M33 and star colour.

[decoyp]

I've been wanting to image M33 for a while and have only had brief chances to get any time on it at all. Last week I managed to get an hours worth of exposure at ISO 800 using a Canon 300d with a 500mm telephoto M42 thread - (Image attached). The optics are not great - the lense was very cheap and I'm starting to realise why. But my main gripe with the image is the loss of star colour. I know there are a few other issues. Perhaps I should have used a lower ISO to reduce grain but that would increase the subs and I am relying on my mount (HEQ5) to do the buisness for 3 minutes. I think it looks OK but I have stretched the contrast a lot. Any tips would be helpful. I know the lense is not the best, but I've a wife and 2 kids to support ..... Blah Blah drone drone

How do you guys retain the star colours with DSLR's ??

Thanks

Richard

[mint_mark]

I think the image is good, but the stretching has made a bit of a gradient around the galaxy.

As for star colour, I hope some experts will be along soon with a proper answer. One explanation I have read goes like this... all stars are points, so when perfectly in focus through a good lens all the light is concentrated into a tiny dot. Over a long exposure, that dot blows out to maximum intensity in all channels and the dot ends up as white. Worse than that, if the lens is not perfect then around the dot you get coloured halos resulting from aberrations in the lens.

I get red halos and sometimes purple form camera lenses, but not from my small telescope. I think it's because even at best focus the stars are bigger (in pixel terms) from the telescope compared to the lenses, probably due to the longer focal length (magnifying atmospheric scattering?). I need to study the images more to see if that theory is correct.

Anyway, two possible solutions... use something called a fog filter, which blurs the stars a little. The outer areas of the blurred star will not be blown out and should preserve the colour. Alternatively, take a few images with the lens slightly defocussed. I haven't tried either method yet.

[decoyp]

Thanks MArk. I'll give your suggestion a try. It's a bit frustrating I think I have a lot to learn. I've just taken a look at the recent image of M33 by Dave Moulton in this forum. It looks as though his setup is DSLR based as well. He's using a 350d but its not too different to mine however the final images are worlds appart in quality.

I have a feeling its not just the lense that's letting me down I think my processing skills are not up to it either. I'm going to try processing the RAW files again and then stack them manually in Photo Plus (Sans serif) its a free package and its all I've got at the mo'. I'm not sure DSS is giving me a fighting chance or perhaps it's the way I'm using it.

I'm going to ask Dave what he did to achieve his result .

[skye at night]

How do you guys retain the star colours with DSLR's ??

Thanks

Richard

Hello,

If you are referring to the 'pink' colour from the amazing nebular areas ... they need not only quite a lot of exposure (some folks spend 6,7,8 hours or more on M33) to be properly 'picked up'...

also your camera (if not modified) is rejecting most of the important wavelengths of 'red' that you need to put the colour into your image...

It's good though, never been brave enough yet to commit myself to time on M33.

Steve

[decoyp]

Steve,

Its not only the pink areas that I can't get but I tend to lose star colour completely and end up with just Blue stars. It does look as though Dave's 350d has been modded for Ha. How much does this cost and can you still use your DSLR for conventional photography after the mod ? I tend to do a bit of both types of photography as I suppose most others folk do too.

Richard

[skye at night]

Steve,

Its not only the pink areas that I can't get but I tend to lose star colour completely and end up with just Blue stars. It does look as though Dave's 350d has been modded for Ha. How much does this cost and can you still use your DSLR for conventional photography after the mod ? I tend to do a bit of both types of photography as I suppose most others folk do too.

Richard

You can modify yourself or send your camera off to be done OR, buy a pre-modified one.. so cost can be from a few tens of pounds for the replacement filter to 'lots'...

Might be worth (if funds allow) maybe picking up a 1000d and modifying your 300! Might seem expensive, but astrophotography IS.. FACT!

Modifying your 300d yourself would be the cheapest option..OR

longer and more exposures.. the data you need is there. just being filtered, so upping the exposure will help to some extent.

Steve

Many folks just modify an older 300 or 350d (even the 1000d) to just use as a dedicated astro-camera...

The canon filter can be removed and left unreplaced, but makes the camera a no-go for daytime...

Replacing the filter correctly can keep the lens to sensor spacing right so you can continue to use lenses, but the white balance needs to be adjusted or a further filter can be used for daytime to adjust for the 'red' that will now be entering the images...