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Just a word of caution about OAGs and fast scopes. Size of OAG prism is important and distance of it to focal plane. With fast scopes - you want your OAG to be very close to sensor - otherwise prism will act as aperture stop. In principle you want your OAG to be closer than F/ratio * size of pick off prism. Say you have 8mm pick off prism and you image at F/4. You want your OAG prism to be closer than 32mm to focal plane (or rather - imaging sensor). If you put OAG too far away on small fast scope - you'll make it work as if stopped down. 8mm prism at 64mm away from focal plane and Esprit 80 will work like F/8 guide scope - with just 50mm aperture and 400mm FL. You'll limit light to guide sensor by factor of 2.56!

M45 looks fine indeed. M31 - shows similar effect on some of the stars. It's not easy to see - but I think it is there. For example in this zoomed in patch: Or maybe here on the other side of the frame: All valid points. Both temperature and gravity can have impact on the lens. What sort of flattener are you using and does it have threaded connection to the scope? Just went back to original post - it is x0.79 FF/FR. I have this FF/FR and I had issues with 2" clamped connection - it would tilt as the scope tracks the sky. Here is green sub: That is bottom left corner - stars are defocused - clear sign of either field curvature or tilt. Bottom right corner shows astigmatism: Ok - blue shows similar thing - bottom left is defocused, bottom right is astigmatic - but to lesser extent than green. My guess is that you shot green first, then blue and after meridian flip - red. There is a bit of play in FF/FR and I think there is a bit of tilt as well since even Red is suffering from this - but in top corners (meridian flip - 180 degrees rotation). Here is red sub - top right corner: Not as pronounced but still there - focused core and defocused skirt. Top left corner of red sub is rather tricky - stars are faint and it is hard to tell, but I think there is a bit of astigmatism there as well: I'd say - FF/FR is first to be checked. Maybe switch to threaded connection if you don't have it already or think of getting tilt plate to fix any tilt?

I don't think there is issue with red channel - I think there is issue with green and blue. It looks like collimation issue - or something similar. Look at these stars for example - this is montage of R, G and B channel (from left to right). Red stars are nice looking, their peak is centered on star profile. Green and blue are distorted. They have center of the star displaced with respect to star profile. Stacking works properly and aligns star centers since those carry the most "mass" - peaks should be aligned. Problem is "skirt" that is not symmetric in green and blue. I've seen this problem before in simple photographic lens - as you move away from the center - you get stars that look like that. To be honest - I don't think above is down to filters. To me it looks like poor optics / collimation issue. I remember @Ken82 having similar issues on his scope.

I think that it is best to do the opposite - keep the EPs warm. At least I have that problem - as soon as I bring my eyeball close to cold EP - it starts to fog up. I guess it is due to moisture in my eye that condenses real fast on cold EP eye lens. As for "tube currents" - I don't think it is important as wavefront is really "dense" in the eyepiece - unlike in telescope tube where it is spread over almost all of interior space of the tube. EP is also much smaller by volume and there is not much room for gradient to form - that amount of air is going to equalize in temperature very fast.

Eyepieces are really personal taste and unless you try something - you won't know if you like it. I've found that I don't really care for fields past about 60-70° and really value sharpness of the eyepiece. I don't mind tighter eye relief, but sometimes eyepiece is not comfortable in use - my eye gets tired fast and I don't like that. At this point of my life I'm perfectly happy to go with cheaper eyepieces that offer 90-95% of performance rather than spending much more for last few percent of performance.

Indeed. There is also difference between eyepieces of the same type. Cheap Chinese plossl might be good value for money but I'm certain there will be difference to notice between it and Televue plossl. Quality control, attention to detail like level of polish or choice of coatings does make a difference.

Both high quality and very expensive. It's a bit like Pringles - once you pop, you cannot stop sort of thing - and being expensive - fair warning is in order - "Your wallet may suffer" I would just like to point out that you have two slow scopes and you really don't need expensive eyepieces to get very good performance. Notice those posted diagrams and difference in spot sizes between F/10 and F/5 scopes. Most eyepieces work rather well in slow scopes.

https://www.telescope-optics.net/ very comprehensive material on amateur telescope optics. F, e and C are Fraunhofer lines - often used in optics to denote particular wavelengths. https://en.wikipedia.org/wiki/Fraunhofer_lines F is hydrogen beta line e is mercury 546nm line C is hydrogen alpha line

Nice video. Just a few remarks, if you don't mind: - PDS version sports larger secondary besides dual speed focuser and more back focus. This is important to provide larger fully illuminated field for astrophotography, but planetary contrast will suffer somewhat because of that. - thin spider vanes throw same intensity spikes as thick spider vanes. Intensity of spikes depends on length of diffraction edge - rather than thickness of light blockage. - You can image larger targets like Rosette and M31 with such scope if you do mosaics.

Maybe a bit too much information. Telescopes suffer different type of aberrations that cause image to be less than perfect. Field curvature is inherent in almost all telescope designs and represents the fact that focal plane is actually not a plane but rather curved surface. Sometimes this reflects by inability to have both center of the field and edges in focus at the same time (or center of the image and edges when taking a picture thru a telescope). Either center is in focus and edges blurry or vice verse - edges are in focus and center is blurry. This happens with very wide field observation or imaging (2" long FL eyepieces or large sensors). Coma is another type of aberration that is inherent in parabolic newtonian telescopes - but sometimes happens in other designs as well - like in SCTs. Both of these were mentioned by @Louis D as a contrast to the fact that both primary and secondary mirrors are spherical - and spherical surfaces tend to introduce spherical aberration - but in case of SCT this is corrected by front corrector. In the end SCT does not suffer from spherical aberration (by design - in ideal case) but does suffer from field curvature (as do other telescope designs) and from coma (similar to newtonians).

Yep, all the math involved is just above table that @Rusted posted condensed in easy to use formula for calculating aperture mask diameter. Numbers 5 and 8 come from 25.4 / 5 = ~5 and 25.4 / 3 = ~8, where 25.4 is number of millimeters in an inch and 5 and 3 represent Conrady and Sidgwick standards. This can also be used for astrophotography purposes, but in that case - one should consider using fringe killer type of filter as well as sensors are more sensitive than our eyes. Here is example of different aperture masks with and without filter: Columns are just different levels of stretch so same star image only stretched less or more. Each pair of rows represents certain aperture mask and same aperture mask + wratten #8 filter. First row is no aperture mask, third row is 80mm aperture mask, 66mm aperture mask and last one is 50mm aperture mask (odd rows are just repeat + filter). This was done with ST102. Even Conrady standard in this case showed some blue fringing - but this is because of sensor sensitivity - visually you would be hard pressed to see any of it. On the other hand 66mm aperture with filter has almost no CA even photographically.

There won't be much wisdom here - except the fact that I simplified the process / math for you. You need to make aperture mask - but what size aperture mask is right for your scope? Process is rather simple - take focal length, multiply it with number between 5 and 8, where 5 is CA free view, and 8 CA noticeable but not really ruining the view (choose 5 if you are sensitive to CA) and take square root of resulting value. Have ST120 and want to observe planets and the moon CA free? No problem - we start with 600mm FL and multiply that with 5 to get 3000. Take square root and result is 54.77mm - use 55mm aperture mask for CA free views. If that is too small for you and you want max aperture - go with number 8, in this case it will be 69.3mm or round that to 70mm. In the end to reiterate - aperture mask diameter = square root ( focal_length * 5 ) (replace 5 with 8 to get "upper limit" of aperture size mask with some CA).

Goto is usually "upgrade" to tracking mount. Once you have motors that are capable of tracking the sky - you can add small computer to use those motors to navigate the sky. With AltAz mounts - this usually comes together as there is no simple way to track the sky without controller (small computer that calculates needed motor speeds depending where scope is pointing). Benefit of AltAz is that your eyepiece is pretty much in same position with all scopes. With EQ mounts, newtonians are a bit awkward to use as eyepiece and finder rotate with tube and can end up in weird positions - you need to rotate OTA in rings to get eyepiece in comfortable position. Goto EQ is almost a must for astrophotography (at least long exposure) - as Alt AZ suffers from field rotation. Completely irrelevant for observation but ruins long exposure. Planetary astrophotography will not suffer from field rotation as exposures are very short in comparison to rotation speed. If comfort of use is primary concern - go with Alt Az. If long exposure photography is in plans - consider EQ No - about the same. EQ does require polar alignment, but for visual that can be rather crude. With both types there will be procedure to complete that lasts about 3-4 minutes (maybe a bit longer due to polar alignment on EQ). Procedure consists of selecting an object (usually star but can be planet) - mount will try to locate it and then you have to help it by centering selected object in your eyepiece. This is repeated 2-3 times with different stars. If every minute counts - go with Alt Az as you only need to make sure mount is level and scope is pointing north. With EQ mount - you'll also need to make sure mount RA axis is pointing at NCP (north celestial pole - close to polaris), and that can take additional minute or two.

You are probably confused by magnifying secondary mirror. It is not flat like in newtonian - it has curve and that curve does similar thing to a barlow lens - it diverges light rays coming in from primary mirror and amplifies primary mirror focal length.

Which camera is color and which one is mono? Scopes are both F/7, one is 60mm and other is 100mm, right? Focal reducers are x0.8 and x0.6? Interesting fact is - FOVs are best matched if you have x0.8 on both scopes or x0.6 on both scopes. Pixel suitability calculations you can just ignore and use 2"/px with these scopes and not less than that. You'll probably have to bin in software. I would be inclined to match 100mm scope with ASI183 and x0.8 if ASI183 is mono camera and 60mm with 178 (presumably osc in that case).

Are you sure it weighs that much? That is extraordinary weight for 8" compact scope. My RC8" weighs less than 10Kg and my HEQ5 can carry it. I was under impression that SCTs are even lighter than that. According TS website 8" ACT OTA weighs only 7Kg: https://www.teleskop-express.de/shop/product_info.php/info/p5825_Meade-8-inch-f-10-ACF-OTA.html Other sources quote it at even less - 14.1lbs or 6.4Kg (at least for lx200 model, not sure if there are differences between LX versions in OTA itself - I guess it's mostly down to mount?) In any case, for compact scope of 15Kg - I would be looking at EQ6 / EQ6-R / AZEQ6 or CEM-60 mounts (for some reason discontinued - replaced by more expensive CEM70). Maybe GEM45 by iOptron as well.

Looks very much like this one: https://www.teleskop-express.de/shop/product_info.php/info/p11386_Artesky-Alt-azimuth-Mount-with-fine-adjustment-for-instruments-up-to-7-kg.html except for color scheme

Now now, some of us need to be early adopters in order to pass experience to the rest of us

Do pay attention that most lenses are far away from diffraction limited optics and while we can talk about theoretical MTF - most lenses will have much lower values. As far as pixel size is concerned - in post on previous page of discussion I produced MTF of pixel. Pixel convolves point sampling - so FT of pixel will multiply FT of original signal. We just need to produce FT of pixel pattern and multiply with MTF of ideal aperture. This is MTF of pixel where pixel is matched in resolution to lens (optimum sampling rate - or pixel size twice smaller than max wavelength produced by the lens). In order to see complete MTF of pixel, here is case when pixel is much larger than optimum sampling rate: First thing to note is that pixel FT does not have rotational symmetry - and it will impact MTF50 depending on direction. This is profile of FT in X direction. We can see from this graph - that MTF50 is equal to 0.8 of frequency related to the pixel size (frequency equal to wavelength of single pixel). Problem is however that such frequency is not recorded in our image as sampling records only those frequencies that have wavelength twice that of pixel size as pixel size is related to our sampling frequency. We can talk of hypothetical MTF50 at focal plane dictated by pixel size - but never on the image recorded with those pixels. Record image with large pixels and there will be some frequency that such pixels will reduce to 50% of their original value - but those frequencies will not be in recorded image as those frequencies are much larger than sampling rate will record. Increase pixel size for any lens and you'll only get sharper image - never "softer due to large pixel size".

Is that flattened or regular AT72ED. It has rather short focal length - field curvature must be strong in the young one . ST120 will have quite a bit larger radius of curvature if we take 1/3 of FL of each to be approximate radius of curvature. If above is flattened field then I can only expect worse from ST120 unless I have young eyes that compensate for FC (I'm starting to need reading glasses when working on computer - so that boat is probably about to set sail).

I wonder why 40mm one is no longer made. It has best performance at the edge of the field of the three. In any case, I can see 35mm having enough of field stop to give me 4° in ST120 (together with 7mm exit pupil) - good to know.

30mm is focal length of eyepiece, while field stop diameter - is well diameter of that "baffle" that limits how much of focal plane "eyepiece" sees. Roughly, for astronomical eyepieces, following relationship holds: field_stop = focal_length * AFOV, where AFOV is expressed in radians. For terrestrial eyepieces this has a bit different form: field_stop/2 = focal_length * tan(AFOV/2), again AFOV is in radians. Although eyepiece is said to be 30mm and it is said to have 69° AFOV - these numbers might not be correct. Eyepiece can have FL of say 30.7mm and still be called 30mm EP, or could have 67.2° and still be said to have 69°. This happens mostly due to marketing and "rounding" up - whole line is marketed as 69° eyepieces but in fact there is small variation in actual numbers. In any case - field stop is true indicator of how much of true field of view in the sky you will see with particular telescope. For that reason it is good to know that value - but it is often not stated for eyepiece. If, for example, I'm looking for eyepiece that will give me 4° TFOV in 600mm FL telescope, then I need eyepiece with 41.9mm or larger field stop - regardless of focal length and apparent field of view of eyepiece. Once I have narrowed my selection of EPs, then I can think about focal lengths, exit pupils, AFOV and all of that. That is one way of looking at it - not necessarily the best way, but it has its uses.

To be honest, it sounds too low. That is field stop of 32mm 52° plossl. It should be closer to 36mm, maybe it is a typo in your post?

It is not hard to measure if you can access inside of the barrel - just be careful not to scratch any elements. Here are a few images to explain it: You can use either calipers or ruler. Ruler won't be as precise - but there is less chance for anything going wrong. You can put it against the barrel and eyeball field stop diameter against it.

Your astigmatism is going to affect all stars in the field equally, so that is not it, but it can possibly be coma? In F/6 scope - you don't see much of it and only time it can be seen is with wide field eyepieces with this one. @Carl Au Nice report. Any chance of field stop measurement?