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I really like how you managed your background - it is very natural looking. This for example shows it nicely: That faint background galaxy is by no means distorted (as usually ends up being - if one over does denoising). I like the fact that there is very subtle grain in the background as well. I just wonder, why is there so much blue bloat in the image? If you show this crop to someone, asking them what type of scope was used to get this image: I'm sort of afraid that most answers will be - achromatic refractor? It is of course not due to scope, but I think it is down to filters perhaps? This crop is rather telling in that regard - blue halo looks like a huge reflection.

Above sort of behavior is very consistent with differential flexure. Poor polar alignment won't do that - even if guide scope is somewhat misaligned with main scope. Indeed it is due to gravity, but most people blame rings and telescope / guide scope fixtures for this. I would rather look at camera / scope connection. If it is not threaded, but simple compression ring - this is where I would look first. Either on main camera or on guide camera. Here, cable snag can also play a part more than on OTA/mount connection.

I did not actually measured PE error precisely from this mount, but we can do some very basic assessment of how it behaves. Here is animated gif that displays effects of tracking errors over a course of one hour (60 subs of 1 minute): Ra is roughly aligned with X axis and Y is DEC (there is very small drift in DEC due to rough polar alignment - I just used finder scope and pointed it to Polaris - not actual polar scope and did not account for offset). I counted 6 "returns" of the star over the course of one hour - that corresponds with my calculation that worm period should be 10 minutes / 600 seconds. According to this page: http://eq-mod.sourceforge.net/prerequisites.html It has 144 RA teeth - and hence each tooth in worm is 24 * 60 / 144 = 1440 / 144 = 10 minutes. Stepper resolution is given as 0.0625" - which I believe is error and should be 0.625" according to other numbers. About x3-x4 should be guide RMS if I compare that to my HEQ5 that guides at 0.5" RMS and has ~0.143" step. Although, I've heard a claim that AzGTI does not use stepper motors but plain DC motors instead, coupled with encoders? Not sure (I did open my mount to fix backlash, but did not pay attention. There are 4 wires to each motor if that means anything). As for magnitude of the error - it goes like this, I measured 13px p2p on x4 zoomed in image, that makes 3.25px p2p on original image. Original image is roughly 4.8µm pixel at 85mm FL, so 11.65"/px. This means that p2p is roughly around 40" - not unexpected given that entry level Synta mounts like Eq3 and Eq5 (even odd Heq5) have similar p2p values (30-40" p2p). From what I'm seeing - it should be ok to guide this mount at about, well, maybe not 1-1.5" RMS (because of stepper resolution) but 1.5-2.0" RMS certainly.

I'm awaiting for delivery of that AstroEssentials 30mm guide scope from FLO, so I'll be able to see how well AzGti guides. I'm hoping for 1-1.5 total RMS guide figures - something tells me mount should be able to achieve that, but we will see. If it indeed guides up to 1.5 RMS total - I would not mind imaging in 3-4"/px resolution with it. In fact - I plan to do just that for fun - use 1300mm FL scope (mak102) and image at about 3"/px with it What I wanted to say is - it's not the focal length, it is sampling resolution that counts.

In my view - it is rather limited in performance without auto guiding - but not unlike other larger mounts as well. I used it with resolutions of around 10"/px and it is good for about 1 minute exposures in those conditions. Your setup is much more "zoomed in" at 1.53"/px and I would not even try to image without guiding if you want to have sensible sub duration. Also, given that you have 73mm of aperture - use super pixel debayer mode to give you around 3"/px. With guiding and around 1-1.5" guide RMS, that should be about right sampling rate.

In which part is that disagreeing with what I've written above? I also assessed that 30% CO will be visibly blurrier than 20% CO and we know that up to 20% CO there is no really difference to unobstructed that can be seen (12.5% in above image is in that range). Take aberrator and make simulations of 4" APO vs 8" 25% CO Newtonian on same image and see the difference - which will be sharper to again confirm what I written above. In fact - you don't need aberrator - you can do it above with the method I showed you in ImageJ.

Ok, I'll show you why it is not a very good solution and you decide if you agree with that. From a single number characterizing quality of the view of a telescope in terms of blurriness, I would expect at least these two things: 1. Given two scopes number performs consistently - meaning if number suggests that one scope should perform better than the other - that it really performs better then the other scope (in terms of resolution / contrast / blurriness). 2. That there is correspondence between quantity and described attribute. If number suggests that difference between two different scopes should be small - we should not see much difference, and if number suggests that difference should be large - majority of people agree that there is significant difference. With above given definition of Contrast ratio - it fails point number one in following case: Take first telescope to be 76mm Newtonian with 20% CO and take 8" Newtonian with 20% CO. Both will have Contrast ratio of 3.27 - which suggests that they should perform the same, but actual resolution / contrast / blurriness will be miles apart between these two scopes. For point number two, let's compare 4" APO scope and 4" 20% CO Newtonian. One has Contrast ratio of 5.25 and other has contrast ratio of 3.27. That is 60% increase in number. What is the optical difference? - pretty much none. Let's now take that same Newtonian with 20% CO and compare it with another Newtonian with 30% CO. Here we have Contrast Ratio of 3.27 and 2.03. That is increase of 61%. Here, you will be able to tell the difference in quality of the view and contrast. For same difference in Contrast ratio between scopes with same aperture we have different perceived improvement in quality. Does such number strike you as useful measure of something? If I tell you that I have two scopes, one has Contrast ratio of 4 and other has Contrast ratio of 3, and I don't tell you anything else - would you be able to tell which one is going to provide better view and by how much?

I agree, but they are solution to the problem you posed: "I need a number that will tell me if telescope is going to show more blurred image depending on size of central obstruction". This is why I asked - what do you want a number to represent.

I think that platesolving manual scope is going to be a challenge. How long exposure do you think you will use? Let's say you use some "regular" gear - like 3.75µm camera and 50mm finder scope as "guider". These finders have about 180mm of FL, thus working resolution will be 4.3"/px Sidereal rate is about 15"/s if I'm not mistaken, so near equator, one second exposure will lead to ~4 px trailing. These both deform stars (harder to plate solve) and also smear light over more pixels - requiring longer exposure to get good SNR - and increased exposure further smears stars in little streaks. Don't know if you'll be able to plate solve such image successfully. You don't really need great precision between plate solved location and scope pointing, so you could perhaps use fast short lens instead? This will reduce working resolution (reducing blur) while still giving you good precision? Something around 40-50mm of focal length perhaps? Old M42 50mm lens with appropriate adapter?

Indeed, many conflicting requirements. For best visual on planets and DSOs - you want telescope to be large. For ease of transportation, you want telescope to be small. Those that are in between tend to be expensive If your budget is ~500e, then have a look at this scope: https://www.firstlightoptics.com/maksutov/sky-watcher-skymax-127-az-gti.html If you have significantly higher budget - this might not be bad option: https://www.firstlightoptics.com/se-series/celestron-nexstar-6se.html

In principle - it could work, but there are a few issues that need to be addressed. 1. FOV and prism placement. Sensors are usually rectangular and often smaller than field stop of eyepieces. If you just take off camera and put diagonal and eyepiece - you'll likely have vignetting / OAG prism shadow on one side of the field of view. This really depends on field stop of eyepiece - but it needs to be taken into account 2. Back focus distances / issues. Simply unscrewing camera and putting diagonal is not going to work as you expect. There is significant difference in optical path between the two. Diagonals can have up to 100mm of optical path over cameras that have maybe dozen of so mm from thread to sensor. This also translates to OAG - you need to push guide camera away the same distance. This can be done with extension tubes (T2 extensions or whatever) - but prism further in optical path will certainly start to vignette and you'll effectively have aperture stop on OAG. It will just require longer exposure to get enough stars to plate solve 3. It is Off Axis guider after all - so you need to account for that - if you plate solve OAG to particular part of the sky - eyepiece will actually be centered on different coordinates than those resolved by plate solving. You need to figure out this displacement direction and magnitude and account for it when directing scope and plate solving. With all of the above in mind, I have couple of observations: 1. Too much hassle? 2. Maybe setup with ONAG will serve this purpose better 3. Maybe smart phone + prism will act better as sort of DSC - to tell you where you are - Celestron seems to have this in their beginner telescopes like these: https://www.celestron.com/collections/starsense-explorer-smartphone-app-enabled-telescopes Not sure if there is open source or at least stand alone application that does the same (no reason why there would not be one) 4. Guide scope can probably serve the same purpose and solve most of the problems listed above

With that sort of focal length - binning is always an option. I guide with ASI185mc - color version. It's a bit larger sensor that my OAG can illuminate - but who cares - as long as it works. It has 3.75µm pixel size and I regularly bin it 2x2 (there is option in PHD2) - so my guide resolution ends up being around 1"/px. Even if I don't bin - I always find guide stars with it, binning just improves SNR.

Yes, I mean - that is how I usually do it - it separates into different images - then you can close off R and G image and just keep working on R image (truth be told - you can do same with layer - just delete R and G layers - not sure if I contributed anything clever - both ways seem equally useful ).

I'd rather RGB decompose to separate images ... This will also give you mono image - use red channel of course.

Well, mono cameras tend to be a bit more expensive - so if you can stretch the budget: https://www.firstlightoptics.com/zwo-cameras/zwo-asi-183mm-pro-usb-3-cooled-mono-camera.html for £979 or FLO is having discount on some items that were pre owned / serviced: https://www.firstlightoptics.com/offers/offer_zwo-asi1600mm-pro-usb-3-mono-camera_190239.html for £1,147 These are both very good options. Starting now with just those filters is fine, and you can add narrow band filters later on.

What OS do you have? Here is page for downloading of actual files (standalone version): https://sourceforge.net/projects/starnet/files/v1.1/

Well, your budget is the limiting factor. Going by your interests and scope that you already have - I was going to recommend mono camera + filters and narrowband filters in particular. However, such setup is way out of your budget (you'll struggle to find even cooled camera to fit your budget let alone all filters and filter wheel). I can't tell what is available second hand, but if you want to go with new, this is probably the best that you can get with that sort of budget: https://www.firstlightoptics.com/zwo-cameras/zwo-asi-183mc-pro-usb-3-cooled-colour-camera.html

I think it will also depend on binoviewer in question. I've never used binoviewer so do take my answers with a grain of salt (hopefully somebody that uses binoviewers will be along shortly to confirm or give alternative advice) but I do think that catadioptric telescopes don't have any issues with binoviewer use. This is due to fact that they focus by moving primary mirror and have very large focus range. This means Maksutov and SCT telescopes (which are good planetary performers and as I understand it - most people use binoviewers for planets primarily). Newtonian scopes are rather poor at back focus and almost none will work with binoviewer without modification - which means moving primary mirror up in tube closer to focuser. For refractors, I believe that imaging refractors stand better chance of being binoviewer friendly as they are made to accommodate many accessories like focal reducers / field flatteners, filter wheels, OAGs and such - which means a lot of back focus. Some models are even "modular" - with extension tubes that you can remove when binoviewing.

Problem with binoviewers is that they use up a lot of optical path. Sometimes telescopes don't have enough back focus distance to accommodate binoviewers and in those cases you need to use glass path correctors with them - optical elements that add magnification but push focus plane further out enabling you to reach focus. Binoviewer ready/friendly just means that focus position is further away than in "regular" scope and focuser has enough travel to be able to accommodate regular diagonal / eyepiece and binoviewers.

Yep, differential flexure - not showing in single subs, but it does show during the course of imaging session. When I was using guide scope, I had drifts like 20-30px in couple of hours due to this. For each sub this is less than one pixel, but over time it adds up. One way to look at it - it's sort of natural dither and if it's not causing too much issues - just don't worry about it (but do keep in mind it's there - so you don't get surprised)

Ah, excellent - only distinction is more blurry (or blurrier - which one is right, how does blurriness compare?), then take contrast factor to be 100% - CO%. 25% CO will have "contrast factor" of 75% 30% CO will have "contrast factor" of 70% Hence, scope with 30% CO will give more blurry views than that with 25% CO. Problem solved

Well, look at Nigel's post above ( @Astrobits ) - he explained why you see 8 spikes in the video. What is misleading is mask in AliExpress product description - it shows regular B mask with 3 different edge orientations. One used in video is different - it has 4 different orientations: As given in above answer - spike in the image is perpendicular to any straight edge in B mask and hence it creates two spikes - each one at 90° to edge but at opposite sides. 4 different edge angles will result in 8 spikes.

You are using it on newtonian scope so other spikes are just regular newtonian spikes two of them being aligned with two spikes from B mask and hence hidden while other two being visible for total of 8 spikes?

I'm wondering, what would you like that number to represent? Can you describe it's meaning - the way you would expect it to work? Let's say for sake of argument that you have one telescope that has "contrast ratio" of 1 and another telescope that has "contrast ratio" of 0.8 or 80%. Can you describe what would you expect differences between the two scopes to be?

All I can say is that I don't doubt the math, but: I don't really understand how "relative peak diffraction intensity" is defined, further, I would not call it contrast ratio, regardless of its definition. As for Strehl ratio - you can look at Strehl ratio in two different ways - both suitable for certain case. Strehl ratio is ratio of energy contained in Airy pattern up to first minima in comparison to perfect theoretical telescope. Choice of perfect theoretical telescope will give you Strehl ratio that is suitable for certain comparison. For example, you have two Newtonian telescopes with same central obstruction and you want to see which one is better. You calculate Strehl ratio of each in comparison to perfect aperture with same central obstruction and compare those. If you want to put two scopes against each other that have same aperture size, but different or non existent central obstruction - you calculate Strehl against perfect unobstructed aperture. However, once you start doing that - you will get misleading results. Similarly misleading results as comparing two telescopes of different aperture. For this reason, I would suggest that Strehl ratio be used only as measure of how good optical figure is. To get that value, you should really compare it to perfect aperture with same central obstruction - regardless of central obstruction size. If you want to compare performance of two telescopes - of different design or different size or different design and size - use MTF graphs for those two telescopes overlapped. That will give you the best idea of how two telescopes differ in views they provide (or rather theoretical views they are capable of). MTF can be used for real telescopes as well - one just needs to asses their wavefront error and this can be measured - for example by Roddier analysis. I'll give you example - you can't really compare performance of 4" APO refractor (we will assume it is perfect in every way) and 6" Newtonian with 25% CO (again perfect in every way) by using Strehl or above "contrast ratio". On the other hand, making MTF diagrams is really not hard if you follow what I've written above. Here is result: Black line is 6" with 25% CO and red line is 4" without CO. Is there any contest between the two? Can you decide which one will throw more detailed image in same conditions (same magnification and seeing good enough to show what scopes can deliver)? We can throw 5" APO into the mix - common "wisdom" says that 6" with 25% CO will in fact perform as 4.5" clear aperture scope. This means that 5" should beat it? Huh, tough one, isn't it? In low frequency domain, 5" unobstructed, here in blue, will in fact have slight edge over 6" with 25% CO, but in high frequency part - 6" will still show more. This translates into following description: "View of Jupiter thru 6" looks more washed out with colors being not as saturated as in quality 5" APO, but 6" Newtonian shows hints of those tiny ovals and ripples in belts that 5" APO just can't produce".