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RGB/LRGB/HaRGB and a filter wheel question

m37

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m37

m37

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I understand why my cheap achro has some chromatic abberation and to be honest it doesn't bother me that much because I love my scope and my mount very much. That said, I have a real thirst for knowledge and I like to learn about things like CA and overcome problems or at least understand why I can't. (Also I can't afford an apo on an HEQ5 synscan yet so there. :p)

With that in mind, and please understand I am aware that my setup has limitations and I'm quite realistic about what I can achieve, I have a couple of questions if you would be so kind... :grin:

1. Am I right in thinking that if I put in a R, G and then B filter I am essentially isolating those colours at the expense of all others? i.e. red filter allows mostly red light through etc.

2. Am I right in thinking this is theoretically a way to overcome some of the chromatic aberration?

3. Would you not have to refocus for each colour as the different focal points is what is causing the CA in the first place?

4. In practical use how the jiminy jillikers would one go about such a task? Surely changing the filter on the eyepiece in between exposures, even if very careful, would move the scope a little thereby scuppering the framing?

5. Would a manual filter wheel help with 4. or would that still involve too much manual intervention to remain accurate? There is no way I am buying a motorised one as that money could go in the imaginary apo fund..

6. Also, what is the deal with LRGB and HaRGB. Is it L+R, L+G, L+B and Ha+R, Ha+G, Ha+B respectively or does each one involve 4 different exposures?

thanks and sorry for so many questions. I have some RGB filters already that I got with another purchase, I've found a manual filter wheel under £60, and wifey wants to buy me something astro-related for our anniversary so Ha and L filters are a possibility!!!

7. Sorry, just though of another one. If one wanted to combine 2 filter assortments (R,G,B and Ha, Moon, IR) could 2 wheels be stacked one in front of the other?

Sorry again, I have ordered MEPC I promise!

Chris

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johnrt

johnrt

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The cameras you have are all colour I believe, and therefore you will see no benefit from placing coloured filters in front of them. Colour filters are used to build colour images with mono cameras, or used for planetary/luna imaging to increase contrast.

You are seeing chromatic aberration in your images due to your scopes inability to focus all colours of light at the same point and control light dispersion - which shows up mainly in the blue channel. This is why you will see bloated stars with blue halos.

If there was an easy fix to this, like putting a colour filter in front of a camera chip, we'd all be doing it and using the cheapest optics available, there is no easy fix I'm afraid other than better glass that controls the CA.

You could try a Hydrogen Alpha filter in front of your DSLR, but without it being modded you would struggle to pick up any signal.

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Ant

Ant

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The simplest way around it to get a MONO camera and then use the filters.

But that does involve multiple imaging runs - each run in a different colour filter. I use narrow band (Ha, OIII and SII) to make up a final colour image.

But RGB would work as well.

As John said there is no easy (cheap) fix for this and regardless of the route you take you can get to the end result. But the more expensive path you choose the less faffing around you have to do.

You specifically mentioned framing. A motorised filter wheel will help out with this a lot, but it costs money. You can do it without a filter wheel but that involves removing the camera, changing the filter and putting the camera back. Refocusing and re-framing the target. This is the cheapest route. A manual Filter wheel will make life easier and will set you back a little cash and electronic FW is the best and will cost you more.

To be honest with you, sit down have a think about where you want to go with the imaging. But be honest with yourself.

Look at your skies from home (or your regular observing location), are they light polluted? If they are then a colour camera is the least preferred, and MONO and RGB will suffer from light pollution and will be affected by moon light etc.

The advantages of narrow band is that they are significantly less affected by light pollution and certainly in Ha you can image during Full Moon.

But the good news is that you can start without buying all the stuff. Get the MONO camera and one Ha filter. You'll have great fun with that. When you have an extra few quid, buy the OIII filter. Then you can re-visit all of the targets already imaged and add more data to them. Then when you have a few more quid add an SII filter or filter wheel.

Be clear where you want to go, make sure that the local conditions will actually let you get there. Make a list of the things that you need and start buying them.

I know poeple that image with little ST80s and ST102s, these are achromatic and will all show a nasty blue halo - but narrow band (or RGB from a darker site) will get around it.

Getting an APO is not the only solution.

Ant

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  • 4 weeks later...
m37

m37

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Thanks guys. Is there a minimum exposure time that is worth bothering with in narrowband? I imagine the reduced light coming in through a narrower band would need longer exposures and my maximum is about 30 seconds on an alt/az go to.

Took your advice Ant, got a manual 5 position wheel and a Ha filter. I presume it will let me see through cloud? ;)

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johnrt

johnrt

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Thanks guys. Is there a minimum exposure time that is worth bothering with in narrowband? I imagine the reduced light coming in through a narrower band would need longer exposures and my maximum is about 30 seconds on an alt/az go to.

Took your advice Ant, got a manual 5 position wheel and a Ha filter. I presume it will let me see through cloud? ;)

I shoot nothing less than 10 minutes using my sensitive mono CCD camera in narrowband, and others will say that's too short by a mile. I don't think you're going to have much joy with 30 sec exposures on an unmodded camera. You really need to expose for longer, there is no substitute and for than you will need to ditch the alt/az mount and get an equatorial one.

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Zakalwe

Zakalwe

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1. Am I right in thinking that if I put in a R, G and then B filter I am essentially isolating those colours at the expense of all others? i.e. red filter allows mostly red light through etc.

Correct

2. Am I right in thinking this is theoretically a way to overcome some of the chromatic aberration?

I doubt it, otherwise everyone would be doing it.

The chromatic aberration would also mean that fine detail is "smeared"?

3. Would you not have to refocus for each colour as the different focal points is what is causing the CA in the first place?

Yes. But you would do that anyway as filters are not perfectly parfocal.

4. In practical use how the jiminy jillikers would one go about such a task? Surely changing the filter on the eyepiece in between exposures, even if very careful, would move the scope a little thereby scuppering the framing?

A filterwheel is probably the best. The framing moving wouldn't be a big problem unless you were imaging edge to edge (impossible at this level I would say)

5. Would a manual filter wheel help with 4. or would that still involve too much manual intervention to remain accurate? There is no way I am buying a motorised one as that money could go in the imaginary apo fund..

Don't go into imaging unless you have money to spend! A manual FW isn't a bad compromise, though it can make taking flats tricky (as it would be difficult to get the filters into the exact same place again).

6. Also, what is the deal with LRGB and HaRGB. Is it L+R, L+G, L+B and Ha+R, Ha+G, Ha+B respectively or does each one involve 4 different exposures?

Yes. You expose for the RGB Ha and Luminance. The Ha is normally blended into the R data.

thanks and sorry for so many questions. I have some RGB filters already that I got with another purchase, I've found a manual filter wheel under £60, and wifey wants to buy me something astro-related for our anniversary so Ha and L filters are a possibility!!!

No problem. I hope that these answers were of some use.

7. Sorry, just though of another one. If one wanted to combine 2 filter assortments (R,G,B and Ha, Moon, IR) could 2 wheels be stacked one in front of the other?

In reality, you would run into backfocus issues, as normally you would have a field flattener or focal reducer in the train.

You are trying on an alt-az??? Then you need a wedge. Or an equatorial mount. Imaging needs longer subs than 30 seconds. Narrowband a lot more (I use 15 minutes and probably need to go to 25). You can, in theory, use 30 second subs and take a LOT of them. You can, also in theory, go to work on a unicycle. Whilst it's possible, it's wouldn't be much fun and I guess that the novelty would get old pretty quickly!

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ollypenrice

ollypenrice

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I'd support the answers you've had other than over a few trivial points. Manual filterwheels are fine and all my own images until this spring were taken using them. Flats are not an issue, they worked perfectly for me over several years. They are also more reliable than electric! I run three makes of electric wheel here and they all have their moments... :BangHead:

Good filters like Baader are truly parfocal and don't need a refocus provided your optics are perfectly corrected. If they are not then, yes, you'll need to refocus. (I've only used the Baaders down to F3.9 in saying this.)

Short exposures in Ha or other NB really would be a waste of time.

In visual use there are an assortment of Fringe Killer and Minus VIolet filters intended to exclude the badly defoused short blue/violet wavelengths afflicting fast achromats. They work well but are not much talked about any more since apos have become (slightly) more affordable.

There really is no way around the fact that DS imaging is expensive. The best you can say is that it's cheaper than going motor racing...

Olly

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cantab

cantab

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If top gear are to be believed motor racing can be done on the cheap - probably cheaper than what many astro imagers have spent!

Meanwhile I've seen people riding unicycles on the roads. More than once actually.

/offtopicness

On topic, have you tried imaging with no filters and removing the haloes in post-processing? I doubt the results would be as good as with an apo scope to start with, but you might still get some nice results.

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ollypenrice

ollypenrice

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I've been motor racing and I've been imaging and in imaging the kit doesn't suddenly explode because it feels like it, nor do you fling it into a wall because you had a silly moment... (I've never ben hit from behind by another tripod, either, come to think of it!!!)

As for believing what Top Gear say, well...

Olly

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m37

m37

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If top gear are to be believed motor racing can be done on the cheap - probably cheaper than what many astro imagers have spent!

Meanwhile I've seen people riding unicycles on the roads. More than once actually.

/offtopicness

On topic, have you tried imaging with no filters and removing the haloes in post-processing? I doubt the results would be as good as with an apo scope to start with, but you might still get some nice results.

I'm into cyclocross so a unicycle would pretty much suck. Be easier for carrying over hurdles though I suppose. :)

I've had quite a bit of luck using a Baader fringe killer, I was dubious so had fairly low expectations but it really takes the edge off the halos and doesn't seem to affect the colour too badly. It pretty much gets rid of them on smaller stars. I also have a more laborious method of putting a black and white level adjusted layer copy on top of the final image and using a mask to reveal all but the blue halo. They work a treat in combination.

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m37

m37

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Awesome, hello fellow nimbus aficionados. Keep meaning to get in touch with East Midlands Stargazers.

Apparently we have an average of 3 hours' uninterrupted sunshine a day.

glorious :mad:

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