Good work Doug...and I thought I was the only insane individual that enjoys rental scopes around here.
Hybrid composites, i.e. the matching of data from different focal lengths can be challenging, though it depends on what you're trying to achieve. At the focal lengths you're talking about 280mm for RGB and 530mm for Ha, the differences are not large so IMO not too difficult. Your woes of different FOVs can be addressed. I've matched RGB data against 5 to 6 times the focal length with good results i.e. 530mm RGB against 2900mm Luminance. I believe I've provided information in previous posts on these forums. Anyway, here are a few pointers;
- I'm not sure on the tool you're using to align/register the data from the different images. Most image processing software will handle the task relatively well, but one tool stands out from the crowd - Registar. It only does a few things, but does them exceptionally well.
- The quality of the data with dictate how far you can upscale it to match. Poor data upscaled delivers mottled noise, but usually can be handled with low frequency noise reduction techniques.
- Don't upscale the luminance, you'll loose resolution in the process.
- Down scaling the luminance to match the RGB is extremely effective delivering impressive resolution. This works really well if you've got a wide field RGB, but the extend object is lacking "punch". Down scaling a higher resolution luminance i.e. taken at a longer focal length of the extend object will improve its quality.
- You'll probably find it easier if you match a low resolution RGB to a higher resolution luminance, if you down scale the luminance first, tweak and flatten to produce a "SuperRGB" layer, then reintroduce the luminance. This assists in managing the overlay of colours (and also provide the opportunity to performed additional colour tweaks such as dodging, etc.)
- One of the biggest challenges of hybrid composites is matching the stars. Upscaled data causes the stars to bloat. For RGB data, this isn't a huge issue, but you can end up with halos. With an LRGB composite, all the details including star sizes are coming from the luminance, thus you can safely use the minimum filter on the RGB layer to control the halos. This will not result in nasty deformed stars as we're not applying it to the luminance. For large, bright stars in the FOV, the star pinch technique will do a better job. Though for wide field images, the minimum filter will suffice.
- Conversely, depending on what data you're matching, it can also help to use the maximum filter to introduce more colour to the stars, giving them a more pronounced profile.
I admire your efforts on this. Thinking outside the square and pushing boundaries. I like what you've come up with. HaRGB blends can be complex even with data acquired at the same focal length. Keep at it and you'll nail it.
