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If you are referring to the diagram that I posted - its purpose was to demonstrate that GSO response did not make sense - illumination of secondary by faster primary. It is not part of actual explanation why aperture is obstructed. I specifically meant to address this claim: Maybe I'm not understanding what is being said properly, but I'm reading that sentence as: faster primary "requires" larger secondary, but larger secondary will impact contrast and we've chosen to use smaller secondary and that is the cause of aperture being stopped down. I gave that diagram to show that faster primary in fact requires smaller secondary (if both are placed at the same distance). We can calculate (a bit of guessing is involved) if there will be some stopping down due to secondary size. If we take primary to be F/4 and secondary magnification to be x3 (giving F/12 scope), then primary will have 800mm FL. 33% linear obstruction means that diameter of secondary is ~67mm. For it to be illuminated by primary without any loss - it needs to sit at distance of: x : 800 = 67 : 203 => x = 67*800 / 203 = ~264mm inward from focal point, or 800 - 264 = 536mm distance between mirrors. Now we have couple of issues - as different websites give different figures: - TS website states 33% linear obstruction ( 203 * 0.33 = 66.99 ) and says tube length is around 620mm (not sure if that includes focuser or not). If we assume those figures, then everything checks out - there is enough distance between mirrors (if we account for cells and mirror thickness - ~20 + ~20 + ~20 + ~20 + 540 = 620mm). - Agena Astro lists different figures all together - secondary mirror obstruction as 58mm (that would be ~28.6% CO), and tube length as 21.1" = 53.594mm (or just about same distance as needed between mirrors). That is really interesting ... I have no idea how long the scope is, I suspect it must be longer than RC 8" - which is about 465mm tube + about 90mm. I did some measurements for some people that make bags for telescopes, so I know figures of my 8" RC. Focuser should be the same so it is about 90mm. This sort reconciles tube lengths given by TS and Agena Astro, as 536 + 90 = 626 - close enough to 620. If tube is 536mm long, than I would say separation is probably about 80ish mm smaller so 465? If that is true, then secondary mirror size needed to fully illuminate on axis is 335 : 800 = x : 203 => x = 335*203/800 = 85mm 67 / 85 = 0.788 while 7.34/8 = 0.9175 Not sure it matches. Maybe primary is not F/4 but rather F/3 with secondary being x4?

I think I like it like this (although this is not what I wrote above - scientifically correct color):

TS one is quoted to have 99% dielectric coatings ...

1. color balance 2. no, image capture has almost nothing to do with that 3. yes, "editing" - or rather data processing has almost everything to do with why they are different 4. depends on "raw" camera sensitivity - each camera will have different relative sensitivity between color components. There is a bit down to sky conditions - levels of LP and type of LP 5. I think I like the best scientifically accurate color version (almost nobody processes their images like that, but if you find image that is done in PixInsight and author says they used stellar color calibration - then it should be close to proper color). Here is quick manipulation of second image to make it look more like first image:

As far as I can tell - it's some sort of projection device that uses smart phone to image stars reflected on that projection device and do plate solve for aiming information. I think something like that could be done in DIY for any sort of manual mount if we could figure out how projection device works.

No, it's according to GSO rep who answered Larry's inquiry (via Agena Astro). To me that explanation simply does not make much sense - and it is question if person who offered such explanation really knows what it is all about. Just let us address one point in this reply - short focus parabolic mirror and size of secondary as means of stopping down scope. For same mirror size, faster mirror (shorter focus) will require smaller secondary, not larger. Look at this image: Shorter focus mirror will need less of secondary (intersection with line is shorter than that of long cone). If there is point that has 100% illumination - that means no light is blocked at that point - there is no aperture stop before primary mirror (as it would affect whole field and no part of field would be at 100% illumination from light reaching front of scope aperture) - that means aperture stop needs to be after primary mirror - light is already bent at that point and any aperture stop will act as vignetting rather than smaller effective aperture. Just use eyepiece with field stop less than 15mm - and your scope should work as regular 8" scope without aperture stop as that part of field is 100% illuminated. In any case - offered explanation in my view does not explain anything. Much more plausible explanation would be that some sort of inferior mirror coatings was used. Celestron uses Starbright XLT coatings for which they say reflects ~97% of the light. If GSO used coating of 89%, then for two mirrors of both systems we would have total transmission of: 0.97 * 0.97 = 0.94 and 0.89 * 0.89 = 0.7921 Ratio of the two is ~ x1.187 Now let's examine ratio of 8" vs 7.34" light gathering surfaces. 64 / 53.8756 = ~ x1.188 There you go - use inferior coatings and it will behave as if you used 7.34" instead of 8" aperture.

It just does not make sense to me. If there is fully illuminated part of the field - let that be 15mm diameter - then any aperture stop needs to be on primary mirror itself - and there does not seem to be any. Any aperture stop that happens after primary mirror will be at some distance from focal plane and will impact field illumination - but if you have piece of fully illuminated field - then that light reaching that part of field is not stopped down - thus saying that mirror size is effectively 7.4" and there is 15mm of fully illuminated field - simply does not makes sense.

Two reasons to use 2" eyepieces: - first and most important - certain combinations of focal lengths and apparent fields of view simply can't be made in 1.25" format. Longer the focal length and wider the AFOV of eyepiece - larger field stop it needs to have. Maximum field stop in 1.25" eyepiece is dictated by barrel size - which is 1.25" or roughly 31.5mm. You need at least 1mm for barrel walls and filter tread and such and it leaves only something like 27-28mm for field stop. If eyepiece design requires larger field stop - there simply is no option but to go for 2" eyepiece (in fact 2" also has the same limit and there are even 3" accessories - but are rare - usually observatory class gear for very large scopes). - some people prefer 2" format because if feels more secure in 2" diagonal - better clamping for heavy eyepieces. This is the reason some 1.25" eyepieces have 2" adapter that can be screwed on (look Baader Morpheus range or their zoom). Fact that 2" eyepieces have larger eye relief is not related to the fact they are 2" eyepieces - but rather to the thing that makes them 2" eyepieces - focal length and design. There are plenty of 1.25" eyepieces that are long eye relief and comfortable - take 32mm plossl for example (this is by the way - about the max field stop in 1.25" eyepiece - 32mm combined with 50 degrees AFOV).

Ah, that is easy: Here we see the "Great Southern Wave" Constellation or Harkoona Chatyu as Hotamte people call it. It is loosely translated as "Wave from the star". Legend has it that star fell from this part of the sky (it is believed that there was another star located roughly in bottom row) into the great waters and raised enormous wave that flooded the lands and created great plains of the south. This was before even time of the elders - legend has it.

I'm not sure what are we supposed to do? Identify few stars in above attached image, make them a constellation and tell the story of it, or produce similar looking image that depicts our fictional constellation - and again tell a story related to it?

Hi Alex, and welcome to SGL. I'm not sure what the question is? To calculate ratio of photon flux at 1um and 0.55um - you don't really need magnitude of the star. Stellar class, or particularly it's temperature is enough (if you do black body approximation). https://en.wikipedia.org/wiki/Planck's_law You plug in numbers for 1um and 0.55um and take ratio of the two. That will give you energy flux ratio. If you want photon flux ratio - you need to account for energy of photon at particular frequency (0.55um photons will be more energetic than those at 1um).

I think that you will be better served with this scope instead: https://www.teleskop-express.de/shop/product_info.php/info/p10753_TS-Optics-8--f-12-Cassegrain-telescope-203-2436-mm-OTA.html A bit cheaper, more aperture, according to reports - very sharp and good for planetary visual (which means it will be good for imaging as well), a bit less focal length - better for wider fov. If you want even wider fov than with native resolution - I think you can utilize focal reducer as it has pretty decent flat field.

Fact of life is that focal length and aperture are tied together in a "mysterious relationship" often called F/ratio - some say even that it is a mythical thing . In any case - larger focal length - larger the aperture and hence light gathering of telescope. You don't necessarily have to go with higher sampling rate as well - as long as you have big enough sensor and you can bin - there is real benefit of using large scope. I would rather image with 12" at 0.9"pp than 5.5" at 0.9"pp. In fact my dream setup is in fact 20" in form of 4x10"

You can still try with jpegs, but results will not be as good due to lossy nature of jpeg compression algorithm and the fact that it records data in 8bit format.

Here is what I would do: - pick shortest exposure time with highest ISO setting. It might not look the best as single sub, but when doing planetary imaging that is what you want to do - go with shortest exposure to freeze the seeing and use highest ISO (gain on planetary cams) as that provides the least amount of read noise. - use PIPP to pre process your images. Hopefully you shot them as raw images rather than .jpegs or similar. PIPP will do debayering for you and save them as 16 bit color png files - Download Autostakkert! 3 and load images in it and stack them - works pretty much the same as Registax - you assign anchor points, analyze frames, select number of frames to be stacked based on their quality and stack - Load resulting stack into Registax for wavelet sharpening - Load result of sharpening into Gimp for final touch ups

Mine is regular tube, not truss tube, being 8" version, and I pretty much don't pay attention to mirrors at all (give them occasional look but about the same as with my newtonian - once in couple of months). Have both scopes for a few years now and it never occurred to me that I should clean my mirrors. Don't worry about mirrors being dirty. That dust is so out of focus that only difference it can make is a bit of dimming at the focal point. It is usually less than 0.1% of 0.1% or something like that - in fact, in order for light loss to be 1% you need something like black stain 1cm x 3cm somewhere on the mirror for 8" scope, and even that can be compensated with couple more minutes of imaging - so for imaging purposes, you just don't need to worry about dust/dirt (unless of course it is severe). In fact, even for visual, you really should not worry about dirt on mirror (again, unless severe). Here is point where I would consider cleaning my mirrors: Btw, cleaning mirrors - not very complicated and while you have to be careful of how you handle mirrors - it is half an hour job at max (I cleaned mirrors on my previous newtonian - although they were nowhere near dirty as above image - but at the time I also wanted my mirrors to be "always clear" )

I would go for all mirror design. In fact I have 8" RC and I'm happy with it. Collimation is not such a big issue (at least it was not for me). I collimated my scope in about 10-15 minutes. It was made easy by use of CMOS camera - my imaging camera is CMOS and has fast download times, in fact you can have live image on computer from it (high fps, fast download). This and software that has focusing aid - star FWHM measurement was all it took for quick collimation. I'm not sure why you say that RC has larger stars. There will be spikes, but I don't think it will produce larger stars.

Only advantage 2" eyepieces offer is larger field stop. That means larger AFOV at longer focal lengths. In order to fit larger field of view / lower magnification in eyepiece you need wider field stop (surface at focal plane that lets light in - larger it is - larger field of view it will allow). At some point you simply run out of space in 1.25" eyepiece format - at about 27mm - you need a bit of space for eyepiece body and filter thread and soon you are at 31.5mm - or 1.25". To circumvent that - eyepiece makers use 2" format. Some 1.25" eyepieces have 2" "converter" - it is just fitting so you can use them with 2" focuser / diagonal - it is essentially the same thing as you already have in your focusers / diagonals - 2"-to-1.25" adapter - just a piece of hardware used to hold EP in place - it does nothing optically. In any case - don't choose eyepieces based on 1.25" / 2" format. Eye lens will be larger if eyepiece has larger AFOV (apparent field of view) and longer eye relief. If you like eyepieces with larger eye lens - you could be in fact liking eyepieces with longer eye relief. These are often described as more comfortable to use as you don't need to get in too close with your eye to use it. Too long eye relief can also cause problems, especially on smaller exit pupil - it can be hard to hold your eye properly positioned and you can experience blackouts because of that. Anyways, here is my list of EPs that should suit your scopes good, provide you with what you want and cost less than TV or Pentax: budget: BST Starguiders Upper tier: Explore scientific 68 degrees and 82 degrees series (I can recommend 5.5mm 62 as well - managed to finally try out mine and I like it). Top tier (close in prices to two mentioned brands but not quite that level): Baader Morpheus I think that you can safely go with ES 68 / 82 but be aware - that will depend on how much you value eye relief. While ES eyepieces do have longer eye relief - It is not always as comfortable as can be - for example 82 degrees 11mm and 68 degrees 16mm - although they have 15.6mm and 11.9mm eye relief respectively in their specifications - in use they feel about the same in terms of eye relief - on edge of comfort that long eye relief provides.

This is really valuable. Next on my list was to test what sort of reduction can I get without sacrificing sharpness. Our setups are quite alike - only difference is that my scope is 102 and not 90mm (Antares reducer is same as GSO, just rebranded). At about x0.45 reduction we are talking about F/5.85 - at one point I thought of getting it to F/6 and seeing how sharp it is - so that sort of matches. At 600mm it will be 1.65"/px after super pixel mode, and I'm afraid that is slow for 4" and OSC sensor - that means another bin x2 after for resulting image size of 750x500 and 3.3"/px. That is ok but a bit small image size - maybe if it is sharp enough it will be decent when resized to fit computer screen? ASI183 type chip would be more suitable as it would provide different "magnifications" - but I feel it is too expensive for basic setup.

Main idea is to create affordable setup that can "do it all". I noticed that many people come and ask for advice for a scope that can basically do it all - as they often put it: "I want to observe both planets and deep sky objects, and I want to be able to record what I'm seeing - to take a picture of it". Some give a bit better explanation of what they want to image - but point is - they are limited in budget - usually up to 1000 of dollars/pounds/euros (I guess it is mental barrier of four figures spent on gear). Of course - you can't accomplish that in said budget or with one scope, but I wondered if one is ready to cut some corners - what sort of setup it would require and what sort of budget would cover it. Since I already own some bits and pieces of such setup, and I fancied idea of having a small Mak as grab&go lunar scope - I got myself new setup for both purposes - to test out "do it all" budget scope and to have lunar scope for quick peek. In lunar role - this scope is beyond my expectations. Mount works good enough to track the target and hold the scope. Setup is less than a minute, and scope delivers sharp views beyond x200 power. What more does one need for grab&go lunar? In the mean time - I'm fiddling with EEVA with this setup as that is the key for "do it all". It will aid observing in light polluted areas, but it will also provide base for "take image of DSOs" that I see (well they will see them when doing EEVA). In order for this scope to be good performer for EEVA - we either need large sensor ($$$) that will provide large FOV or if we go with small sensor CMOS camera that will be both for planetary imaging and EEVA/DSO imaging (and cheap) - we need means to exploit all corrected and illuminate field that scope is providing - that means some sort of focal reductor. We also need quite aggressive reduction if we want to get widest field possible out of this scope (and make a good match in resolution vs pixel size for available cameras that have pixels in range of 2.4um to 3.75um). I've identified 4 different means to achieve focal length reduction: - EP projection in "reducer" configuration ( focal plane of scope is at sensor and eyepiece lens acts as focal reducer - see first post - I could not achieve this successfully because I could not get sensor close enough to eye lens of eyepiece with this adapter). - EP projection in "regular" configuration ( focal plane of scope is at focal plane of eyepiece and eyepiece is again bending - reimaging at sensor). I tried this approach today and while it works and you can "dial in" required/wanted reduction - edge correction is disastrous - there is so much blurring at the edge of the field. - Regular reducer - very limited choices there given that most reducers are T2 or 2" and we have 1.25" option only here (maybe I could look for T2 reducer?). This seems to give most usable results so far. - Afocal imaging. This uses eyepiece the way they are meant to be used - focal point of telescope is at focal point of eyepiece and eyepiece produces collimated beam at exit pupil. Lens is then used again to focus that light onto the sensor. This combination is the worst in terms of number of glass surfaces - but will potentially provide best correction / best sharpness because both eyepiece and lens will be doing what they are designed to do. Problem with that configuration is - attaching everything together and matching eyepiece focal length and lens focal length and getting lens that is good enough (there are a lot of security camera / industrial type lenses out there that match sensor size - but most are basic and not well corrected lens - usually marked as 2MP - for this application we really need sharp lens - marked as at least 6MP or higher - 10MP, and of course lens performance needs to match the label - and still be cheap / affordable). That is left to be tested and I'll test that as soon as I get lens to test with. This was rather long answer, short one is - we use EP projection as one way to get focal length reduction because we want wider field and better matching of pixel size to focal length with this scope and camera I'm using.

That FOV seems to be too small. Ah, I get it - you are looking at ED80 FOV without flattener / reducer. With x0.85 FF/FR it is wider - here is comparison:

Why not consider a bit larger scope instead - that will give you target focal length (and as a bonus collect more light). Something like this is rather good and won't break the bank (in fact there are few "second hand" items on sale now - showroom pieces that were on display but are treated as second hand and reduced in price): https://www.teleskop-express.de/shop/product_info.php/info/p11871_NEU--TS-Optics-PHOTOLINE-115-mm-f-7-Triplet-Apo---2-5--RAP-focuser.html That scope is 800mm focal length, but when you add suitable FF/FR (I would consider Riccardi x0.75 FF/FR) - you will get 600mm and F/5.2 setup I don't own this scope, so I can't speak as owner, but I do own TS80 APO and know what sort of fit&finish is on photoline scopes and I can say that 2.5" R&P is very adequate focuser for imaging and you can easily access 10:1 shaft to attach motor focuser (or maybe use belted connection).

While testing proper EP projection today, I also managed to test x0.5 reductor again in more detail. - I added 5mm extension ring to get closer to target F/4.5 instead of F/5.1 from first post. - I also wanted to check level of vignetting in this configuration - in first test, image seemed a bit blurry so I wanted to check sharpness of this setup - both on axis and in corners (because EP projection showed severe degradation in corners). Here are results: Reduced image shows nice definition across the field. Tile measurement shows reduction factor to be around - x0.36, or F/4.7 (closer to target F/4.5) - here is image of tiles cropped and 100% zoom (but super pixel debayer - not debayered but rather just binned): I found setup a bit difficult to focus, and I don't think it is because it is fast. Maybe there is some issue with tilt - as nose piece with reductor can't be fully inserted into 1.25" receptacle on scope - there is baffle on tube that prevents this. I might try threaded connection in the future. I checked corners as well (slew mount - no change in focus until tiles that were in center of FOV are now in the corner), and they seem to be as sharp as center: and other corner Here is vignetting: It shows that field is not quite centered on sensor. This is 5% steps - white is 100%, and corners are at 55%. Actually right side is at 45% because vignetting is slightly moved to the right on sensor. I was shooting at cloudy skies and some of center illumination is actually shadows on clouds that were in focus rather than true flat field - but this gives idea of what vignetting is - it indeed falls of to about 50% at the edges and flat fielding is necessary. I wanted to get idea of how sharp this setup really is - so I turned the scope to very distant high contrast target: This is top of TV broadcast tower - it is roughly 10.7km away. Here is what 100% center crop looks like: Now this might seem like rather good image of something that is 10.7km away. It might even look like it is due to ground seeing and that you can't really get good sharp image of it. That is in fact not true - this scope is very sharp. I tested it at night - it is collimated properly and it gave very sharp images of Moon at x236 power. I decided to take again this tower in prime focus. Here is image at 100% without binning / debayering. Bayer matrix is still visible in this image, but presented like this - it is actually at critical sampling for this scope - best resolution that this scope can provide - and it shows Yes - antennae elements can be clearly seen (and sharp? right?) all along the central column. In fact I think I can make out ladders used to climb to top of this tower at the back side. Again - this is 10.7 km away. This little scope is truly sharp In any case - wanted to compare the two so I reduced image at prime focus to rough size of that taken with reducer: Difference is clearly visible - left is prime focus reduced and right is image with reducer. Reducer introduces quite a bit of blur. I wanted to see what sort of blur we are talking about here, so I tried gaussian blur with different sigma, and it looks like sigma of 1.9px is about right: Now I know what sort of simulations to run in order to asses what sharpness will I get out of EEVA images. I know - it is far easier to just go outside and do EEVA session instead of running sims - but one needs clear skies in order to do that BTW, I'll try threaded connection to see if it will improve things and also reversing lens in the cell. I did that once before when I used this reducer with F/6 scope - and it helped. Maybe at F/13 better image will be from lens facing other way around?

Small update on this thread. I managed to test out proper eyepiece configuration, and I'm must say I'm rather underwhelmed by how it performed. Just a bit of background first - I used TS eyepiece projection adapter: It is standard item that has compression ring (and 4 screws) and it is quite wide inside. In fact there are three models I believe - for different eyepiece barrel diameters. I ordered one to fit my 32mm plossl (by GSO). Adapter fits fine except - there is an issue with it. Eyepiece barrel has a slope at the top: while adapter fits with flat surface. This makes EP hard to center. With adapter - you get 3 different self adhesive band - and no manual, so I just took one that fits best and inserted it into adapter. This makes EP body fit nicely within adapter but does not remove issue with tilt completely (although it is reduced because you don't need to slide adapter all the way down). This creates repeatability problem - it is hard to position camera at proper / same distance each time. It is also a bit problematic to center eyepiece in adapter as you have 4 screws that you need to "balance" (pretty much like collimation of scope / adjusting finder or similar). In any case - here is EP with adapter attached: Centering is done by making sure - there is same amount of space between eyepiece top and sides of adapter (eyeballing it). Other than centering and positioning issues - adapter works quite well. It easily carries camera attached to eyepiece and everything feels sturdy enough. Attached to scope it does create balance issues but for this test AzGti was not complaining: I ran some indoor tests with this configuration - where distance between eyepiece and sensor is provided by extension T2 ring and at that time 15mm extension seemed to provide wanted reduction factor when used in "proper" EP projection configuration. In the meantime I forgot which extension I used, so I started first with 10mm. This pushed system to the limit - I'm not sure if I was in fact able to properly focus as I reached end of focus range when I took test image: At this close distance - reduction factor is rather severe. I then remembered that I had actually determined that 15mm extension is proper one, so I switched it and here is what FOV looks like with that: (images are binned x2 but not debayered - hence mono look. I plan on using super pixel mode so actual resolution will be same as this - although this image is reduced to 33% to show whole FOV and save on upload image size). This is center crop. According to measured tiles I in fact got x0.25 reduction factor for F/3.3. I'm not sure why is this - either I was wrong in my indoor tests (although I tested it on a ruler and had 2.4cm map to 0.89cm which gives x0.37 reduction or F/4.8) or there is simply issue of repeatability of adapter. At these extreme reduction factors even 1mm of distance change can have dramatic effect on change of reduction factor. I was also less then impressed with edge of the field in this setup. Maybe at F/4.5 - F/4.8 things would be different, but I'm not holding my breath for that. Here are roof tiles at the edge of the field (I just slewed scope so that tiles that were in focus in center of FOV become positioned in corners): other corner I shot two images in opposite corners just to avoid this being tilt issue due to adapter centering - but both corners show same thing - blurring is so severe that we can distinguish tiles any more. This shows that proper ep projection configuration is also - "no go" in this case. What remains is afocal with 12mm lens - I just need to wait for lens to test that out.

Yes it should be if it is adjustable. I know that ASI cameras have that - not sure if QHY ones do as well. You will be able to test your darks for proper offset without shooting other subs and if you still have raw darks from that session - you can test if it is affected by wrong offset. You won't be able to fix that session though. Changing offset requires redoing your calibration subs and darks taken at different offset (one that is not affected) - will not match lights. If offset is issue - lights are "affected" as well (not really affected because LP provides offset and there is great chance you won't have clipping in lights but if matching darks are affected - it will create artifact regardless).