Hi, can I eliminate CA using a filter wheel?

[matt42s]

Hi all, I'm new here so I'll introduce myself - or you can skip to the CA question
I'm a photographer with plenty of DSLR equipment, 5DmkII, 1Dmk3, mk2, a bunch of L glass and a couple of TEC/peltiers which will soon be installed into a 300D. I've had a long time interest in astronomy but the toy my parents bought me 30yrs ago still has not been upgraded. I recently moved out of Sydney's inner west and rekindled the idea of owning mown scope.
Somehow I've managed to restrain myself so far and I've bought only the mount, an NEQ6 and a breadboard dovetail. Enough to get a started with my 5D, 70-200 and 1.4 teleconverter.
First light was only slightly better than abysmal. From NW Sydney, with light cloud cover to the south, polar alignment came down to a rough guess. I couldn't see anything other than red light through the polar scope (I have now found the dimming option in the hand controller) The few 30 sec exposures I took all had horrific trails, it was until the morning after that I found the hand controller option to turn on tracking. I went out with the intention of making MY mistakes so I'm ok with the fail.
Future equipment: Initially I was looking into a 10" newtonian but now I'm thinking of first getting an ED80 or 100 and following it up with a GSO 8" RC. I know the RC is a challenge, esp for a beginner but I like biting off more than I can chew. I'm also prepared for the 300D/DSLR route to fail and eventually buy something like the QHY9, or whatever the comparable product is in 2014.

The CA question.
Generally speaking, CA is caused by different colours focusing to different points, a doublet will focus 2 colours to the same point, a triplet, 3. (simply speaking) Are there downsides to using a cheap refractor, a mono CCD, colour filters and refocusing for each colour? Is it simply too difficult to achieve critical focus with the different filters? I strongly suspect I'm missing something, but can somebody please tell me what.

Thanks for reading
Matt.

[multiweb]

Using a mono camera with a filter wheel means that you can focus and capture every color separately. So short answer, yes - no CA. On the other end an OSC will need a high quality APO or some kind of fringe killer + UV/IR filter to get rid of color artefacts and halos.

[Shiraz]

as well as helping with CA, using multiple filters also largely gets around the problem of differential atmospheric dispersion at low altitudes - a bit of a pain with an OSC. Regards Ray

[Moon]

Think of light colour as a countinum (not discreete R G and B) and you will see what you are missing. For example, look at the images I took on this page with R G and B filters.
http://deepspaceplace.com/ed127.php
Look closely at the one with the blue filter. The blue filter has a 'passband' - a range of frequencies if you like - that this scope is unable to bring to perfect focus. So you are still going to get bigger stars when you use the blue filter, and still get some blue halos when you combine into the final R+G+B images. In fact the point where the green filter ends and the blue filter starts, the light is focused to almost the same point. So there isn't much benefit in refocusing between images at all.

Narrowband filters (such as Ha) have a very narrow passband, so this is a slightly different story.

I hope this makes sense!

James

[pmrid]

You can certainly use a filter wheel and focus each colour separately. Nothing wrong with that idea at all. If you are using filters, you might want to consider using narrowband filters (the usual trio are Hydrogen Alpha, Oxygen 3 and Sulphur 2) with the narrowest bandpass (width of the spectrum it will allow through). A bandpass of 5nanometers is fine although there are some available with even less). That way, even in the burbs, you can image deep space objects without having to be too bothered by street lights andn such.
Having said that, you might also want to think twice about buying a cheap achromat instead of a triplet. It is probably a false economy and you will quickly become frustrated by its limitations.
The things that will turn your imaging from ho-hum to whacko will be your guiding though. The mount will track reasonably well but for imaging you need to help the mount along by using a guide camera as well. These are either attached to a separate telescope mounted on top or beside the main scope, or they are built into an off-axis guider that does away with the weight, cost etc of a second scope.
My best suggestion is buy as good a triplet as you can now afford (they hold their resalem value well) and an off-axis guider and guide camera such as a QHY5 or similar.
Then you have to familiarise yorself with them and with the methods of aligning your telescope to the celestial south pole.
It's a learning curve but a rewarding one. And there will always be a heap of advice available in these fora.

Peter

[Poita]

The main thing I'd say is you won't save much as you would have to invest in a filter wheel and filters which gets pricey.
You could get the ED80 triplet in the classifieds here on IIS for under $600 and use your DSLRs to capture full colour images.
It will be way easier to learn by not starting out with mono, and way less frustrating, and uses the gear you already have.
Then if the astrophotography bug really bites you, you will still have a high grade scope, and can add filters, wheels, mono CCDs etc. at that point without having to upgrade the scope.

[matt42s]

Thanks for the replies everyone.
James - the passband was exactly the thought I was overlooking, thanks.
Poita - that's exactly what I did, bought the ED80 triplet :)