vlaiv

Not a can of worms - regular question and answer is simple - reflectance. https://en.wikipedia.org/wiki/Reflectance Most materials have dependence only on frequency (that is what we call matte color or diffuse reflection), but some have on other items as well, that is why we get specular highlights for example - dependence on angle of incidence. There are special types of material that even have highlights that depend on direction of light. That is why we have special "candy colors" popular in cars: This is actually the same color - that changes depending on observing angle. What color is this object then? (check out this short youtube video as well: https://www.youtube.com/watch?v=oqNNdCWs0c4) In any case - there is similar curve to filter response curve. Filter response curve specifies how much of each wavelength is passed thru while reflectance curve specifies how much (what percentage) of particular wavelength is reflected. If you take single light source and apply different filters to it - you'll get different color of light coming out at the other end. In the same way - use same light source to illuminate two different materials and reflected light will be of different spectrum. Formula is the same - light spectrum * reflectance curve (or in filter case: light spectrum * transmission curve). It is this reflected light that we see as color of object. This is why object appears to have different color under different lighting as base spectrum is different (but modulated with same reflectance curve). It is in effect light that we see as color and not object itself.

Of course I'll use OP image - the latest one. I don't know how photometric star colour calibration works in Siril - I'll have to examine that. I've looked at PixInsight feature called the same - and unfortunately it has nothing to do with actual color. It uses B-V index as color (which is also called stellar color) and calibrates against that. That is not useful if you want to make accurate images. By the way - by color accuracy - I mean following: Imagine daytime (or nighttime, it does not matter) scene that is illuminated by some sort of light and you take picture of it. I want to reproduce actual colors as they were and not "color balanced" version of the image. For example if you have a camp fire that is illuminating the scene and you have white piece of paper in that scene. I don't want that paper to end up being white in final image - but rather orange because it is illuminated by camp fire. Many people get confused by color balance / white balance in the whole story and that is not what we need here. White balance is way of showing objects as they would appear under what is common illumination to us. I maintain that object don't actually have color - they have color under certain illumination. It is light that has color - and as such it does not depend on illumination. But this is completely different topic. I'll post steps of my processing with emphasis on color handling.

I'll show you what I believe is "correct color processing", rather than correct processing of the image (don't want to get involved in discussion on "correct processing" of astronomical images ) - at least basic version of that - one star color calibration. Multiple star color calibration is much more involved and I would rather use dedicated software for that than do it "by hand". I would also rather have pre measured RAW to XYZ matrix for that - which I don't since I don't have that particular camera. Will post my processing in a few hours.

You could try barlow lens. F/2.8 is going to be extremely difficult on any eyepiece. I guess that only central part will be sharp and rest quite astigmatic. Barlow lens should clean up things. Maybe get x2.5 or x3 one. Don't get telecentric - I don't think that they extend focus position as much as regular barlow. Get barlow with longer focal length (if you can - look at the specs and avoid shorty barlows - get regular one). Even in that combination it is going to be very hard to get diagonal in place. Your best bet is prism diagonal as it has shortest light path - yet these don't do well on fast beams like F/5 - F/6 as you'll have with barlow. https://www.teleskop-express.de/shop/product_info.php/info/p9870_TS-Optics-T2-90--Star-Diagonal-Prism-with-28-mm-free-aperture-for-Observation-and-Photography.html That has only 40mm of optical path - but that is between T2 threads. 1.25" adapter is certainly going to add another 25-30mm so you'll have 70mm total not counting the barlow lens. You could try all of this - but I'm not sure it is even going to work, so if you like experimenting then ...

Indeed - it will act both as a dew shield but also as a light baffle. Due to design - faster Maks have a bit of light leak into baffle tube and that can cause subtle contrast issues on Lunar for example. Nearby light sources with light falling on corrector plate also cause light scatter. This image taken by member of cloudynights is rather telling:

Again - that really depends on a person using it. I can tell you my experience, but it is questionable if it will apply in your case and how much. I have Skymax 102mm and I purchased 6.7mm ES 82 eyepiece to use it as high power eyepiece in that scope. I also have 5.5mm ES 62mm which was primarily purchased for F/10 102mm achromat as high power eyepiece (but also used in my 8" F/6 dob). 5.5mm is simply too much for me - view is distinctly soft. This is not down to the scope - Mak is, I believe, sharper than my F/10 achromat. I'll do side by side comparison at some point, but that is the impression I have so far with limited time with both scopes. 6.7mm is again excellent eyepiece - and I feel is too soft really. I wish I went for 8.8mm ES 82 instead - and I'll probably add that one as well at some point. 11mm ES82 is exceptional EP - very sharp and I'm really happy with Mak102 and that one. I do feel that there is a bit more magnification to be squeezed out of the scope. I observe with only one eye that is fairly sharp - probably 20/20 or a bit sharper than that. Other has severe astigmatism that can't be corrected even with glasses. That one is all blurry. I don't wear eyeglasses (in daytime nor when observing - but started to wear them when reading recently - I was farsighted as a kid and presbyopia is starting to kick in). If it was me - I'd choose around 8mm to be my high power eyepiece. In fact - there is interesting eyepiece that I would like to pair with my Mak102. There aren't any reviews of it so I don't know exactly how good that eyepiece is - but on "paper" - it fits with F/12 - F/13 scope nicely: https://www.firstlightoptics.com/ovl-eyepieces/hyperflex-9-27mm-zoom.html That one + 32mm plossl eyepiece for widest views possible in such scope. You'll already have very decent 25mm in case narrow FOV at high focal length bothers you. For BSTs - I only heard good things and I think it is very good eyepiece. I would choose 8mm over 5mm as highest power EP.

@rojach78 On account of those dust particles - your flat correction seems to be working fine regardless of the fact we are seeing these. These just mean that dust has moved between lights and flats - or that you changed something to your setup between lights and flats. You should take flats in exact same setup as lights - even focus position. Don't rotate anything, don't adjust clamping, don't fix focus position - everything should remain exactly the same for flats as it was for lights. If you loose focus during the night and refocus while taking the lights (this can happen due to cool down if temperature drops over night) - use last good focus position - the way you finished your lights session and take flats straight away at the same ambient temperature.

I personally think it is wrong in how it is doing color processing in the images. I had quite extensive discussion with the author of StarTools here on SGL and we did not seem to come to conclusion. He is under impression that I've got it wrong and similarly, I'm believe he is wrong. I don't want to get into that sort of discussion again, I'm just going to say what I believe is wrong - there are no teal or greenish looking stars. Here is what main sequence stars look like in color: Or perhaps this image: This is so called Plankian locus - what sort of colors you can get from black body of a different temperature. Stars are very very similar in their spectrum to black body and hence will have very similar color to Plankian locus (in most cases indistinguishable to human eye). Good color correction for astro image should produce majority of stars with above colors. Having teal or greenish tint in stars is unrealistic. Mind you - we are talking here about accurate color representation - and not artistic side of things. If someone wants their stars to be pinkish and feels it is their artistic expression - then by all means, but I'm talking of color matching and accuracy.

Yes that is quite normal for raw data and it needs to be corrected. Simplest way is to assign weights to each channel. You can find F3 class star and make sure it is white - measure pixel values and scale each channel so that F3 class star is white.

That is interesting - auto color balance in StarTools usually produces that sort of colors, so I assumed you used that. In fact - I've seen what I believed to be StarTools processing in many of your images. I was obviously wrong, but it is interesting none the less.

I think that flats were taken and applied and did a good job - except for that one dust particle that moved between lights and flats - which is causing that emboss effect. I don't know - that is typical star tools coloring of the image. There are no teal stars out there, nor greenish ones.

There is one special case where it is beneficial to have guide scope and imaging scope point in same direction. This special case should be avoided - so in most cases it should not pose a problem if your guider is not perfectly aligned with the main scope. It is when either imaging or guiding low down close to horizon. Due to atmospheric influence - and rotation of the earth - apparent positions of the stars change when light is refracted thru a lot of atmosphere. Effective speed of stars is higher than sidereal when stars are setting and lower than sidereal when stars are rising. If you guide on star that moves at sidereal but image stars that are rising or setting - you'll get trailing. Similarly when imaging regular stars and guiding on setting or rising stars - you'll again get trailing. This of course is not a problem if you guide on the same star that you image - or when two scopes are aligned.

Focal reducers usually move focus point towards the telescope - if you use extension tubes - you might need to remove some of them. Also, be careful to get distance between focal reducer and sensor right - changing that distance also changes reduction factor - more distance you put between the two - larger reduction you get - which in turn means: 1. you'll need even more inward focuser travel 2. more aberrations can appear on outer parts of the image (you are trying to fit larger field onto same sensor size - and larger field might not be as well corrected).

Something is still not good with stacking. Two 400mb images have been drizzled for no good reason. You should not turn on drizzling. Also, when exporting data, don't apply stretch that Deep Sky Stacker makes: In above image: - Use 32bit format (that is OK - you did that) - Prefer Fits file format for linear data (like when posting here on SGL) as fits is standard for astronomical data (it keeps actual values and is easier to diagnose and work with in other software than PhotoShop and Gimp). - Use compression for TIFF (makes smaller files and nothing is lost - use ZIP) if you want to post TIFF - Embed but do not apply adjustments In any case - something is wrong with stacking. What stacking parameters did you use? Here is histogram of green channel data in Needle.TIF image: Histogram should not look like that and this implies that something wrong was used for stacking - maybe median stacking instead of sigma clip or similar?

I'm having trouble downloading those two 400mb versions - which should not be nearly that large. 3000x3000x3x4 = ~103mb sounds about right - anything over that is probably x2 drizzled (for some strange reason - one should not drizzle unless under sampled - and even then, it is questionable if it works as expected).

I've looked at CGX-L when it came out - and for some reason I just did not warm up to it. There is not much detail on how it performs (that I could find) - and to me, it looks more like 0.6" RMS mount than 0.3" RMS mount like iOptron CEM60/120. Maybe it is just my imagination, but I classify mounts by their guide performance and CEM60/120 are 0.3" RMS class mounts. HEQ5/EQ6/EQ8 are 0.8" RMS (stock) or 0.6" RMS (modded). Mesu is 0.2" RMS range. I could be very much mistaken - but those are sort of figures that are in my mind when someone mentions those mounts. For large aperture scopes it really is worth having mount at 0.3" RMS or below (of course - payload should be respected as well and it is always better to "overmount" then "undermount").

There is one more mount that is in the pipeline and looks very much like iOptron offerings. Not sure when it will be available, nor if it's going to be good budget option. https://www.primalucelab.com/world/equatorial/skywatcher-cq350-pro-synscan-mount.html although it does say that it is CEM60 replacement - so not much better than CEM70.

Depending on what type of DSLR you'll be using - with that scope you are petty much at the edge at prime focus. Say you use modern DSLR with around 4µm pixel size - with F/16 you are pretty much at critical sampling rate (with 3.75µm - it is F/15). That scope is F/12 so very close to critical sampling rate. In fact images close to critical sampling rate feel soft if you don't use special processing and sharpening (see lucky imaging and wavelet sharpening for example). You can use eyepiece projection to get more magnification - but it will be "empty" magnification without further detail. You can get same effect if you simply enlarge your image in software - it will be zoomed in but blurry. This happens due to physics of light and there is no way around it - except to get large scope. This really depends on how sharp your eyesight is. If you have sharp eyesight - you might find that using higher magnification does no yield sharper image. People with very sharp eyesight can happily use x1 aperture in mm - so around x120 is enough for them (your scope is effectively around 120mm - although it says 127mm). If you don't have that sharp vision (about quarter of people have less than 20/20 vision even with eyeglasses) then you can use eyepieces down to 6mm to get x2 per mm of aperture - or about x250 power. I think that x2 barlow is not needed for Maksutov type telescope as you can easily get decent short focal length eyepieces and eye relief is not issue with these scopes (secondary acts as barlow and makes eye relief longer).

CEM120 or Mesu as higher end offering. Since Mesu is way out budget wise - then I guess CEM120?

From what I've heard 115mm triplet does not need serious cool down time. Most people are able to use high power views after only half an hour of cool down. All the reviews and remarks on that particular model have been highly favorable.

No need to take the bias if you took darks that match in exposure length. One of the subs or multiple have either bad column kind of artifact or row of dead pixels or similar. Softer edges are due to subs alignment and bilinear interpolation that DSS uses. If it is only single sub - sigma clip stacking should get rid of it - but it can be present on all the subs - in that case you need to "interpolate" defective pixels.

It is usually best not to process data in DSS other than stacking. You can export your data from DSS in 32bit fits format and then process it in Gimp for example. If you like, you can export data and attach it here and people can give best tips on hot to get the processing right.

Excellent information given by @Budgie1 That is only the first step. You'll need to weigh your color information later ( ASI533 is very strong in green and images tend to have green cast that you must balance out). If you have issues with pale colors - it could be that DSS messed up things. There is option on background calibration and certain options tend to wash out colors: If you notice issues - try changing background calibration setting (but do pay attention that sigma clip stacking depends on frames being normalized - and you can get worse results if your LP levels change significantly during session and you don't normalize the background between frames).

You need to debayer your images as part of your workflow. What software are you using for stacking and what is your workflow?

Not sure if it is the case: