Telescope Photography question

[adebear]

Hi,
This is my first post here :-)
I have just recently purchased a ED80 + EQ5 mount and love them.. I have taken a few shots of jupiter (shown as my avatar) and it came out fairly well, if not small.. it did get me thinking though.

these cheap achro telescopes ie 150mm skywatcher's ..
if your taking shots with a B&W with colour filters and use a micro-focuser..
the fact that the different wavelengths don't focus at the same place wouldn't be an issue if you re-focus between each filter.. is that right or is there something I am missing..

just looks like if you spend the money on a good B&W and a good micro-focuser you wouldn't have to fork out $1000's for a really good scope?

really good forum BTW...

thanks,

Adrian

[jjjnettie]

Good question Adrian.
Welcome to IIS.

[little col]

welcome to iis adrian and enjoy your stay
colin

[iceman]

Hi Adrian, welcome to IIS!

What do you primarily want to take photos of? Deep space objects or planets/moon?

What's your budget?

[rmcpb]

Welcome to IIS Adrian :)

As for your question you are basically right. The chromatic abberation of the "cheaper" scopes is only a problem if you are viewing or photographing in the complete spectrum. Using one wavelength at a time would get around the problem. Maybe not the best BUT if it allows you to get going within your budget then its a goer. BUT make sure that your colour filters are really good ones though!

Cheers

[adebear]

well I was looking at about $1000 for the focuser, about $600 (prob Canon 300) for the camera..about $4-500 on good RGBL + auto changer
but a really good corrected refractor is going to be about $5-6K.. and that will take a while to save up for.. so I figured spend $900 on a 150mm skywatcher and the laptop can auto re-focus between shots...

infact I plan on using the toucam as an autotracker.. I plan on setting the scope up and letting it go.. it's too damn cold in winter in Canberra :P

[Doug]

G'day Adrian, Why not stay with your ed80? It is a good scope and while not as fast as some larger optical systems it is capable of good things. If you want a longer f ratio, invest in a good powermate or barlow. One thing to keep in mind is that a 150mm Acro is going to set you back 3~4k anyway.
for around 3K you could get yourself a 127mm Meade ED OTA and mount that on your EQ5. If your EQ5 is not a goto model then you could upgrade with the money saved by settling for a 127mm ED. Refocusing for each colour is as has been said practical, but it is also a pain in the thumb. Another thing to consider is that if you do invest in a 300d body, this is a one-shot colour platform and the money spent on filters ans a filter wheel or such will be wasted.
In summary, learn to wring the most out of your ED 80, concentrate on getting something like a canon 300D or 350D body, get that EQ5 upgraded to a Goto if it is not already there.
Then when you have that equipment mastered decide what road into the future you wish to travel.

I can also recommend the Lumicon easy Guider for mounting your web-cam guider.

cheers,
Doug

[Doug]

Another point to consider; if you're planning on getting a 150mm f8 OTA to swing on your EQ5 it will be a bit dodgy for accurate astro-imaging. I'm assuming f8, a shorter f6 or a mac should be ok but at f8 you might be disapointed with the end result. The 3~4K I mentioned was with the heavier EQ6 mount.

[Terry B]

I agree that it should be possible to refocus the scope to get around the achro image but I have tried it with mixed results. I have a 120 x 600mm achro as a guide scope but have tried to image through it. See http://www.iceinspace.com.au/forum/showthread.php?t=20710
As you can see, I was still unable to get the blue image to come to sharp focus. Having read some more since these images were taken I think the problem is that the filters leak somewhat into the violet and near UV. The camera is not very sensitive at those wavelengths but still enough to give you some ghosting. I havent tried yet but adding on of the minus voilet type filters should help but this is adding further glass to produce reflections ets.
If you use narrow band filters then there should be no problem but they are expensive and I don't own any.
With Jupiter you also need to be pretty quick taking the separate colour exposures as Jupiter rotates pretty fast. I don't own a filter wheel and have to manually change the filters. This would take too long.
The way around it of course is not to use a refractor but a scope with mirrors for your imaging.

[Stephen65]

Just musing on this - theoretically if you used a filter that excluded everything but the exact wavelength you were focusing to then you could get exact focus on that wavelength despite chromatic aberration.

But...that would have be a very narrow band filter and you'd be excluding all the light between the wavelengths you chose to image so you would probably end up with a very dim image. On the other hand, if you made the filters a bit wider to let in more light around each wavelength then the range of wavelengths you allowed in wouldn't have the exact same focal lengths either so you would be trading away sharpness of focusing for brightness.