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To put that into perspective - ~20µm of spot diagram is 6px across - without influence of seeing / tracking. In comparison 90mm of aperture at F/4.4 has Airy disk diameter of 5.5µm - or x4 less than geometric radius. Star shapes are not the issue here - it is too small pixel size. If you reduce image by factor of x2 - then you get rather nicer looking stars. These seem ok, right? That is far corner of ASI2600MC image

No, I used ASI178mc and Skymax102 maksutov telescope. Yes, it was mosaic, I think at least 6 panels. I use Autostakkert!3 for stacking - as it is easiest to use and gives best results. Other than that - Pipp (which is planetary imaging pre processor - handy tool). Registax wavelet sharpening and Gimp for touch-ups In this particular instance I used Microsoft ICE for stitching panels since I used AltAz mount and there was some field rotation between panels, but often I use ImageJ and its plugins for mosaic stitching.

That was taken with Atik314L+ which is 11mm diagonal sensor - even smaller than ASI183

It really depends on context. If we are talking about planetary sharpness in refractors then I'll say that Takahashi is probably justified, but that kind of sharpness is useless in context of imaging and you'll be hard pressed to tell the difference between Strehl 0.8 and Strehl 0.98 scope in long exposure images because there seeing and mount dominate. If we are talking about F/7 doublet scope - then yes, if you want to have visually CA free image - you need FPL-53 as for example FPL-51 simply won't be able to do it at that focal ratio. I would not mind having FPL-51 F/7 doublet for visual as I'm not that much sensitive to CA for visual - but that is different from imaging. If we are talking about color correction - then yes, triplet over doublet. There are very good doublets - but with triplet good color correction is much easier to achieve. Again - there are exceptions and I'm certain that there are triplet scopes that don't have good enough color correction even if they are triplets. That is probably most sensible option for that focal length - and you might even find that you don't need to stop down that scope - that it is sharp enough for you as is.

That is why I asked about your budget. With 4/3 sensor - you can use 1.25" filters - so that reduces cost as well. There are three popular choices for 4/3 sensor: https://www.firstlightoptics.com/zwo-cameras/zwo-asi-294mm-pro-usb-30-cooled-mono-camera.html https://www.firstlightoptics.com/zwo-cameras/zwo-asi1600mm-pro-usb-3-mono-camera.html https://www.firstlightoptics.com/atik-cameras/atik-383l-plus.html I personally have ASI1600mm camera and I'm happy with it, although some people report having microlensing issues with it. It seems to depend on F/ratio of the system and optical components and their spacing (filters, flatteners and such). ASI294 is a bit better - a bit larger, better QE, no issues with microlenses - but it is a bit more expensive. KAF8300 is old CCD technology that requires really long exposures - but is otherwise as capable in producing excellent images. If that is still expensive, then you are only left with this option: https://www.firstlightoptics.com/zwo-cameras/zwo-asi-183mm-pro-usb-3-cooled-mono-camera.html but that is only 16mm diagonal. It also has rather small pixels so you'll need to bin your data after acquisition - that is really not big issue once you get used to it. I do it all the time on my long focal length scope and ASI1600. And those would be really all sensible options. I would personally advise you not to go with non cooled models as cooling is essential for proper calibration.

On that page you linked that describes the product - there is example image of M31 taken with that setup. Don't know much about that scope - but according to this recent discussion: It might need Astronomik L3 as well.

It is good if you don't care about star shapes. There is better option - but not in focal length you want. Celestron has its own model called RASA. https://www.firstlightoptics.com/optical-tube-assemblies/celestron-rowe-ackermann-astrograph-rasa-8.html

If you use larger chip then yes, you want flattener - with or without reduction. These scopes have curved focal plane and sensor is of course flat - which means that odds are, corners are going to be out of focus and show bloated and astigmatic stars on larger sensors (smaller sensors will be enough in focus for this not to happen). Only problem that I see with said reducer flattener is back focus. Sensor needs to be mounted 55mm from back thread of flattener. This does not leave much room for all accessories like filter wheel, rotator, OAG if you plan to use any of them (well, you do plan to use filter wheel but that will use up only 20-30mm of optical path - so you are good there unless you need OAG and or rotator). Btw there is no such thing as standard 42mm filter thread - there is T2 thread which is 42mm and there is 2" or M48 thread and 1.25" or M28 thread which are filter threads This seems to be very good camera: https://www.firstlightoptics.com/zwo-cameras/zwo-asi-2600mm-pro-usb-30-cooled-mono-camera.html But its relatively new and there is not much feedback on it - so others might step here with relevant information. You'll also need filters of course - probably 36mm unmounted with that sensor size, and suitable filter wheel.

If you don't care about speed - then just get 60mm F/6 doublet with focal reducer and if you happen to have any issues with chromatic blur - then stop it down to say 40mm - that will sort out chromatic blur. Chromatism in telescope is also factor of aperture size - if you stop down aperture you reduce color issues. You can stop the lens down in the same way you stop down a telescope - by aperture mask. If aperture mask is round - you won't get spikes. If you like your ST80 + reducer that much - why not use it? just stop it down to say 40-50mm and it will reduce CA problems significantly. This image was taken with ST102 and 66mm aperture mask (I cut aperture mask by hand so it was not perfect circle - that is why there are bunch of tiny spikes on that sharp star - but you can 3d print proper mask or have it cut for you).

You already got answer about that focal reducer back in the thread: There are couple of things I could object to that image - but it has no CA issues. You need to distinguish between atmospheric / optics scatter or any filter reflections and CA induced halos. What that scope / lens has is - lateral chromatism - that is not good either. For example - look at this part of the image: It looks like blue channel is further away then red - like stars are smeared. Let's try that technique where we inspect mono image. What do you think of star quality here?

Quintuplet is just triplet with built in field flattener / reducer. Either get triplet and separate field flattener or if you want all in one package without the need to get the spacing right and all of that - get quintuplet. Type of glass does not impact quality of colors. If you have triplet scope - it does not matter much if glass used is FPL-51 or FPL-53 or FPL-55 or something totally different - if telescope is good - it will be good regardless of the glass type. Simple fast achromat will have other issues - astigmatism, spherical and such. How sharp it is will depend on host of factors. There is a good chance that good ed doublet or triplet will be better scope even for Ha than simple achromat - but sometimes simple achromat if optimized can perform equally well on Ha. If you don't know how to test it or how to optimize it - then it is lottery if you get decent enough fast achromat to be useful for Ha imaging. Just forget about such aggressive reduction. If you want to image on those focal lengths and that sort of speeds - just look at camera lens instead.

I guess it is up to you to decide. Mind you, if you are not going to shoot color with this scope, only luminance - then you can judge if image is acceptable for you - by converting it to gray and just looking at it and deciding based on star shapes and overall sharpness. If you for example find this image to be sufficiently good for your taste: Then you don't have to worry about slight residual CA at all.

Budget, mono + filters vs OSC preference?

Sorry that I was not more helpful with that - if you can find, ideally you want small 60mm triplet, but if you can't find one - look at images taken with OSC cameras and 60mm doublets without filters and choose telescope that gives best/sharpest images without any trace of blue halo around stars.

With Ha filter - you can pretty much use any decent scope as that is single wavelength of light and as it is single wavelength of light - it will always be in focus with itself - provided that you focus it properly. With Luminance filter - you have a problem. You are trying to shoot many wavelengths of light at the same time. With doublet scope - only 2 of them will be in focus at the same time, and with triplet scope - only three of them. Advantage that triplet scope has over doublet is that other wavelengths will be only slightly defocused - not enough to make blur, but with doublet scope, it can happen that some of wavelengths are so out of focus that they create blur or even halo. Look at this graph: This is focus position versus wavelength for doublet type scope. At any time you can have two wavelengths at zero position. All other colors of the rainbow (or wavelengths) are slightly out of focus - and further towards red or violet part of spectrum you move - more defocused they become. This is same graph for triplet: It also has some wavelengths being out of focus - but out of focus is much smaller for triplet than for doublet and also, because this curve crosses focus line 3 times - most of the wavelengths are only very slightly defocused. This means that you will have better / sharper image without halos with triplet than with doublet - when shooting Luminance - as lumanace records all those wavelengths at the same time.

Fact that there is gap between 550nm and 650nm is irrelevant for my reply on blue halo with 4" f/7 doublet scope and use of filter.

Luminance is no different than shooting with color camera - all the wavelengths are recorded at the same time. In case of mono + luminance - they all end up in gray value of the pixel in case of color camera - they get distributed between pixels with different filter on them. You would get the same if you used mono + color filters - and shoot without focus change on each filter and then add all channels together. Only advantage that mono + filters has is ability to refocus on each filter (it has other advantages - but in this context only one is that one).

It does eat into violet part of spectrum a bit - so might be part of explanation

Yes, guess what this is: Here is the answer:

Even singlet lens can be great with Ha filter. Ha is single wavelength of light and it will be in focus by itself - regardless of the lens type as far as lens optically decent (not much spherical aberration or similar). L filter is the worst as it covers whole spectrum from 400 to 700nm. It will include all wavelengths and it will show any defocus in wavelengths as either increase in FWHM or even halo around bright stars.

Indeed - I was surprised as well to see that level of CA as well. I do know that some images taken with ED80 also show slight CA halo and it is also FPL-53 and smaller objective lens and slower than 102 F/7 scope. I do wonder if filters have something to do with that? I know that proper filters can remove CA blur even with not so well corrected scopes - that is why there are Astronomik L2 and L3 filters.

Your image really is a beauty. I just love that level of sharpness and detail in an image.

What is confusing you about the speed? I believe that scope will be very good at DSO AP - wide field or otherwise. I mentioned above 102 F/7 scope as it is doublet with claimed excellent correction. I believe that to be true for visual - but not for imaging. I ran across that issue few times now - people having that scope (or same optics from another vendor) and results are not looking nice in some cases, for example see here: Or if you don't want to go thru the thread - here is exact example: Look at those stars and blue halo ... Many people will not see this and will happily use their scope simply because they shoot mono + filters or do narrow band. This was taken with OSC sensor - where one shoots all wavelengths at the same time and any difference in focus is not compensated between filters (like in mono + filters). Granted, above can be corrected with use of say Astronomik L3 luminance filters which cut furthest parts of spectrum - and that is fine for the price - you get 4" scope for half or third of the price of 4" triplet - but there are some things that you need to circumvent. On the topic of speed of scopes - well, speed of optics - like F/ratio really does not have to do much with speed of image acquisition. "Slow" scopes can image fast and "fast" scopes can be slow when imaging. In fact - it is better to have slow scope in terms of F/ratio as aberrations are much easier to control with slow scopes. Less issues with filters / reflections / edge correction and such. Just make sure that that you pair your scope with proper pixel and sensor size. Best definition of the speed is "aperture at resolution". You set your working resolution and then throw as much aperture as you can on it. Slow scopes - or rather scopes with larger focal length - have only one drawback, they don't record very large field at once - FOV is function of focal length and sensor size. You can do mosaics in order to go around that.

There is a limit imposed by physics of light and size of aperture on level of detail that you can record. For any given pixel size - there is maximum focal length that you can use on certain aperture. In most cases, limiting factor is atmosphere and if you want to shoot highest resolution images possible for a given telescope - you need to use lucky imaging approach. Again, in order to do that - you need very specific type of camera - dedicated planetary camera. For dedicated planetary camera, few things are important: - QE of sensor - low read noise - fast readout rate Pixel size is not important as you can use barlow lens to get required focal length for any pixel size. Moon does not have any significant color detail so you can use mono sensors that are more sensitive. ASI224 is excellent choice because it has great stats on above quoted three properties. It has good QE at around 80%, it has very low read noise and it can deliver extraordinary high FPS. Say you go for camera like ASI224 and you use 3.75µm pixel size. You then need ~ F/15 telescope or in your case x2.5 barlow. Use exposure length of about 5-6ms and get tens of thousands of frames. Stack the best 10-15% (judge how many based on quality of your frames) and apply some sort of frequency restoration process (say wavelet sharpening). This will get you very close to theoretical limit of 200mm aperture. Using that technique, I captured this image: http://serve.trimacka.net/astro/Forum/2020-07-30/moon.png (SGL no longer allows embedding non https images - so I'm providing link only - but you can view it in full resolution by enlarging it to 100%). Above image was taken with 100mm telescope. This means that you should be able to go twice higher resolution with 200mm telescope.

Yep, my bad - I did not read carefully, I thought that it was triplet lens. I don't really agree that Takahashi blows away that scope - but am happy to be corrected if you give me an example of that.