[Peter Ward]

From my short time on ISS, I have to say it is an very enthusiastic group, polite, but often not very critical. By critical I am not suggesting you go around calling someone a boof-head!

When I look at a deep sky image the things I like to see are, excellent focus (this is the one thing you can nail every time), excellent tracking (i.e. round stars) accurate colour balance.
There are other aspects, but for now the above three really are essential to a good image.

As a case in point you would not be happy with a picture of your family/dog/cat that was out of focus, blurred and had a strong green colour cast from having the wrong white balance setting.

Focus and tracking are self evident. But getting the colour right on an object you can not physically see in colour with a warm human eye at the eyepiece is a little more tricky.

Happily there are many professional images out there, for example the treasure-trove of David Malin images taken with the AAT. A good deal of effort has been made to get the colour right in these images and can be considered as the gold standard of colour accuracy.

A case in point, the Tarantula Nebula is not blue, or cyan as rendered by many DSLR’s.

It’s RED.

Hence to post an image of it looking blue, unless you have used a narrow-band Hubble telescope like filter set, is just plain wrong. So how do you get the colour right?
After all, images you take in daylight look spot on.

The problem stems from the internal I/R cut-off filter used by most DSLR manufacturers, which only passes around 20% of the critical Hydrogen Alpha light in which most nebulae glow. As a result emissions from elsewhere in the spectrum will swamp what little red data is being let through. So you end up with a blue-green spider. There are 3rd party companies that will replace this filter for a more suitable one. Canon also made the 20Da for a time which has a factory fitted version.

Modifying a not inexpensive DSLR would make many hesitant. However armed with the knowledge your image data has very little deep red signal, you can use software like Photoshop to boost the curve of the red data and once again approximate reality. If you have not already done so, I suggest you give it a go. You may be surprised by the results!