky as possible but this is particularly important with the blue data as this suffers the most from atmospheric refraction.

This is not something I had considered or read before I don't think - thanks.

Seems like I need to experiment. A filter a night seems like something to work towards... I will be wanting to practice and image a few different targets, and as a setup/teardown man, I simply can't see myself doing that for quite some time. I think I will be doing hour stints, or maybe 45 mins and then changing/focusing each time. will just have to see how things pan out then.

A motorfocuser with known focus differences between filters that can be programmed in to the sequence does sound appealing given the information above

[PhotoGav]

Realistically you have to accept that imaging a target with a mono CCD is going to extend over more than one night. You can of course grab a bit of data in just a few hours and make an acceptable image, but to really take advantage of the quality on offer with the ccd, you will want to gather as much data as possible to minimise noise and make a stunning image! That, along with our lovely weather, will have you out there night after night trying to finish an image!

I would seriously recommend an Ha filter from the start. Many LRGB images will benefit from some Ha and there are lots of Ha rich targets to image in mono. The number of extra imaging sessions that an Ha filter will open up to you make it an invaluable addition. Moonlight is much less of an issue, though don't expect good results if the target is too close to the moon.

As for what order to image the filters in, I like Steve's suggestion to go L first so that you have something and then add RGB at a later session. However, I tend to grab an hour for each of R G & B, and then as much L as possible, so I normally capture my RGB filters first. I try to get the B data with the object as high as possible and am happy to grab R data when the object is low. I normally grab the G data last as you can get away with generating a synth Green channel from R & B data if the weather really lets you down.

As for focus, aim to get it spot on for all subs!

Good luck with the new toy when it arrives and I have no doubt that you will love it!

[MattJenko]

I need to solve the plate solving conundrum on my Mac then it seems as my multi night image alignment manual efforts have been distinctly amateur

[PhotoGav]

I need to solve the plate solving conundrum on my Mac then it seems as my multi night image alignment manual efforts have been distinctly amateur

Definitely, it makes everything much quicker and easier!

[peroni]

I need to solve the plate solving conundrum on my Mac then it seems as my multi night image alignment manual efforts have been distinctly amateur

I use Astrotortilla with Cartes du Ceil. Typically I select the target in CdC and then plate solve to get the image spot on target (sometimes using 'force flip' in EQMOD to save a meridian flip midway through session).

When I return to target another night it's easier to find the same position as before.

There are other ways to find you last know spot. APT I think has a tool, possibly Artemis. I've not tried any others though as I'm too keen to start the imaging runs. 

[strutsinaction]

So, if you refocus for each filter, does that also mean that a different set of flats are needed for each filter?

For my first LRGB I focussed using the L filter, gathered all the L, R, G and B subs during the one session (without refocussing) then grabbed the flats later on (taking care not to move the camera or focuser). Maybe I need to rethink this workflow?

Regards

John

[frugal]

If you are only doing one filter per night, why bother with a filter wheel?

In my case it would be so that I did not get sticky finger prints on them, or chance dropping them in the veg patch trying to change filters at 3am

[Earl]

So, if you refocus for each filter, does that also mean that a different set of flats are needed for each filter?

For my first LRGB I focussed using the L filter, gathered all the L, R, G and B subs during the one session (without refocussing) then grabbed the flats later on (taking care not to move the camera or focuser). Maybe I need to rethink this workflow?

Regards

John

Olly Penrice just uses a Luminance based flat for all filters and all looks good, iirc, not tried that myself

[PhotoGav]

Olly Penrice just uses a Luminance based flat for all filters and all looks good, iirc, not tried that myself

That method does work, but comes with a warning: it won't remove dust bunnies if a filter has different dust particles on it compared to the L filter. So if all filters are sparkly clean, it can be fine.