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I'm firm believer in capability of HEQ5 Yep, that is 200mm F/6 OTA + 60mm guide scope on Heq5 balanced by 3x 5kg counter weights. Now, that is experiment I would not repeat, but I did manage to take an image in that combination. Not the best image in the world, but image still ... Here is HEQ5 in different configuration (sorry for poor quality images): Yap, that is 100mm scope being used as side by side guide scope for 8" 1600mm RC scope. This time balanced only by 2x5kg - although they were pushed as far down as possible. Heq5 if well adjusted / tuned / re-lubed can handle up to 12Kg of load for imaging without too much issues. I would put 115 on my heq5 without any hesitation, especially since it's been additionally modded since above images.

There is a thread that I started some time go about all the details, but in simple terms - because it wrongly represents when you over/under sample and does not take all important parameters into account. Above is copied from Astronomy tools CCD suitability calculator. There is mathematical relationship between star FWHM in the image and ideal sampling that I could show you (but it is quite a bit complex and I don't want to overload you, and I've written about it as well) that goes like this: star FWHM / 1.6 = close to optimum sampling. Now, if seeing is in range of 2-4", then star FWHM in the image will certainly be larger value than that, because seeing FWHM is only part of the blur that impacts star FWHM. Other bits are - tracking / guiding precision and aperture size. Smaller scopes will have larger star FWHM then larger aperture scopes for same seeing conditions. In any case - all these things add up to increase FWHM. Even if we did not take that into account and took 2-4" FWHM at face value - optimal sampling would still be in 2" / 1.6 - 4" / 1.6 range or 1.25"/px - 2.5"/px That is quite a bit different result than suggested by CCD suitability calculator. I can show you step by step how above is derived, and also show you where in CCD suitability calculator explanation things are wrong, for example this: That is taken from CCD calculator website as rationale for their calculations. However - it is wrong on several accounts. First - it does not properly specify what Nyquist criteria is, and that is: For band limited signal, in order to completely and faithfully restore it - one needs to sample at twice maximum frequency component of that signal. So we are not talking about "any frequency of analog signal". Then it goes to equate FWHM with that frequency and use half of that - which simply can't be done as FWHM is not directly related to max frequency component of signal. It is related in non trivial way and Fourier transforms must be used as well as certain approximations to be able to arrive to particular number (which is x1.6 that I mentioned above if one uses 10% energy cutoff in frequency domain because there is no clear cutoff for Gaussian approximation to actual PSF - that is serious Math stuff that we can touch upon if you wish). Then it later goes to say - its better to use 1/3 instead of 1/2 - for no particular reason and giving only vague explanation of pixels covering the star image - which are simply wrong as Nyquist gives full explanation and no further work is needed except properly understanding and applying it. Overall - it is very unlikely that any amateur setup will need to sample at 1"/px or less (I personally consider 1"/px to be useful limit for 99% of imaging) - yet CCD suitability will recommend combinations of camera / FL that go higher than that without warning nor mentioning that in such circumstances one should bin their data.

In principle, field flattener - flattens the field. It has no idea what sort of optics sits in front of it, but is design to handle only certain level of field curvature. If element is pure field flattener - then it will work more or less ok with range of optics that produce field curvature that is ball park of what this flattener corrects. Tweaking the distance of field flattener to focal plane will tweak correction for a given telescope. Some items passed off as field flatteners are correctors in general and are designed / matched to particular scope (they correct for more than curved field). As such - they produce the best results with said scope, but that does not mean that: a) said field flattener can't be used effectively with different scope b) different field flattener can be effectively used instead of that one with scope first FF was designed for Issue is that it is very hard to make optical element that only corrects for one type of optical aberration. They all introduce some additional aberrations to some degree. Optical aberrations go into "positive" and "negative" direction - and ideally you want to match them between optical elements so that they cancel out - one brings in positive part and other brings in negative part. This is why "matched" correctors work well - they are designed so that their residual other aberrations for the most part cancel out with those of matching scope. Now if you use such matching corrector with different scope - a range of things can happen. That different scope might have same / similar aberrations as original scope in which case aberrations will still cancel out to some degree, or scope can have unrelated or even same aberrations as that corrector - in which case aberrations can "enhance" rather than cancel out. Unless you know optical design of both scope and corrector - there is no way of telling how good a match they will be (unless they are designed to match each other). That is why it is basically trial and error of using generic field flatteners - you need to try with your optics, or take advice of someone who used that combination and found it works satisfactory.

I guess that you would need to make sure that mount is operating out of specs - which might not be the case. Do you have specification for the mount anywhere - like max P2P error, or maximum RA drift error? First you need to see what is specified as "normal" for that mount and then to show that one of parameters is outside of those normal ranges. You would need to show that without additional gear on the mount, so there are several ways to do it, but I'm not sure which would be considered "definite" proof by your retailer. 1. Using just guide scope and guide camera with PHD2 to create star motion log without actually guiding. This is usually done as a part of PEC correction file preparation and is a way to characterize periodic error. It will provide you with information on peak to peak periodic error and max RA drift rate among other things 2. Using high precision encoder coupled to RA shaft. This is best way to do such analysis as it will give you very precise information without all the noise from seeing or other sources. Problem is that you need at least ~20 bit absolute rotary encoder (you need to be able to at least measure 1" of deviation if not more precise than that) but ideally 24bit one. 3. Using some other means of measuring how precisely mount works - there are several ways to do it, but none are "out of the box" and will require some "software hacking" (finding what software can be used to get results without writing dedicated software). 3.1 Using laser on a white screen in a distance. With this method you can record with camera and lens position of laser pointer mark on a screen and then with help of some software that does motion tracking extract position and in spreadsheet make a plot of how the mount tracks (you'll need some trigonometry to get actual angle from point position) 3.2 DEC tracking vs RA tracking. This one can be done with single exposure or with a video film. In either case you'll need a way to measure a line in an image or point position in a video. You turn the mount to "its side" so that RA actually tracks vertically, and you record star at meridian and equator (due south at celestial equator). Star will naturally drift in RA due to earth's rotation and mount will track in DEC with sidereal speed. This will create line at 45 degrees in long exposure (or dot that drifts along 45 degree line in video). If your mount tracks perfectly - the line will be straight, but since there is periodic error - line will be bent in the shape of periodic error. From the shape of line - you can calculate all the needed info on periodic error 3.3 Using two scopes and artificial star indoors. You place artificial star at focus of one of the scopes (will need some adapter to hold it there - it can be DIY / 3d printed - whatever works) - and this will project artificial star image at infinity (effectively create collimated beam from artificial star - like it is placed at infinity) - you aim that beam at guide scope mounted on your mount and set the mount tracking. Again use software like PHD2 to create log of star position or any other software that is capable of motion tracking and producing coordinates from frames. This is like method 1 above - but it removes: - need for clear sky and night to perform the operation - it can be done indoors in darkened room or basement during the day - removes seeing influence Downside is that it requires artificial star (which you can DIY from a strand of optical cable and LED - look it up online) and way to mount that artificial star to a telescope. Another thing is that it is limited in how long you can record periodic error for. You need to take focal length of your guide scope and pixel size and calculate "/px - or pixel scale. Then combine that with pixel resolution of your guide sensor. Say that you have 5"/px guide resolution and 1920px across. At sidereal which is ~15"/s - mount will move 3 pixels per second and since you have 1920px - that will be 1920 / 3 = 640 - or about 10 minutes of periodic error recording. You can gain a bit of time if you orient your guide camera so that star image moves diagonally - that way you'll get longest distance. That would be about all I can think of. Probably best way to start would be to do point 1. You have the gear for it - just take mount and guide scope that we mentioned (one with 240mm FL) and laptop with EQMod and PHD2 and produce guide log with guiding output disabled. This will "trace" your periodic error with mount alone - no gear attached so no problems with wind or sag or cable drag. Once we analyze that - we will see how much out of "specs" the mount really is (and it would be good idea to find those specs for your mount - I failed to find anything after a quick search).

@matija I just saw that EQ8R-Pro has PPEC, or permanent error correction. You should be able to train that even with ASIAir - as it is feature of synscan controller. Can you check that?

So mount behaved the same as it is behaving now (more or less)?

This is from PHD2 log file that you posted on google drive: If above is correct and you have 240mm entered as focal length (maybe you used 60mm F/4 guide scope with 240mm prior to switching to OAG and forgot to change focal length?) - then your error is exaggerated by x8 and in reality it is much much less. Try checking what FL you have entered in ASIAir and correct if needed and then see what sort of guide RMS you get (it will still behave the same - but will probably report much smaller numbers - like 0.5" peaks instead of 4" or whatever the value was).

Something is off here. What guide camera are you using? From this data it looks like you are guiding at about 5.2"/px which would mean that you are using 52 micron pixel size? For the sake of argument - 3.75um pixel size at 2000mm focal length will give you 0.39"/px, so error of 0.52px translates into ~0.2" RMS - which should be considered superb guiding performance. Can you confirm that you have FL and pixel size correctly entered into guiding app?

Also, it would not be bad idea to try running PHD2 on some computer just for diagnostic purposes - to see how everything behaves. Run guiding assistant - it will tell you amount of backlash as well.

That is normal - it is due to projection on guide sensor. You can image Polaris without tracking as it moves only very little in a small circle over 24h - that is just a handful of pixels for 24h, so rate of movement in pixels per seconds is very small. This means that relative error also gets smaller (how much mount trails or leads actual position). Most severe guiding requirement is near equator at DEC 0. How stiff is your scope and connection of scope to the mount? When you take it by hand and shake it - the scope that is, can you wobble it by hand or is it firmly sitting on the mount?

Skywatcher mounts are hit and miss affair, I'm afraid. Almost no works very well out of the box for such a demanding application and if you want good performance - you need to do two things: 1. mechanically improve the mount 2. tweak guide settings to best suit your particular mount. I had to do a lot to my HEQ5 to make it behave in satisfactory way. First order of business is stripping down mount, changing all the bearings for high quality SKF ones and cleaning and re lubing the thing. Next to that, I did belt mod on my mount, changed saddle plate, added Berlebach planet tripod, but you probably don't need to do that as your mount already has belts and decent saddle plate? Make sure that you also "tune" the mount when assembling it back together. Tune out any backlash / play in gears by adjusting tension where it needs to be adjusted. Maybe lookup EQ8 stripping / tuning videos on youtube. I'm not sure if there are any, but I've seen such tutorials for smaller mounts and they are helpful. Next is guide settings. I've found that skywatcher mounts need to "accelerate" slowly when doing corrections. They are not very stiff / solid mounts. This means slow correction speed - like x0.25 sidereal. You'll also need to play with aggressiveness settings and other parameters. Min motion parameter (I'm here talking about PHD2 params - not sure how ASIAir handles those things) is also very important - and is often misunderstood one as it is expressed in pixels and not arc seconds. It therefore depends on guide system. As far as guide exposures are concerned - I'm firm believer in longer guide exposures - but this can be used only if mount is smooth enough, so mechanical tweaking is important. One thing that you can and should do to tame your periodic error is to do periodic error correction. This will reduce total deviation and will make guide system work less to correct for errors - which is a good thing. Ideally guide system should be minimally employed. From above guide graphs - I think that you have significant amount of backlash in the mount - and that is something that you need to tune / tighten up while re greasing / changing bearings. whenever you see pattern like that - it might be sign of backlash issues. In ideal world - one correction is enough to return mount to where it should be. In reality it often takes 2-3 corrections of varying intensity as there is a lot of noise in the data, but as soon as you see that you need a lot of corrections - this means that mount is not responding to correction properly. Either correction is wrong, or mount "responds" to it - without really moving. This is what backlash does. Instead of correction moving the mount it is "spent" clearing the backlash before teeth properly engage again. It can also be sign that there is a lot of lag / flexibility in the setup. To move large mass - you need to accelerate it and it takes time for that to happen. If everything is not rigid and there is flexibility - then this time goes up (think of pulling something with inelastic and elastic rope - with regular rope it will start moving right away as you pull it, but with elastic rope it will take some time for it to start moving). Hope this helps

Camera window is too close to focal plane and anything on it will be highly localized (think dust shadows). Sensor usually does not ice or fog up evenly and this leads to distinct pattern on the image - either on one side (icing up) or over whole field for fogging up: or ice would look like this: or often in the corner of the sub. Closest to above pattern is dew forming on objective lens on the front of the scope - but that tends to "soften" up the stars and creates halo around them rather than distinct pattern. It looks a bit like this:

Here is an interesting discussion: https://www.astrobin.com/forum/c/astrophotography/deep-sky-processing-techniques/bright-stars-seem-to-have-some-kind-of-flare-happening/ Conclusion is that "some scopes just simply show this phenomenon". I'm not entirely sure I'd buy into that conclusion.

Forgot to say - best way to check focuser is to simply turn scope at a white wall or towards the sky (just be careful not to point near the sun) and look at the focuser side with your eye - just empty focuser, don't use eyepieces or anything - you want to see the silhouette of focuser tube edge against the light coming from objective. Be sure to move focuser through its focusing range as that can change how much it clips incoming light beam. If you don't find anything suspicious - then just try couple things on your next imaging night: 1. aperture mask 2. placing bright star in corners vs center of the field to see if any of those change diffraction pattern.

Next place to check would be focuser tube on far end. If it is focuser tube, then this pattern will only show in certain parts of the image. For example - bright star in center of the frame won't have it, but same / similar star closer to the edge will have it. In general - if there is issue that is equally affecting the whole image - it is likely originating in part of the setup that has light rays parallel (before or at lens in front). If it varies with position in the field of view - then cause is in part of the optical path where light is converging. Focuser sits in converging beam, and as such will produce artifacts (again - look for some sort of protrusion in optical path - either screw or jagged edge or similar) only in some parts of the image, and not in other.

Hi and welcome to SGL. That pattern is most likely produced much further up the optical path - probably in lens cell. Lens cell has collimation screws and screws holding things together. Those are probably main suspects for this pattern. People with newtonian telescopes get similar issues from clips holding the primary mirror or when focuser tube protrudes into optical path and blocks some of the light. This is often solved by shortening focuser tube or using aperture mask over primary mirror that hides mirror clips. Since you have refractor - neither of the two suggestions will help you - but you could try using aperture mask in front of objective lens to stop light hitting sides of lens cell. You don't need to mask off too much - just a few mm on each side - for example, make 90mm aperture mask for testing purposes. Next time you are under stars and imaging - just pop that mask on and perform single (or few) exposure on a bright star to see if there is difference. If there is - then cause is most likely lens cell. Not sure how to proceed from that point in fixing it - but it will probably involve shortening some screws (for example if they are grub screws - using slightly shorter ones) if they protrude too much into the light path. Alternative is to just look at front of the lens and see if you can spot any strange looking screws sticking out. I just thought about another thing that might be the cause (again - mask will show this) - there are objective lens spacers. Small pieces of material that keep two elements of objective lens at correct distance: Those could have ragged edges or be too much inside optical path to create diffraction effects. Ok, I finally found suitable image of ED100 lens to show you what to look for: Spacer bits and any screws that stick too much in the side of the lens cell (ones at the front are not important - as long as they don't sit in optical path - they can't cause issues

Just use long dew shield - it will block any stray light and won't block light or cause diffraction issues.

CA rapidly grows with aperture size. If you look at "CA" index as first approximation (even if we use exotic ED glass), then 60mm F/6 scope will have CA index of 2.54 (that is F/ratio divided with aperture size in inches). Now - we are not interested in achromatic performance of such scope - but we can compare two scopes by CA index if they use same glass elements. In another words, we can see what sort of 100mm doublet with FPL-53 glass we need to match CA index of 2.54 and to have same correction as small 60mm F/6 scope. 4" scope, in order to have CA index of 2.54 needs to be 2.54 = X / 4 => X = 2.54 * 4 = F/10.16 or ~F/10 scope in order to have same correction as 60mm F/6. Yes, we know that SW 100ED/900 has very good color correction and is virtually color free. We also know that 4" F/11 with FPL-51 is also virtually color free. However - we do know that both FPL-51 and FPL-53 4" F/7 scopes have some residual color - especially in photographic applications where sensor is more sensitive to short wavelengths than human eye.

That would be my choice. I can't really say why, it's just something about that scope that is appealing to me.

Oh I see. I was under the impression that 102 in question was a doublet. In that case - choose based on other criteria. I think that both scopes will have level of correction that you need for astrophotography, so that won't be an issue.

115 F/7 with FPL51 is triplet lens and will perform better than doublet FPL-53 glass because of that. I'd personally choose 115mm over 102mm for both visual and imaging. Only drawback is somewhat slower cool down due to amount of glass involved.

Ah, my favorite "happy" drink. I start of with 5-6 shots of it to get me in the mood and then, after all that lemon and salt - I continue sipping cold beer for the rest of the night

Hi and welcome to SGL!

But it is in case of RASA. Given such a fast system and need for correction over large field - that system is far from diffraction limited. In fact - even without seeing effects you are limited to sampling of over 2"/px due to RMS of spot diagram (which really just translates into RMS of blur of optics).

I meant longitude , but yes, it's summer time when the living's easy