vlaiv

For me, there is clear distinction between the two. In case of stacking images - there is precisely defined mathematical workflow that has been proven to be correct. In second case - it's much like giving someone a list of numbers and asking them to guess which number comes next. Sure, it is educated guess (AI has to be trained) - but it is still a guess and as such - not 100% right all the time. When solving a mathematical equation, you don't say - here, this is solution, but I'm 87% confident it is the right one , If you apply correctly mathematical principles - you can be 100% sure you have the right solution. Since I consider astronomical imaging to be more than producing pretty pictures - even if that one step above is just to "document" what is out there (rather than doing additional measurements and analysis) - I value the workflow that does not utilize guesswork.

I would not otherwise post this in dedicated threads regarding said scope, but I think I can post it here because of they way thread topic was phrased. Is it too good to be true? Well - consider this: You can't really purchase 80mm F/7.5 achromat for less than say 250 euro - but I managed to purchase said lens for 35 euro of AliExpress. I won't go into detail of how much other components cost - except to say that I'm confident that my DIY scope based on this lens will be less than 100 euro. Rest is just labor costs and profit margins. If someone decides to cut down their profit margin from say 100% to 60% - what you get is a bargain scope

Stars are effectively at infinity so it really does not matter at all. You can have your lens mounted at any sort of angle with respect to rotation of the mount and at any sort of distance - as long as lens follows mount's rotation - you'll be fine. From what I gathered in this thread - you really have two options: 1. guiding 2. encoders First is more involved but less expensive, second is less involved but probably way over your budget.

@JoeKitchen I think I understand your requirements a bit better now. I'm afraid that I will need to "swamp" you with a bit of technical stuff here, but as far as I can tell - you'll be able to follow it. First thing to get a grip on is pixel scale and there is simple formula to do that: pixel_scale = pixel_pitch * 206.3 / focal_length Where pixel_pitch is in micrometers and focal_length is in millimeters. Above will give you pixel scale expressed in arc seconds per pixel. In astrophotography, given the state of sky, you can expect stars to be between 2" and say 5" (that is arc seconds) FWHM. FWHM is Full width at half maximum and is measure related to roughly Gaussian shape of star image. You don't need to go over FWHM/1.6 in sampling rate as that leads to over sampling. To put this into context - let's take your current 6um pixel camera and say 250mm lens. Let's also say that you hit 4" FWHM stars in your image (actual star size will depend on several factors - sky conditions / how still or turbulent atmosphere is, then how good is your tracking and how sharp is your optics). Ideal sampling rate for such stars is 4 / 1.6 = 2.5"/px With 250mm and 6um pixel size you'll be at 6 * 206.3 / 250 = ~4.95"/px - so you won't be over sampled, you will be under sampled - but this is not a bad thing. In fact this is very good sampling rate for wide field. This is also number that is important for some other considerations - namely mount that you are using. Every mount, if not guided will suffer two main types of tracking error. DEC drift due to error in polar alignment and RA drift due to periodic error of drive. DEC drift can be calculated by using following calculator: http://celestialwonders.com/tools/driftRateCalc.html Say that you are off by 10 arc minutes from the NCP in your polar alignment and you are shooting target that is at 30 degree DEC. Your total drift during exposure will be 11.34 arc seconds - which is about two pixels if you shoot at 4.95"/px. This might be acceptable as it won't create star streaking - just a bit oval shape and it depends if that will be noticeable in your backdrop for set or not (it depends on how well resolved background will be). In any case - above will give you idea if you can shoot unguided for 5 minutes. Second thing that happens to mounts is periodic error. It is there because imperfection of gears in mount drive train. They are not perfect circles and as such (think a bit egg shaped) and they rotate at different speeds - or rather they have constant angular velocity but have different radii at places so transfer rotation motion at different rates at times. In any case - this periodic error is expressed as peak to peak periodic error and something called worm period. For example HEQ5 class mount out of the box has about 30 arc seconds p2p periodic error and its period is 638s (if I recall correctly). This means that mount will be on spot then lead a bit then return to correct then trail a bit then return to correct position in period of about 10 minutes and total deviation will be 30 arc seconds. Another way to look at it - it will "travel" in error for 60 arc seconds in 10 minutes so average RA drift will be about 6 arc seconds per minute or 1 arc second every 10 seconds. Mounts never have such smooth periodic error and sometimes their drift is less and sometimes it is more than average. This leads to people being able to take longer exposures than average drift rate suggests - but also that they need to discard some subs as well (depending on where on the worm cycle image was taken). Here is graph of such cycle for EQ6 mount: Above graph is used to create general correction - or what is called PEC - periodic error correction (average of several cycles of such leading / trailing - btw, if you read the numbers above mount has about 26" p2p). Now, you can compare periodic error and p2p with your working resolution. If you for example use 28mm lens with 6um then you'll be working at 6 * 206.3 / 28 = ~44.2"/px Above p2p error is simply insignificant in this case - as it all falls on single pixel - you won't see any elongation, however - for first case and 5"/px - you will see elongation in some exposures. Bottom line - compare your working resolution with what mount can deliver - in some cases you'll need to use PEC, but you can always guide, and even simplest guide setup (which consists out of guide scope, guide camera, usb cable and software running on your laptop that is connected to mount - about $300-$400 additional expense) will give you performance that will be more than sufficient for your working resolutions. I haven't even touched on the fact that sharpness of the lens will contribute to how big FWHM of stars in your image will be - but just to say that it lowers sharpness of the image and increases FWHM so it decreases need for high precision in mount and guiding. Hope above gives you some idea of what you should be considering and sorry for the long post.

Problem is that we don't really understand what is requirement for images. I can imagine taking a very large format image and using it as drop in background for panning shot for example. In that case - image needs to quite a bit larger than the actual size of the shot (think scrolling background). On the other hand - how big stars need to be in that shot? Tiny - barely visible or larger? How much of distortion per star is acceptable? Is pixel level precision required or maybe it does not matter because image will be resampled for use and stars will end up tiny?

Fact that you work as photographer works against you in this case. Many of day time photography concepts are useless and even misleading in astrophotography. Given that you work with 6um pixel size and use short focal length lenses - most star trackers will do good a good job, but you really need to think in terms of: 1. pixel scale - or how many arc seconds per pixel you want to have in your final shot 2. what sort of sharpness lens provide. Camera lenses are optimized for close focus or rather range of foci, while astronomical telescopes are always optimized for infinity focus. They give much sharper image that is only limited by physics of light rather than design of the lens itself. Do look up above concepts and learn a few things about wide field astrophotography as it will benefit you, but in order to get started quickly - I agree, look into small portable mounts that utilize strain wave drives. These will suit you the best. https://www.firstlightoptics.com/harmonic-drive-mounts.html Small enough for good portability Able to carry enough weight and Precise enough for what you need. I'm afraid that I can't recommend any specific model as I have not worked with any of them nor taken keen interest in their performance.

Just remember that pixels are not square pants

That is because MTF has minimal effect when observing extended objects. MTF shows what sort of sharpness in transition you can expect when going from very bright to very dark and vice verse - think stars (how large airy disk is - transition from bright core to black background of space, or planetary detail - again transition from bright details to dark details - regardless what you are observing - craters or festoons). It is important at high magnification and most observing of extended objects (by that I think of DSOs that have surface brightness) is done on low power where differences in MTF between scopes are negligible. To address the second part - we can see Cassini division in 3" scope because it has nothing to do with diffraction limit. Diffraction limit is measure of how much we can resolve - not how much we can see. It is akin to expecting not to be able to see the stars - because stars have much smaller angular size than Cassini division, in 3" scope. We see stars that are tiny - just fine. What we can't do in small scope is resolve close binaries. We see them still - there is bright spot, but it is not quite clear if that is one star or two (or perhaps something totally different - a Sponge Bob shaped bright object in the sky ). Same thing happens with Cassini division - we can see that there is some sort of dark feature on bright background - but we don't have clue what it is - is it just one line or several lines close together or row of dancing monkeys that hold hands. Resolving power of the scope is about ability to resolve - and more aperture is needed to resolve smaller things - but to see contrast (and this is partly related to MTF as MTF dictates how abrupt that contrast change is) - even small aperture is enough.

These two would be my choices: Budget: https://www.firstlightoptics.com/evostar/sky-watcher-mercury-707-az-telescope.html A bit more money and a bit more serious instrument: https://www.firstlightoptics.com/evostar/sky-watcher-evostar-90-660-az-pronto.html Both should be fairly easy / straight forward to use and look like telescope is "suppose to look like" They will also work for day time observation (although left and right side will be swapped if you use stock diagonal, but there are accessories like this one: https://www.firstlightoptics.com/diagonals/skywatcher-45-erecting-prism.html that will put eyepiece in more comfortable position for daytime viewing and also provide correctly oriented image (but for night time observation stock diagonal will be better choice as it will give you better image and viewing position will be more suited for objects high in the sky).

Small update. Tested it today and it looks very promising. Even placed on a table with eyepiece (32mm Plossl) hand held about 555mm away from the cell it gives surprisingly sharp image.

Some time ago I decided to purchase doublet achromat lens from AliExpress. It arrived in great condition, but I haven't managed to find time to do anything meaningful with it until now. Here it is mounted in 3d printed lens cell. Hopefully, I'll get aluminum tubing for OTA and dew shield in a few days so I can start assembling this DIY scope. Btw, lens is 80mm F600 one.

Smallest exit pupil is very personal thing. I tend to be happiest with about 1mm exit pupil. There are several things that contribute to what people find suits them. Existence of floaters for example. Floaters tend to be noticed more below 1mm exit pupil. Another thing is visual acuity. Some people have sharper vision than others. Those that do - don't like too much magnification as it makes image look soft. Others on the other hand find that they can see more detail with increased magnification. It will to some extent depend on target too. Some targets look too dim with smaller exit pupils, but others have plenty of light and don't cause such issues.

A lot will depend on camera model used and lens / telescope used as well.

Data is nice, but SNR is rather poor - total integration time needs to be much longer.

There is a point where going with longer subs gives you diminishing returns because of the way the noise adds. It will depend on your conditions and equipment. As long as you have noise source that "swamps" read noise after some time - going longer is not feasible. There are two major noise sources that can swamp read noise. Thermal / dark noise (noise associated with dark signal) and LP noise (noise associated with sky background glow or light pollution). If you use cooled astronomy camera - first one is non issue and what remains is LP. Even if you use DSLR - in most cases LP noise will be larger of the two (unless you are somewhere really dark and work at higher sampling rates). Look up how to determine max useful exposure length from one of your subs. There are several videos on Youtube that describe this and you can also find threads here on SGL that do the same (I believe that SharpCap also has functionality to do this automatically for you). After that - decide if you want to go even shorter because of other factors (mount performance, chance of ruined subs and so on), or perhaps longer to save storage space (but that won't contribute to image quality).

I think that comparing Mak and refractor, F/ratio and thus often used exit pupil have more contribution than central obstruction. Central obstruction is small enough (even large one) to fall below just noticeable difference (which is around 7-10%). Take for example 30% CO - it makes only 9% by surface or light gathering area. I think that having two mirrors and corrector plate - removes more light than that central obstruction (or about the same if coatings are enhanced at 96% reflectivity).

I think it really depends on type of object in question. For stars - it is almost straight forward - faintest magnitude has direct relationship to aperture size (if conditions are fair and optics is good). Doubles and planetary - again, it is related to resolving power of telescope - and here we have direct relationship between resolving power and size of aperture. For DSOs - well this is very controversial topic, or rather topic where we don't have full understanding of what is going on. Simplified model says that there should be no difference in surface brightness or contrast between two different apertures as long as exit pupil is kept the same. However, there are other factors that contribute that have nothing to do with aperture of telescope directly (but more with other properties) - like size of object in question, and minimum photon count per resolving area element to trigger psycho physical response of the brain. In another words - for same exit pupil, sometimes more and sometimes less magnification can give better view (which involves focal length of scope or F/ratio rather than anything else). Sometimes small telescope, although having the same exit pupil won't show object because there is not enough total light for brain to "allow" seeing the object (again, this is somewhat controversial as it would mean that putting object half way out the field stop should make it disappear - but that never happens. Maybe because once brain "sees" the object - it keeps it "switched on" because brain has tendency to keeps reality consistent). Anyway, that is difficult and probably not fully understood topic, so not sure if we can determine rule of thumb here.

Mount Even modified 30mm or 50mm finder scope and web camera will get you 90% there. It is fully functional system on any cheap mount. Your problem won't be guide system - your problem will be mount.

How did you mod your DSLR? There are two types of modifications: one is removal of IR/UV cut filter and other is replacement. UV/IR cut filter that comes with camera has particular response that is crafted so that camera produces correct colors (or to be easier for it to produce correct colors). Here is comparison: Blue curve belongs to Canon stock filter, and red to Baader astronomical filter replacement. This modification is performed mainly due to emission type nebulae astrophotography - as you can see the difference in Ha line sensitivity (green vertical dashed line). In either case you have two options - get regular UV/IR cut filter that is front mounted - but this is redundant if you did second type of astro mod - replacement of UV/IR cut filter. In this case - you need to look up ways to calibrate your images for new response (derive new color correction matrix and so on). Alternative is to find front mounted UV/IR cut filter that has curve like Canon's one in above image - and I'm not sure you'll easily find such one.

Looks very nice. What material did you print it in? I've found that sometimes steppers can run really hot, but on the other hand, night time use means lower ambient temperature and open design means good cooling. Even PLA based material should be able to handle that well.

Hi and welcome to SGL This is probably the wrong section and belongs to DIY part of forum, but moderators will decide on that. As far as stepper mounting bracket goes - some images of design would be nice. You can also attach STL here as attachment or post a link on some of popular 3D print model websites where you upload STL

Aperture per pixel - is just F/ratio. In order to truly capture the speed of system we also need relation of pixel to angles in the sky - arc seconds per pixel. We combine the two and we get the speed formula - aperture at resolution. Here is mental helper: 4" at 1"/px is baseline 8" at 1"/px will be x4 as fast 4" at 2"/px will be x4 as fast 8" at 2"/px will be x16 as fast Another way to look at it is by sensor size. This is because pixel size is "variable" quantity - we can bin it to get larger pixels, but if we do that - we reduce total number of pixels for a given sensor size. On the other hand - larger sensor will let us use larger telescope (more light gathering) to get the same field of view (and with binning - we can keep sampling rate - "/px at wanted value). This limit depends on FWHM of stars in the image, and those partly depend on spot size. Spot size in microns is not telling much. Spot size in arc seconds it much better in telling the story of resolution. Fast scope - say 8" F/2 with 4um spot size - is x3 less sharp than say 8" F/6 with 4um spot size, because 4um at three times shorter focal length translates into x3 bigger spot size in arc seconds (or compared to objects in the sky).

If I'm not mistaken @Lee_P images from heavy LP so he might be able to share some thoughts on this?

It is always possible - but takes progressively more time to get the same results, so it gets painfully slow and question becomes - if it's feasible for you. If I recall correctly - every two SQMs equals about 6 more time imaging to hit the same SNR. Being in Bortle 4 skies - you SQM is likely somewhere around 20.5 while in London you will be around 17.5 SQM. - that is 3 mags of difference, or you'll need about x16 more time to get same result. What you've accomplished in one hour will take several nights. Investing in good LP filter, being mindful of sky transparency and imaging only objects directly overhead at the time of the night there is the least LP (majority of people is asleep but morning services have not yet kicked in fully) and being willing to spend multiple evenings per target - and you will be fine

Look at their specs. There are several important factors - quantum efficiency, read noise and so on and compare. You can even throw new CMOS sensors in the mix if you accept that you'll need to bin your data to get appropriate pixel size, and in doing so in software - count read noise as being multiplied by bin factor. If you bin x2 - read noise is multiplied by x2 as well. If you look at it that way - modern CMOS sensors win even if they have small pixel size. Here is example: ASI2600 vs Starlight: Native pixel size of 3.76 which can bi binned x2 you get 7.52um vs 7.8.um - so slight advantage Starlight, but difference is small Both are APS-C sensor, so sensor size is a tie ASI2600 has 50K full well capacity, while Starlight has half that at 24K Read noise of Starlight is "less than 12e, but typically 7e" while read noise of ASI2600 is ~1.4e at unity gain, so double that to 2.8e when binned in software - again less than half of Starlight - ASI2600 wins here ASI2600 has 80% peak QE while Starlight has 50% so ASI wins again. Price of ASI is less than that of Starlight - win ASI Both cool to 35C blow ambient to so that is a tie. Overall - ASI2600 is better option in my view, but I did this more so you can see what you should consider when choosing between cameras.