Combine data from slightly different telescopes?

[dx_ron]

I suspect I already know the answer to this, as I am aware that StarTools does not work well with mixing different exposure times, for example.

I have been imaging with a WO Z61 doublet, and have acquired an AT65EDQ (triplet objective with integrated flattener). The 65 has a slightly longer f/l (420mm vs 360mm) and is slightly slower (f/6.5 vs f/5.9).

I've been imaging IC 410 ("Tadpoles"), and what I have so far still has rather a lot of noise, particularly in the fainter Ha regions away from the "main" nebula. Should I wait to swap to the the new scope (ack! that's as bad as having to wait for Christmas day! ) until I am more satisfied with the result, or can I get away with registering and stacking subs from both scopes (with cropping to the smaller FoV of the 65, of course)?

[admin]

You can definitely combine data from different scopes, just not (well) for the purpose of luminance/detail (the noise signatures and point spread functions would be all over the place, limiting how much detail you can recover from the resulting stack).

So, for example, you can definitely use narrowband accents or color from one scope and combine it with luminance from another.

It depends on what your goals are. For example, if you are acquiring Ha right now for the purpose of making a SHO image, you could just acquire heaps of Ha now with the goal of using that for your luminance. You can then later add S-II and O-III purely for the coloring to make a nice SHO image that has all the right coloring, but whose detail is mainly driven by Ha (which is typically the case anyway in many objects).

If you are acquiring Ha for the purpose of augmenting a visual spectrum dataset, then you can use that Ha as narrowband accents, or even luminance for a future dataset. Just shoot RGB later with your new scope.

As of 1.8 you now have 3 parallel datastreams (luminance/detail, chrominance/color and narrowband accents) that do not influence each other - you can use (e.g. mix and match) whichever source for any 3 of those streams as long as you don't cross the streams and as long as they are aligned properly.

Hope that helps!

[dx_ron]

That helps! Not the answer I hoped for, but the one I expected. I am collecting OSC duo-band data, so I must choose between using the new scope vs gathering more data.

"more data" won tonight, but I doubt I will hold out too much longer.

[hixx]

Hi, your challenge will be in the registration step. Both scopes have different fields of view and you'll need to have the stars match up during preproduction before even loading stack in Star Tools. You need to register all lightframes against one master frame - Even using 3 different streams as advised by Ivo will require this.
with different optics I'd recommend using Astro Pixel Processor for stacking.
clear skies,
jochen

[Mike in Rancho]

Ivo, could you expound on that more from above re the PSF? Not that I fully understand PSF other than Airy Disk (barely understand that too!) and diffraction...

There have actually been a few topics/questions bouncing around lately that are similar, users wanting to stack combinations not just of multiple nights, but different telescopes and cameras too!

I have not yet done such a thing, but I do have two telescopes now... I am pretty certain that, if anything, I would only consider such usage for enhancements via layering perhaps.

However, wouldn't a casserole of different cameras and optics lead to stacking trouble for the rejection algorithms, let alone what it might do to the color channels if different OSC cameras have differing CFA's and response curves? I mean, you'd get a result, assuming the stacker can scale and register, but what would that result be? Almost makes me wonder why some stackers even offer that, unless I am missing something.

But really, the PSF's. Is that just something used by deconvolution? And so the mixing of the optics would not allow a proper reverse model to be created?

[Burly]

Registar software can apparently combine data from different setups apparently https://aurigaimaging.com/index.php

[admin]

Ivo, could you expound on that more from above re the PSF? Not that I fully understand PSF other than Airy Disk (barely understand that too!) and diffraction...

A Point Spread Function is (in this case) just a fancy name for a "blur function" or "the-way-a-point-light-is-being-blurred-by-the-atmosphere-and-optical-train".
Ideally, you would have blur functions that stay the same across all frames, so when you stack them, they stay the same in the final stack. This then allows you to model one point spread function for the whole image, and (when doing multi-band color or NB images), you will have much fewer issues with compositing (chiefly color fringing), because the stellar profiles perfectly overlap.

The fancier the alignment and rejection algorithm, the more crucial it tend to become to keep PSFs roughly the same as well. More-blurred stars vs less blurred-stars, can make throw off rejection algorithms. A more-blurred stars can spread its light across a larger area, whereas a less-blurred stars will over-expose quicker, as it is "better" concentrated into a point light. And, as we know, over-exposed areas of an image contain no usable data/measurements.

[Mike in Rancho]

Thanks Ivo.

On its surface it seems a reasonable idea - if a stacker can identically scale the images to each other, might as well toss them all together to aggregate more time and get improved SNR. But if rejection goes awry and modeling is thrown off such that the stars can't clarify properly through deconvolution, I suppose that affects the fine detail also.

But that's for mashing together different optics. However it then occurs that some form of this could also happen when doing multi-session even with the same rig, if the seeing conditions are widely varied from night to night.

[hixx]

Ideally, you would have blur functions that stay the same across all frames, so when you stack them, they stay the same in the final stack.

I think, in reality this is even more complex. The PSF is not constant in all lightframes but varies varies over time, even when using the same camera and optics within one session. Hence, the PSF will be different throughout time, between the frames, even within one frame, changing every few seconds. But there will be one resulting (spatially variant) integrated PSF. So in any case, Integration will produce the resulting PSF, no matter the individual lightframe's PSF's. Of course it helps to reject frames with bad FWHM etc, so only the "best" PSFs will be stacked.
Hence even if using lightframes from different optics, all we get is a resulting PSF which might be more complicated but should be a good target for SVDecon.
I hope I didn't miss anything
cheers,
jochen

[dx_ron]

I'm back with a variation on the same question.

Same telescope - RC6 with AP CCDTV67 reducer. I rearranged the optical train a bit, and wound up maybe a mm short compared to the spacing I had been using. The plate-solved focal length previously had been 877mm, last night was 894mm. OK to combine?