vlaiv

For same total imaging time - less longer subs wins over more shorter subs. This is because of read noise. If read noise is 0 - the those two are equal - then it does not matter how long are your subs - as long as you accumulate same total imaging time. As read noise is not zero - it makes a difference. It makes less difference to CMOS cameras, because CMOS cameras have lower read noise. Whenever read noise is low enough - difference is small enough - you almost can't tell the difference between few longer subs over many shorter subs (again - same total duration). Higher gain - means even lower read noise (by a bit). This is particularly important if you use high resolution / long focal length - as read noise stays the same - but it grows in relationship to other noise sources. 100 x 5 vs 5 x 100 will have more difference at 0.5"/px than it will at 1"/px for same aperture.

Not sure why it would seem crazy? There is only limited amount of detail in the image - if you want to match detail in the image with sampling rate - then you should go for bin factor that is suitable. If you achieve on average 2" FWHM - appropriate sampling would be 1.25"/px. If in turn your native resolution is 0.28"/px, then 1.25 / 0.28 = x4.46. You should bin at least x4 to get to 1.12" sampling rate. Of course - you don't have to - it is a choice, and again, it depends on whether you think you have the detail or not. I don't think that detail is there - but you can also take your image and compare it to some really high resolution ones - like HST images. Match them at different pixel scales and see what sort of detail you have in your image and what sort of detail there is in HST image. HST image will depend only on resolution selected - it has enough detail, but your image might not have detail at high resolutions and might look blurry. Try to match them to see what is the actual resolution of your image - one where you have detail to match.

I think that 20s unguided is viable - you've shown that it works. Question is - does it work better than guided? Not sure if I can answer that one - but I do think that guided longer subs will go deeper and get you better SNR as result for your overall imaging time. One benefit of going deeper is that you can sharpen more without bringing out noise as much. Maybe that will yield better results - not sure. I think it is the way I would approach things - guide, get better SNR and then sharpen image (within limits and until it makes sense - don't over sharpen). Another important thing to do is - well, accept resolution limit imposed on us by seeing. I know that this is hard and we like to think that we can do close ups - and most people try, but as you have seen - although we can get in real close - that does not mean we get the detail. Just checked some of his images with C11Edge - and it does not look like that to me. They seem over sampled as one would expect at those resolutions.

I understand that you think you have detail in that image that is supported by 0.28"/px - but it is far from it. Here is what can be recorded at that resolution (I eyeballed resolution, but I think it is pretty good match): Those things that are featureless blobs in your image are in fact rather interesting clusters, At this resolution you should be able to start to resolve those clusters in individual stars. Look what happens if I reduce that M99 image of yours to 25% of its original size and compare it with same comparison image: Now they look pretty much the same in terms of detail that they show. Fact is - this image might be sampled at 0.28"/px - but in reality contains detail for only ~1.12"/px - which is still great. Not many people can image at 1.12"/px resolution. I used first one. With second one and a bit less aggressive stretch - that one star is easier to spot, but still not quite resolved (in terms of split between the stars).

Aperture size certainly makes a difference - but I think we are hitting limit - both with 8" image and with your 11" image. That is not due to aperture size - but rather due to seeing. There is also fact that as you stretch more - you get more bloat in stars, however - one should be able to detect certain splits - that is what resolution is about. Look at that last image I posted - star sizes are not considerably smaller compared to our images - yet there are clean splits in some cases. There is small star in this group: Here there is no question about it. My 8" version seems to show it, but we can't be certain due to level of stretch: In your image - it also seems to be there, but there is no split - it looks like joined to larger star - as if larger star is a bit deformed: What I'm trying to say is that actual resolution is not supporting pixel scale - similarly to that 80mm image that was enlarged to 300%. Maybe we image at 0.67"/px in my 8" case or 0.56"/px with your 11" - but detail is not there.

I managed to track one really resolved image at similar pixel scale: Same star triplet that actually shows bunch of smaller stars around it.

Very nice image. It can serve to study resolution achieved. I've imaged M13 on few occasions with different gear. This gives us nice chance to do comparison. In particular I want to focus on 3 prominent stars that are close together. This is screen shot from your image. Stars are almost touching. There is also fourth star as well - blue one, in that group that almost seems to be touching one of three main. Here is same triplet of stars in one of my images. This was with 8" scope. Guiding was poor and most of my stars are a bit elliptical because of that. This one is done with 80mm scope. This image has been enlarged to 300% of original (original at 2"/px).

In that case - don't worry much about guiding, your guiding setup is fine. With that mount, I think you'll be happy with about 1" RMS or so. Don't think you'll be able to go much lower than that as it is lightweight mount that is not very precise - it has single step 0.28125" in its stepper motors. Look into binning of your camera data. Not sure what camera are you using - but you can bin either in hardware (CCD) or software (CMOS usually, but CCD can be binned in software as well). There is relationship between imaging resolution and achieved SNR in your image for given time and aperture. Higher resolution you image at - lower SNR you can achieve. This is because light is effectively spread over more pixels and image gets "dimmer" per exposure (similar to using higher power eyepieces - image gets dimmer with increased magnification). Lower signal -> lower SNR. At some point, you enter domain of empty resolution - meaning increased resolution won't help you capture additional detail because things are blurred out - due to seeing and mount and aperture of the telescope. In long exposure astrophotograpy it happens at much lower resolution than the scope is capable of in comparison to lucky type planetary imaging - because of mount performance and atmosphere. In that case - increased resolution is not bringing you anything - and it is costing you SNR for your image. For this reason, it is not good to over sample your data, and if you over sample your data - then you can use binning to recover SNR. How to know how much to bin? Well - that is rather easy - take your linear subs and measure average FWHM you get with your setup. Take FWHM in arc seconds and divide that with 1.6 to get sampling rate. If for example your average FWHM over the course of the night is about 3" - you should sample at 3 / 1.6 = 1.875"/px. If your FWHM is 2.2" then you should sample at 2.2 / 1.6 = 1.375"/px. After you figure out - what your skies and setup is capable of - then it is just matter of "dialing" in as close as you can with binning - with keeping in mind that slight under sampling is better than over sampling as far as SNR is concerned. You can bin x2 to get 1.7"/px - which is nice medium resolution to work with and requires FWHM of about 2.7", or you can bin x3 to get x2.55 - which is nice lower resolution that is good for FWHM of about 4". Hope this helps.

Yes, I flashed firmware that supports EQ mode of operation. Once you switch to EQ mode - you still need to do alignment if you want goto to works for you. Indeed, motion profile is uniform in EQ mode - no change in tempo and only one motor running continuously. That makes for smoother ride - simpler tracking. I did not compare - I did not try to image in Az mode. In Az mode, besides all of this - you have additional thing to worry about and that is field rotation. Target slowly rotates in field of view when you track in AltAz mode. I took some mosaic images of the moon (using lucky imaging technique) and in that half an hour or so - it was really visible. I had trouble composing final image out of panels as panels were rotated between themselves (not oriented the same). This does not happen in EQ mode. Of course there are - but are much more expensive. There are mounts that have high precision encoders that can track 10-20 minute without being guided. Such mounts for example cost $8000+ (look at 10Micron GM 1000 HPS mount). I have much heavier and more precise mount - HEQ5 and I still need to guide that mount if I want to do 60 second exposure at around 500mm. If you want tight round stars - you'll have to guide on almost all mounts - except for those few very expensive mounts that have exceptional precision and encoders.

Hi and welcome to SGL. I don't really subscribe to notion of having certain ration between imaging and guiding resolutions. In the end - one might end up with certain ration - but these two things should be considered separately. For example - given your stats in screen shot above - you'll be imaging at 0.85"/px, In my view that is very high sampling rate. In 99.99% of cases you should not go below 1"/px in long exposure astrophotography. In fact, with 6" scope - you should probably keep above 1.2"/px. What mount do you have and how well it guides? Guide resolution is more related to how well your mount guides and how accurate you want your centroid calculations to be rather than imaging resolution. Guiding at 4.58"/px is probably on the low side of guide resolutions. Say you have mount that can be guided at 0.6" RMS total. By The way - that is poor guiding if you want to attempt resolutions below 1"/px - but otherwise quite decent for budget mount (like EQ6 type mount - in fact in order to get there - you'll need to tune EQ6). Anyway - in order to reliably measure that sort of RMS - you need your precision to be at least 1/3rd of that - so centroid precision needs to be at least 0.2". Centroid algorithms have precision to 1/16th of pixel - which means that pixel can be max 16 larger than 0.2". So we have 0.2 * 16 = 3.2"/px. You need 3.2"/px or higher guide resolution if you want to reliably measure 0.6" RMS total. You might be able to get reading of 0.6" RMS with 4.6"/px guide resolution - but you can't be sure if that is actual 0.6" or maybe 0.8" RMS or 1" RMS - as measurement itself is not precise enough. If we go the other way - 4.6 / 16 = ~0.86" RMS So that guide resolution that you now have is good enough if your mount does not usually guide below 0.9" RMS. In turn - you don't want to image at 0.85"/px with such mount as you'll just get blur at that resolution. That sort of guide RMS is ok for 2"/px or above resolution (depending on seeing). General rule of thumb is that your guide RMS should at least be half of intended imaging resolution of not less. I'd recommend that you bin your images x2 and get OAG for RC6" type scope.

Well, that is what I call a near miss

I'm a bit confused by results presented in the table. Off the bat - people with average visual acuity can resolve down to 1 arc minute. Moon is 30 arc minutes in diameter. 10 arc minutes is 1/3 of the full moon. If I saw a star in the eyepiece being as large as 1/3 of the full moon by naked eye - I would be very upset with such eyepiece - and I certainly would not consider it as perfect. Similarly, Baader Zoom Mark IV has >60 arc minutes spot size at the edge of the field at F/4??? That is more than one degree! That is twice the size of full moon with naked eye - or width of your fingertip when you extend your arm in front of you. I'm certain that people seeing this in their eyepiece would say something online: This is what 1 degree "spot" looks versus ~50° AFOV

Mount like AzGti/Gte is not astrophotography mount. It can be turned into astrophotography mount if you turn it into EQ mount and load appropriate firmware onto it (even then SkyWatcher warns that they are not responsible for how mount performs). One part of turning it into EQ type mount is adding appropriate wedge so that mount can be polar aligned. Difference between AZ type mount and EQ type mount is among other things - in the way mount tracks - or how motors work. With EQ mount, only one motor needs to be working for mount to track - RA axis motor - and that motor needs to move at constant speed. With AZ mount - both motors need to be continually working and they are always working at different speed - not only different speed to each other - but speed that changes every second. Just to give you an example - if you are tracking object just rising in the east or setting in the west - most of motion of object will be in vertical direction. This means that Alt axis motor will do most of the work - and Az axis motor only needs to do small corrections. In contrast to this - think of what is happening to object crossing meridian due south. Here object is moving almost exclusively in horizontal direction - Az axis motor here is doing most of the work and Alt motor - only corrections. Speed of each motor changes continuously between these two "extremes" as object is tracked across the sky. Mount like AzGti/Gte is powered by stepper motors - and those have finite precision. It also has finite precision in measuring time intervals. To be able to track object accurately in alt az mode - you need good precision of both of these things. Ok, this may have been too harsh on little AzGti - it has enough precision to track object for visual applications - but not enough for very demanding applications like astrophotography. On top of that all - mounts are not perfect and they have machining errors / tolerances in their parts so they don't track perfectly even when having enough resolution / precision in timing and stepping. This is called periodic error associated with gears. Most mounts regardless of the type suffer from it. If you want to do photography with your AzGte mount - look into switching it into EQ mode - that will help with tracking, but even then, if you want to have exposures of decent length - you'll need to guide your mount. Btw, if you want to try EQ mode - you can do it fairly easily, here is my setup before I purchased dedicated wedge - I used DIY counter weight / counter weight shaft and ball head as Equatorial wedge (yes, polar alignment was bit of a pain - but it was doable): With this setup, I was able to do up to 60 seconds with short focal lengths - up to 100mm or so. With 500mm focal length - well, that is going to be tough on little mount and I don't think you'll be able to pull much longer than maybe 20-30s without use of guiding.

I think you are quite right - between the two mentioned scopes at the beginning of the thread - better option is one with FPL-53 glass and given that price difference is small - it stands to reason to go for that one. One can also expect a bit of star bloat in blue - little bit less than in example given in a thread above which shows 102mm model - smaller aperture shows less CA everything else being equal, so 80mm F/7 scope will have less of it. It won't be perfect, of course - but there are options available like Astronomik L3 filter for example to be used with OSC cameras.

I don't think he said that. I mentioned that I've seen images taken with the same scope as yours that shows blue fringing when used with OSC sensor. For example - look at this thread: There is stacked data posted that you can examine. I did quick process on it and blue bloat popped out around brighter white(ish) stars.

Given the level of preparation and "enchantments" spoken in this thread - I see no way this can fail

Does it? I think that to start resolving M13 for example - you need x100+ magnification. I think that at these magnifications - seeing effects start to show already. I might be wrong though - I haven't observed M13 in quite a long time. Last time I did it was in 4" scopes .

Just a thought - seeing does have impact on resolution. In poor seeing / not optimal conditions - detail in globular cluster can suffer as much as planetary detail. 6" refractor in good seeing will outperform 12" dob in poor seeing, although 12" will have both more light grasp and better resolving power.

Very nice. Do take extra precaution on binos. Make sure wind can't knock those of and they are held in place firmly. Maybe even a few pieces of sticky tape - just to be sure.

Sure, it is rather easy to do. There are several ways, but I'll show you easiest one. - Either drag&drop your fits onto ImageJ bar (I do it below toolbar - onto status bar - but it will show words drag & drop as soon as you drag fits anywhere on that window) or go File / Open ... and select your fits - make sure you are working with 32bit fits - if not, convert it to 32 bit (just click on menu option to switch data type - it is Image / Type menu) - If your fits is color one - you'll get "stack" (this is ImageJ terminology - not actual astronomy stacked image we call stack) of three separate images - each channel) - move bottom scroll bar onto middle one (being green) and then right click anywhere on the image, you'll get menu: Marked are scroll bar and menu option - duplicate. Once you hit that - it will ask you if you want to duplicate "stack" (several slices) or just one slice: Leave "Duplicate stack" unchecked and this will create new image - copy of just green channel. Close original stack of three channels as we will be working just on green for the time being. - Hit Analyze / Measure menu option You should get stats for your image and Median value should be there (it's not show on screen shot as it is far right but Mean value is shown). If you by any chance don't have median in results - you need to customize your measurements. It is done via Analyze / Set measurements menu: There are quite a few options that you can use - just check Median and make sure you have enough decimal places for precision. Now you can repeat measurement via Analyze / Measure and you should have Median value included. - Note down median value (in my case - it is 0.05964.... ) and do Process / Math / Subtract - After that - you can do another measurement to confirm results: Here we go - old and new value - In the end - File / Save as / Fits ... Repeat with other files ...

In that case - I have no idea (don't own/use PI). I can do it for you however, if you wish. I'm using ImageJ and it's really easy for me to do it. Just attach fits that you want normalized (like you did with 90 and 135 minute exposures) and I'll be happy to do it for you. Alternatively - if you want to install ImageJ / Fiji - I'll be happy to show you how to do it in that software.

It looks like you are working with 16 bit images. Try working with 32bit floating point images when doing all of that. I think it will solve the problem but not 100% sure - in any case 32bit precision is much better.

As simple fix - you can just "wipe" the background with simple process. Using pixel math just subtract median value of each stack from it. After doing that - all subs should have 0 median value and backgrounds should be normalized. There will still be some intensity mismatch due to atmospheric absorption on different altitudes of object during recording - but that is minor thing - you should get usable results from just setting median value across stacks to same value.

First thing you need to do is normalize frames. They have to have the same background level and same star intensity. I think that PI should have this feature - but since I don't use PI - I have no idea how it's performed. After that - when intensities are normalized - linear stretch is simple - you do same levels without messing with middle slider - just top and bottom - on all frames and you should get same linear stretch. In fact - when frames are normalized - you don't have to do linear stretch - as long as you perform same levels / curves (can save that as preset) - you should get compatible result.

Hope you don't mind - I took liberty to process the fits you posted, here is result: