vlaiv

Not really, or rather - it depends. We have to be pragmatic about this and understand what is going on. I'll try simple analogy and then connect that with real world example in imaging. Consider this (I just realize that this may involve more mental gymnastics than I initially envisaged since we are used to different temperature units - but try to convert to F what I say in C) - Difference between half an hour and 10 hours is x20. That is sqrt(20) = ~x4.5 improvement in SNR. Now consider the difference between water that is at body temperature and one that is x4.5 colder - or about ~ 8.2C. There will be wast difference in sensation between the two. Splash 37C water on someone and they wont get too excited - maybe a bit annoyed because they are wet now. Do that with 8.2C water and it is likely that they will jump from their seat - it will feel rather cold. Drink a glass of water on one temperature and other and you will see very big difference. Swim in water with said temperatures and difference will be huge. Now, consider 8.2C and 1.82C water or maybe even between 1.82C and 0.4C. Both are very cold, both are near freezing but we don't ascribe big difference between those temperatures - but they are still x4.5 ratio in value. You know that for each additional imaging time - SNR improves like square root of time spent. If you want to double SNR, you need to spend x4 imaging time on target. At some point you enter region of diminishing returns since we can't tell difference between 50 and 100 SNR but we can tell difference between 5 SNR and 10 SNR. Notice that both are x2 improved. If I do this: Everyone will see the obvious difference between the two: - Stack is lacking hot pixels and cosmic ray artifacts - Stack is much much smother - all the grain is no longer there (SNR is indeed improved adequately for x20 longer exposure) - Images are stretched differently and stack lacks contrast - it is much "flatter" Everything seems to be as one would expect for high signal target and half an hour vs 10 hour of exposure (except the difference in stretch).

Why don't you just try what I suggested to see if it will make any difference to your perception of quality of each image?

Welcome to SGL. This has been discussed number of times before, so check out these threads: There is also calendar entry: (you can access it from home page or via Browse / Calendar menu item.

Not weird at all. You are relying on STF to provide you with same level of stretch - and it is not happening. Stacked data is lacking contrast due to different level of stretch and that does not look nice. If you really want to see the difference - you need to do "split screen" approach. Copy half of still linear single sub data and paste it over still linear stack (have them aligned of course so they form proper image after copy/paste operation). Then proceed with any stretching / processing. This will make obvious difference for given level of processing as both sides of the image will undergo same processing.

It won't be a problem even at 50ms if you use low guide speed. It is high guide speed that will cause issues even at 20ms (you can't issue a pulse lasting less than that, but even at 2"/px guide resolution for MinMo of 0.1 you will need to go lower than 20ms - at 14.8ms to get good correction and not overshoot - with 20ms you'll overshoot by 25%. I'll let you do the math on 0.5"/px guide resolution).

I can see that pdf guide is outdated and that it is indeed possible to set this value lower - but default is still 50ms and it is still doing the same. It is rather obscure setting and most people have not heard about it - hence it will be set to 50ms for most if not all people anyway. I'm sorry to say but logic that PHD2 Developer applied is flawed - let's just see what happens if we use that line of reasoning. If we use 6"/px guide scale, which is according to the developer - too coarse anyway, and MinMo of 0.1 we will get 0.6" threshold of movement (if you want to guide at 0.5-0.7" RMS this is not acceptable, but let's go with it) - indeed at 13.5"/s it will take 44.444ms for correction to happen - and it won't be possible as minimum correction is 50ms. We lower our minimum correction to 20ms and all is fine - no more error. However, like developer mentioned, most people don't use such coarse guide resolution. Let's keep 20ms as minimum pulse width and use better guide resolution - let's say 2"/px? We still have MinMo set at 0.1. Now we have 0.2" as our minimum correction and at guide speed of 13.5"/s that is 14.8ms - again below even lowest possible setting of 20ms. Using finer guide resolution just makes things worse with respect to this issue. If you have lowest pulse duration and you want to do precision guiding with EQMOD, you need to lower correction speed. That is only sensible way to do it. Manual further states: It's talking about setting guide rate "too low". Is this true? Let's see on an example. If we set our correction speed to be fairly low - x0.3 sidereal or about 4.5"/s and need to make very large correction - 4" in a single cycle, can we do it? It turns out that we can. Even with very short guide cycle of 1s (I advocate using at least 2-3s as a guide cycle to smooth out seeing influence and stabilize the mount), it will still have enough time to apply correction as it will take less than 1s to do correction of 4" at speed of 4.5"/s (in reality it can take even longer than this because camera exposure is stopped until correction is finished - it does not "fire" on every second like a clock - I just wanted to point out that correction is shorter than a guide cycle even for very large correction and one should not worry about chasing the error rather than correcting it).

Here it is from their PDF documentation: I'm quite certain that one can accurately measure much shorter periods of time, but this is what they say (and indeed, long time ago in Win95 and similar operating systems, when using simple timer component, one was limited to that sort of timer resolution), however things have changed and now one can measure in nano seconds (processor tick count).

My recommendation for lower guide speed is based on following: - there is a setting in EQMod primarily kept for legacy reasons - minimum pulse duration. It is there because at the time, systems were not able to time properly intervals less than that. It is set at 50ms. This limits minimum correction that one can make, depending on correction speed. With 13.5"/s guide rate, that equates to 13.5"/s * 50ms = 0.675" That is minimum correction that you can make. This alone gives you something like 1.3" of error in each guide cycle. If you are off the target by small amount - you will overshoot by 0.675" - error - in all likelihood that will be at least 0.4-0.5 for OAG as it can measure error down to 0.1". In most cases it will be due to seeing. MinMo is set to 0.51 for both axis. This parameter is still set in fraction of a pixel, so that works out to be 0.2" or there about. Any error in that range will trigger correction - and likely overshoot - that will most likely happen due to seeing. - Heavy scopes on lighter mount have quite a bit of inertia. Using high guide speed will result in them wanting to keep going further then they ought to by correction. For this reason - one must use slow correction speed so that energy in the system is low.

Yes, my recommendation was due to the fact that I was experiencing rather fast ripple in tracking performance - 13.8s one, related to period of single tooth on motor gear. It was due to improper meshing between motor gear and belt. I'm not seeing that in your guide logs - not yet anyway, it could turn out to be an issue after you get good guide results, but for now, if it's there, it is masked by much larger issues. Here are some recommendations that I suggest you try: 1. Bin your camera pixels at least x3 or even x4. There is plenty of them and you don't need to go as low as 0.43"/px for guiding. My camera gives around 0.48"/px and I bin it at least x2 to get close to 1"/px (OAG and 1600mm FL). With x4 bin you'll be at 1.72"/px and that is good enough for best performance possibly offered by HEQ5 (which is around 0.5" RMS). 2. Use ASCOM driver for your 290 camera instead of native drivers - this will enable 16 bit readout mode and improve centroid precision and star SNR 3. You are using quite fast correction speeds. I think you would benefit from lowering those. You have it set at x0.9 sidereal (13.5"/s). Consider lowering it to something like x0.3-x0.4. HEQ5 type mount responds much better to slower/longer corrections than fast short ones. 4. Consider using dark library / dark calibration with PHD2 5. If you think that seeing is a problem - maybe experiment with guide exposures up to 4s long

And always seem to be out of stock with time to delivery being 1+ months.

What FF/FR is usually recommended for Esprit 100 and what is resulting focal ratio?

I don't really subscribe to all that "lickability" stuff, but I understand that some people enjoy it, so here we go:

I remember seeing a thread over here that discussed said scope but can't remember actual detail (good or bad).

Planetary type imaging? Even Eq5 would be sufficient for that - as long as it can carry the weight and point in general direction of a planet - it will do I imaged planets with EQ2 and simple DC motor that even could not track properly - there was potentiometer that regulated tracking speed and I needed to adjust it every 10-20 seconds to keep planet in FOV of camera - but it worked.

@carastro Here is my scope (old image - when it was brand new): As you can see, it has two sets of screws on focuser - silver ones, one set at each side. These screws use hex wrench and don't hold anything on the scope - they are ideal, and in all likelihood meant to hold finder shoe. I use them like that. You can also mount your guide scope on top of rings - they have flat surface and M6 holes (x3 each) - also visible in the image. In the end, I've found dew shield to be rather stiff in motion and unless under heavy load - stays in place. When I'm doing flats - I turn scope vertically and rest flat panel on dew shield. Sometimes it will hold flat panel without issues, other times it will start to slip slowly - I guess it depends on temperature as it probably expands differently then the rest of the scope. In any case, there are three very small hex bolts - best seen in this image (but also visible in image above): I believe that these are used to tighten / loose dew shield sliding action. Not 100% sure as I have not tried - but I can't see them being used for anything else.

I had issues with tilt as well, that is why I switched to threaded connection using this adapter: https://www.teleskop-express.de/shop/product_info.php/info/p9781_TS-Optics-360--Rotation---Thread-Adapter---M63-to-M68--M54-and-2-.html together with this one: https://www.teleskop-express.de/shop/product_info.php/info/p5144_TS-Optics-Adapter-from-M68-and-M63-to-M48---Riccardi-Connection-Adapter.html Very versatile combination - provides many different threads for camera side connection and also a M48 filter thread on telescope side - inside focuser. Useful for both reducers and filters (like 2" LP filter).

Btw, if you want something really fast, close to Samyang and in focal length of above scope, maybe have a look at this: https://www.teleskop-express.de/shop/product_info.php/info/p11082_TS-Optics-150-mm-f-2-8-hyperbolic-Astrograph-with-Corrector-and-Carbon-Tube.html

I have similar scope, but without built in field flattener - this model: https://www.teleskop-express.de/shop/product_info.php/info/p3881_TS-Optics-PHOTOLINE-80mm-f-6-FPL53-Triplet-APO---2-5--RAP-Focuser.html It is very good scope and very well worth the cost. I use it with x0.79 FF/FR. There is also x0.75 Riccardi FF/FR that would make it F/4.5 - so pretty close to above scope. From what I've read - some people had some issues with flat field TS scopes - mostly due to alignment of optics (there are 6 pieces of glass that need to be precisely aligned and spaced), but don't know how often that happens. In my view, probably better option would be to go for regular triplet and then use good FF/FR like Riccardi one, but again - I have no real experience with flat field version - maybe it is hassle free (no need to purchase adapters, fiddle with distances and so on) and that could be a big plus.

Probably not going to help much as I have yet another contender to be added to the list? https://www.widescreen-centre.co.uk/tecnosky-80480-f6-fpl-53-ed-triplet-apo-refractor-telescope.html 80mm F/6 APO - same scope as TS 80 F/6. Largest of the bunch, but I think it will still be carried by SA. If HEQ5 is planned - it is very good match (I have that combo - only TS version).

That is always a good guideline - if price is such that you feel like buying a new item rather than having that second hand item - you should definitively purchase new. On the other hand, if you think to yourself - look what I could be having for only "insert number here" and it is in very good condition - then you probably should purchase that item second hand.

I would go for 130PDS except for intended mount - it is rather limited. In my view 130PDS has edge in AP and visual. Only advantage 72ED has is portability and small size. It is wide field and travel scope. 130PDS has 4Kg without any accessories. Add CC and 600d and you'll be pushing your mount quite a bit. You also need to think mount precision regardless of the weight. AZ-GTI is wide field imaging mount, so wide field scope is really only suitable option (next to of course imaging with lens - which are inherently wider field since they are shorter FL than almost any scope out there).

There is quite a bit of difference with effects of CO on imaging and on observing. For imaging it is not as nearly as important as for observing. We can and do digitally enhance captured images (contrast and deconvolution / sharpening). We can't do that while observing. In fact, for very technical reasons, I think it is better to image with obstructed aperture than clear one (of the same diameter). As a final note, I don't think people really understand what sort of contrast we are talking about here as it is frequency dependent contrast and it is only partially related to notion of contrast that we use in everyday life.

Not really, or rather - it depends on measurement method used. For example Roddier test (WinRoddier implementation) allows you to select if you are using artificial or real star and in case of artificial star - distance to telescope. It will automatically remove spherical term produced with close target so you can have even 1/4 or 1/2 wave of spherical and it will be accounted for. On the other hand - that much spherical with other tests will mask other aberrations so you won't be able to tell how good optics is. Any test that produces Zernike polynomials of your optics allows for removal of appropriate spherical term - you just calculate spherical magnitude like above given aperture size, wavelength and distance and you remove it from appropriate Zernike term and then synthesize wavefront again. This is common practice - and you will often see some terms remove in optics reports - like coma from newtonian parabolic mirrors since it is inherited in design and will be 0 at optical axis (but can have small value if test was done at an angle and is thus removed).

There are two "points" that you need to worry about when using artificial star. First point is point where artificial star is resolved. You want angular size of your artificial star to be at least half that of airy disk in order not to be resolved. This distance is not all that big with very small artificial star. For example - using 150mm aperture telescope and medium sized artificial star - about 20um (most artificial stars are in range of 50 down to 10um), calculation goes like this: 150mm aperture creates 1.7" airy disk, so we want our artificial star to be at a distance so that it presents about 0.8" of angle with its diameter. http://www.1728.org/angsize.htm handy calculator for those things, we input values - solve for dist a nce, angle in arc seconds and we will put artificial star diameter in millimeters rather than microns - so 0.02 Solution is 5156mm, or 5.15 meters. That is fairly close and in general you need to worry about that only if you have larger artificial star and larger aperture (like 50um and 200mm+) then you need 10m+ for resolving criteria. There is second criteria - and that is amount of spherical aberration we want to introduce or rather not introduce. That is important for optics testing. Here we can do following: Let's say we want to have less than 1/10 wave spherical due to distance and we work with 150mm aperture. What is the distance we want to place our artificial star? 1/10th of 0.5um (500nm green light) is 0.05um. What angle will give difference in hypotenuse and side of 0.05 and what would be sides length? Here is a nice image to explain it. Artificial star is at A. B is center of our aperture and C is edge of our aperture. AC edge (hypotenuse) is longer than AB edge (Adjacent side). We know CB - as being half of aperture, or 75mm. We need AC - AB to be 0.05um long. So first equation is: AC - AB = 0.05um => AC = 0.05um + AB Second equation is Pythagoras AC^2 - AB^2 = CB^2 => AC^2 -AB^2 = 5625mm (0.05um + AB)^2 - AB^2 = 5625mm 0.0025um + 0.1um * AB + AB^2 -AB^2 = 5625mm => 0.1um*AB = 5625000um - 0.0025um AB = (5625000 - 0.0025) / 0.1 = 56249999.975um = ~562450mm = ~56.2m We need at least 56.2m of distance for 150mm telescope to have 1/10 spherical from artificial star.

Not really necessary to do that as collimation issues will be equally seen both sides of focus - only mirror reversed. I don't think so. You certainly need larger or at least the same aperture of "reference" telescope to be able to cover whole aperture of measured telescope for optical quality tests, but wavefront error won't be reduced if one is using only part of the aperture of reference scope - at least not random part. If you know that you have very good section of mirror and you make sure you use that section of the reference mirror - then yes, it will help. Another way large mirror helps is that same mechanical surface quality translates in less relative wavefront error on large mirror (at least I think so), but large mirrors are harder to make and on average I guess you end up with same level of wavefront error (basically you get what you pay for and if you want very good figure, it's going to cost in large aperture).