Focal ratio vs colour intensity

[casstony]

I just recently started playing with images again and found that the NP101 at f/5.4 produces much more red in the stacked image than my FC100 at f/6 with WO 0.8x reducer.

The two images are cropped parts of Carina, both 30 x 2 minutes, taken with a Nikon D5600, ISO 400 and IDAS P2.
The redder image is the NP (also softer because of crappy weather conditions).

Is the extra red simply due to the faster f ratio (f5.4 vs f6) or is it because of the different telescopes/reducer?

[traveller (Bo)]

Very interesting Tony.
Assuming the colour balance didn’t change between the two shots, my uneducated guess is it’s more to do with coating on the different glass surfaces; glass v flourite lenses and number of glass surfaces along the light path.
Edit, saw you note about the seeing condition, that would also contribute to the image because of the scattering of light?
Bo

[casstony]

Apart from stacking, the only thing done to the images was to adjust levels to remove pollution.

I've got an old NP image of Carina with a 600d but it's also more red. Also images from an Esprit 80 f/5 were more red. So I'm thinking it's either the different f ratio or different glass/coatings.

Next clear night I might try the FC100 without a reducer: in comparison to the existing FC image, a more faded look would point to f-ratio while more red would point to the reducer robbing colour.

[Atmos (Colin)]

It looks like the colour balance is different between them. The first one looks correct and is a deeper colour and the larger stars are bluer. In the second more orange image the same stars appear white.

It looks like the colour balance is the main culprit between them.

[casstony]

What does colour balance mean and how might it change? Its not something I've intentionally adjusted. Keep in mind I'm a beginner especially where processing is concerned.

[casstony]

You had me doubting myself there Colin so I restacked and removed the light pollution but the FC image turned out the same - washed out.
I haven't figured out how to get the image down to 200kb without massacring it but here's most of the FC image.

[rcheshire (Rowland)]

Here

In StarTools. It's a very rough attempt and not a very good result. But a lot redder. Check out the histogram.

Can you upload the raw image to dropbox. It would be interesting to play with it.

[casstony]

Thanks Rowland. I'm aware that I can adjust the colour, but I'm curious why one set of optics is obviously more sensitive to red. I'm planning to eventually sell one of my refractors and if one gives better colour straight to the camera I might as well keep that scope. Good thing the NP didn't sell recently or I would not have seen the difference.

I'll try the FC100 with no reducer when I get a chance and that might shed some light.

[rcheshire (Rowland)]

Originally Posted by casstony

Thanks Rowland. I'm aware that I can adjust the colour, but I'm curious why one set of optics is obviously more sensitive to red. I'm planning to eventually sell one of my refractors and if one gives better colour straight to the camera I might as well keep that scope. Good thing the NP didn't sell recently or I would not have seen the difference.

I'll try the FC100 with no reducer when I get a chance and that might shed some light.

No worries. But it would be good to have all three channels aligned or nearly so and properly saturated.

If you separate the image into L R G B channels and combine only the R G B, the channels are more closely aligned... as in the image..

Interesting... I notice the brown effect when combining an additional L channel to an RGB image (to increase saturation while having better control over chroma noise).


EDIT: And on reflection, I have the same problem when using a reducer with my FS60CB, with a DSLR. But only with some images.


EDIT: Comparing stars between images, your brown image luminance is stronger than the red image. The only change to your set up is the addition of a reducer from what I can tell. So aperture is a factor..

[casstony]

Originally Posted by rcheshire

But it would be good to have all three channels aligned or nearly so and properly saturated.

Thanks for your comments in general Rowland and I'll keep in mind what you've said about the different channels with future images.

[casstony]

Did Eta again tonight straight through the FC100dc, no reducer, and the image is still washed out looking (before any adjustments to colour) - so maybe the FC simply doesn't provide as accurate colours to the camera as the NP101?