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I can think of one reason why it might perform differently. Field curvature of focal planes of both scope and eyepiece. F/7 triplet and F/7 Newtonian are likely to have different radius of curvature (although that depends on focal length of scope and not F/ratio - I'm assuming regular / available scopes that will have somewhere between 500mm and 1500mm of FL in both cases). In one case field curvature of EP can match that of the scope and as a result - no FC will be present, but that would mean that it won't match the other scope as refractors and newtonians of roughly the same FL will have very different radii (for refractor it is about 1/3 of FL while for newtonian it is equal to FL, if I'm not mistaken).

I have 50mm one and 30mm one that are currently surplus - both unscrew easily. 50mm one has two threads - one that removes eyepiece and one that removes complete back plate.

There is almost exact replacement refractor scope and at the price you are willing to pay. https://www.firstlightoptics.com/startravel/skywatcher-startravel-120t-ota.html Although this scope has "only" 120mm of aperture, in reality it is very similar to 130PDS in light gathering ability. This is mainly due to the fact that 130PDS is reflector with central obstruction. We can do simple math to see what is "equivalent clear aperture" of 130PDS. We can take 94% reflectivity per mirror and 30% secondary obstruction. (65 * 65 - (0.3*65) * (0.3*65) ) * 0.94 * 0.94 = 3397.2211 sqrt( 3397.2211 ) *2 = ~ 116.57mm So equivalent clear aperture would be 116.57mm 120mm on the other hand has 4 glass/air surfaces, and if multicoated - every one of those will have transmission of about 99.5%, so we have 120 * 0.995 * 0.995 * 0.995 * 0.995 = ~ 117.62 of clear aperture So they are equivalent in light gathering, and this scope has 600mm of focal length which is very very close to 650mm FL of 130PDS. Obviously, only problem will be chromatic aberration, so odds are you won't like your images, but this is only solution to sub £300 equation. Here is very good review of this scope by Ed Ting (btw, he started a YouTube channel, and I think it is certainly worth checking out) - Orion version (same as SkyWatcher): Btw, there is another similar scope form Bresser that is a bit more expensive (but probably has better focuser): https://www.firstlightoptics.com/bresser-telescopes/bresser-messier-ar-127s-635-refractor-ota.html

I think I'll just make sensible decision and get TS WA 38 (or rather local supplied clone which somehow looks like TS version but is sold as SkyWatcher brand - don't ask) as this will really be my M31 eyepiece in 4" refractor. For the time being I'll use it at F/10 and after assessing it at F/6-F/7 will decide if I want to keep it when I get faster 4" scope. If these work well at F/8 - then if I really miss edge performance - I'll make aperture mask for F/7 scope to bring it down from 102mm to ~90mm thus creating F/8 - for those moments when I want to marvel stars at the edge of the field. In fact, I might keep both F/10 and get F/7. I'm still yet to do all the things that I planed to do with F/10 and once I do Roddier test on it, if it is good in Ha - it will be delegated to Ha Solar role (again at some point in the future).

Would that still hold at F/7 and could we possibly add another criteria - light weight - what would be options then?

There is so much conflicting reports on these 2" large AFOV eyepieces. I want something with 46mm of field stop - to get maximum sky and I don't really care much about focal length - as long as it is up to 40-42mm. I don't want overly large exit pupil. EP will be used at F/10 and possibly F/7 scope. Currently, this is candidate number one: https://www.teleskop-express.de/shop/product_info.php/info/p957_TS-Optics-38-mm--2----70--Wide-Angle-Eyepiece.html (being the same as SW Panaview 38mm - maybe a bit lighter). Problem is of course that I keep reading: "It is ok in F/7" and "It is not good even at F/7" - all over the place . I'm fairly sure it will be acceptable at F/10, and I'm also aware that Aero ED will be better corrected - problem of course being that Aero ED is no longer available in 40mm FL - only 35mm and that has 10% smaller FOV if I'm not mistaken - about 42-43mm, right? Further problem is that some of these reports depend on people using short FL refractor scopes - sometimes having more issues due to field curvature of the scope rather than correction of the eyepiece. Other times it is newtonian scopes that have coma.

I think this is rather good for only 15x10s with respect to sampling rate.

I'm sorry if I mislead you in thinking that you need better mount for EEVA. I was just pointing out that if you want to achieve certain resolution in your images - and by term resolution I mean sharp enough image at your wanted pixel scale - then you need certain gear / sky conditions. What I failed to emphasize is that for good EEVA - you don't need high resolution, and if you revisit what I've written - you can achieve that with very basic mount and short exposures. This is where I would start off - by keeping AzGTI - even in AZ mode, although you can use it in EQ mode - and switch is rather simple to do (involves flashing suitable firmware, getting wedge and counter weight shaft). Question then is rather - how to achieve 3-4"/px resolution rather than what is suitable larger mount. Just as a side note, ASI294 with 130PDS in super pixel debayering mode will have ~3"/px, and so will ASI183 if you super pixel debayer it and then bin it 2x2 in software. Using x0.5 reducer with ASI224 will also give you sampling rate that you need if you debayer it in super pixel mode (~4"/px depending on how much reduction you achieve by careful distancing of reducer to sensor).

I think you have combination of the two things (it might be one thing that fixes the other as well): 1. collimation issue 2. tilt in focal plain. top left stars: Ellipsis with strong bottom edge Top right stars: These two indicate spacing issue for CC (but it could be collimation as well as it tilts primary mirror) Bottom right stars: These look like small coma marks that have been written vertically (not consistent coma tail orientation - it should be away from the center but it's pointing "up" rather than "down") Similarly bottom left of the stack: This looks more like astigmatism then coma - and if it is coma - it is pointing completely in the wrong way (should be like 90 degrees to this). This is causing vertical spikes to be split in two (this happens when you have poor focus): Which makes them shorter. I would first - check collimation without coma corrector and then see if things improve or do you need to deal with tilt as well.

It is exit pupil - which is related to focal length and aperture. With increased telescope focal length - magnification is increased and for the same aperture - brightness of background sky goes down, but if F/ratio is maintained, then larger aperture offsets effects of increased magnification and brightness remains the same. Since all of these quantities are tied together in one way or another - we say that exit pupil is thing that governs background sky brightness but in reality it is aperture + focal length or magnification used.

Problem of course being that one can't get new Aero ED 40mm - it is just unavailable wherever I look.

I don't see your signature - that makes it a bit hard to determine focal length used to record this. I'm also having trouble identifying stars in the image. Let me see what astrometry.net gives ... Ah, yes, you are a bit off from your target. Back to original question - it is in principle possible if you have more smear in one direction. Spikes get fainter as you move away from the star and if their light is smeared over more pixels - it becomes fainter faster. This would usually be in RA or DEC - depending on which axis is causing trailing issues. With your image - this does not seem to be the case. Elongation is in horizontal direction - This is neither oriented with RA/DEC coordinate system, nor does it align with optical center (not towards nor perpendicular to line that joins star and center of the image). Given that it is not guiding/tracking related (not aligned to RA/DEC), nor is has optical origin (not aligned to line joining star and center of the image) - then it must be processing. Fact that it is oriented up/down - left/right emphasizes this assumption. Can you post raw linear image to see if effect is present there?

Here it is slightly processed from attached tiff: It is nice wide field image of Milky way - centered on Deneb region. I can make out North American nebula. There is some structure to milky way, and unfortunately there is rather strong light source in bottom right corner - but that is the thing with wide field images - any LP will create strong gradient. I would say that this is rather nice image for first time. Well done.

One just needs to smile in particular way

This was what I was really after. I currently have 4" F/10 and looking for eyepiece that will give me good performance and wide field of view. This eyepiece will eventually be used with F/7 4" ED scope that I'm planning as replacement for F/10 model - and that is why I'm looking at somewhat faster performance - but not as fast as F/4 - F/5 range. If 36mm Aspheric has larger FOV - and it does since field stop is 45mm - it might tick the right boxes - at least vs 50 mm / 42 mm SuperView. On the other hand, if I wasn't planning on upgrading F/10 scope - then sure, 42mm Superview would be cost effective wide field to use.

Out of interest, how do you compare edge performance of the two in fast telescope (Aspheric vs SuperView)?

I'm a bit confused now. In original post, you mentioned 130PDS, which is this scope: But now you mention 127 - with added PDS for some reason - there is no 127PDS scope as far as I know, and your signature says SW 127mak which is of course this scope: Now, second scope is very good planetary performer and matched with ASI224 will give you excellent planetary images, but is probably the worst combination with ASI224 for DSO imaging. Natively it will give you 0.52"/px, and it certainly won't be able to show you Crescent nebula: as FOV will be significantly smaller than the target. If you are indeed using 130PDS then it will indeed be as you say - coarser sampling rate will reduce star bloat, but there is also a bit down to processing - which is whole another topic. Sometimes star diameters will depend on the way image had been processed / stretched. Stars are in principle gaussian bell curves (or very similar in shape) - when you observe image as 3d graph rather than 2d pixel brightness (in 3d graph - height is equivalent to brightness in 2d - a bit like old maps where mountains are in one color and valleys in another). Look at this diagram: Horizontal bars represent "white point" or max value - if you stretch hard - then you make that max value be lower and then it makes star cross section larger - star looks like it is fatter - but it is the same star.

Google usually prepares graphs for me so it is simple copy paste (there is a small software called lightshot that enables you to take annotated screen captures and save to file or to clipboard and SGL post editor allows for paste operation) - knowing what to search for, well, yes, you can say that I touched upon this subject once or twice before

Thing with OIII is that you can't really display that color on computer screen or any other device as all display devices have limited color gamut and can't display pure spectral colors. Here is simple way to display color that is closest to OIII. Take XY chromaticity diagram with sRGB color space marked on it and draw a line from D65 to outer spectral locus at 500nm like so: Take first color this line "runs into" - color in sRGB triangle that is closest to 500nm. That is color that is visually closest to true OIII color. True OIII color will be "deeper" / "richer"/ more saturated than this, but this color will have closest hue to it. You can check the OIII color yourself by looking thru OIII filter at a light source (even against sky in daylight), just make sure you keep filter perpendicular to your eye and not at an angle - because interference filters shift they bands depending on angle and you want it to stay on band. Green is often neutralized in astronomical images because indeed there is no green - in star colors. Presence of green cast usually means poor color balance. Star colors are in Planckian locus: You'll notice that sRGB can't show very cool red stars as well and that Planckian locus is below green part - there is no green color in stars: (main spectral classes OBAFGKM) This however does not mean that there are no green objects in outer space - most emission nebulae will end up having some green hue in true color because of OIII in them.

Indeed that is confusing as term resolution is used in so many cases: - resolution as number of pixels in image / on sensor - resolution as in resolving capability of telescope - resolution as in sampling rate - arc seconds per pixel Sampling rate - resolution that I was referring to - depends on pixel size and focal length. It can be thought of as "zoom factor" when taking a image. It depends on how many arc seconds - angular size in the sky, is covered by a single pixel - hence "/px unit (arc seconds per pixel). There is simple relationship between object size in the sky and object size in number of pixels - size_in_sky / resolution = size_in_pixels. For example 200 arc seconds galaxy with sampling rate of 2"/px will be 100px wide in image. Here is how to calculate sampling rate (sometimes referred to as imaging resolution): Online calculator can be found here: http://www.wilmslowastro.com/software/formulae.htm#ARCSEC_PIXEL If you are using 130PDS and it has 650mm of FL, and you are using ASI224 with 3.75µm pixel size, you'll get 1.21"/px Here is like a break down of resolutions (sampling rates) and what they are useful for: >4"/px - this is wide field imaging with lens and such, any star tracker should give you decent results 3-4"/px - wide field imaging with telescopes - good resolution in almost any seeing, any decent mount should be able to do it unguided with relatively short exposures - any mount guided will be ok. Aperture of 50 or above 2-3"/px - medium to wide(ish) field imaging with telescopes - good resolution in decent seeing, better mounts required unguided, decent mounts guided, 70-80mm or above 1.5"/px-2"/px medium resolution. You need good seeing for this. Very good mounts can do this unguided, you want decent to good mount for this even guided, 100-120mm and above. 1.2"/px-1.5"/px high resolution. You need good seeing this one. Again, only best mounts unguided, good mounts guided, 150mm and above 1.0"/px-1.2"/px very high resolution. You need excellent seeing for this one. Only few premium mounts with encoders will do this unguided, otherwise you need very good / excellent mount guided (stock HEQ5/EQ6 and such will not do it). 200mm and above required Above is if you want to have tight stars that is. You can sample at 1.2"/px - even with gear that is not up to it as we have shown above - but you get bloated stars, but if you reduce size of your image by x2 to x3 - which makes it go from 1.2"/px to somewhere in 2.5"/px - 3.5"/px range - we get good star shapes since we are in totally different league equipment and seeing wise.

It is M82 - Cigar. Star bloat in that image is due to several factors: - I used very large telescope for my mount. This was taken with 8" F/6 telescope (OTA from SW 8" Dob) mounted on stock HEQ5. That scope is a sail - any wind will make it move. - I used x0.5 reducer - questionable optical quality. Those GSO reducers are just finder scope / binocular lens mounted in a cell. - I was not guiding and I used relatively long exposures. You can see that star shapes are not the best in the image. In any case, star bloat is always due to inappropriate resolution of the image. In this case I was using ~1.29"/px which is rather optimistic for stock HEQ in average seeing conditions. In fact, stars in that image are quite good compared to some other images of that galaxy I took. Have a look: Above is crop from M81/M82 wide image. This was taken with 80mm apo scope again on Heq5, this time with ASI178mcc (color cooled version) If I reduce above image to approximately the same size, we get very nice pin point stars:

I see. Smaller pixels usually mean that camera will be less sensitive. That is, if you use it as is. ASI183 is good because you have very large number of pixels. It natively has 2.4µm size pixel but has 5496 of them in width. ASI224 has 3.75µm pixel and has 1304 of them in width (if I'm not mistaken). With binning 4x4, you can easily get from 5496px to 1374px and pixel size will grow from 2.4µm to 2.4 * 4 = 9.6µm Suddenly you have about the same resolution as ASI224 with 1374px in width - but pixel size is much larger. This is the same logic I did with FOV above, except this time we did it on pixel level. You can choose which one you want to "keep fixed" - either FOV or pixel count, but in either case, you end up with increased sensitivity. If you can - get pro / cooled version, it really makes significant difference. ASI183 has amp glow and you want to be able to calibrate it out. Try that one as well on ASI224, as you already have it and it won't cost you to try it out. Only problem will be reaching the focus, so you might want to use reducer on 1.25" nosepiece but use 2" barrel of camera to attach it to the scope (sinking nose piece inside focuser to get some back focus that way). ASI224 is small sensor and you might get away without much coma on F/5 scope. I used same sized camera on F/6 newtonian and did not have coma in the corners. This was with that 1.25" reducer.

I think that your setup is up to the task, but it really takes a lot of exposure to make it work. Here is Crescent nebula only starting to show after about hour or so of exposure with similar sized camera (equivalent of ASI120mc) on a similar sized scope - Skywatcher ST102 F/5 achromat (hence blue around stars). Don't replace 224 if you don't really need the funds - add ASI183 cooled version (pro) if you have the budget for it. 224 is the best planetary camera of the three mentioned, but ASI183 has the largest sensor - which translates into larger field of view. Cameras tend to be more expensive than the optics these days and ASI183 will be faster camera than ASI178 if you pair it with proper scope (I know - this really needs very good consideration - larger scope means larger mount and all of that and AzGti can only hold so much). Just for the sake of argument, look at this for example: That is 130PDS and ASI178 and 200PDS + ASI183. Second combination gives you slightly larger FOV, but it is 8" of aperture vs 5" of aperture. 8" will collect much more light. In the meantime, have you tried simple 1.25" x0.5 focal reducer by GSO for ASI224 and 130PDS?

Sure, it is very famous and often quoted as one of the most beautiful if not the most beautiful math equations: https://en.wikipedia.org/wiki/Euler's_identity

Almost everything astronomy related that I want to print has some sort of thread on it - good to know that threads are printable and they work ok.