vlaiv

If that is the case, can you run simple test for me with ASI294, just to confirm? Take two bias subs. On one of those, subtract offset and then analyze it for standard deviation (measure of noise). Then take again first sub and subtract second from it and again analyze it with standard deviation to compare which approach produces noisier result.

Can you explain how synthetic bias produces less noise?

I can get the color. It's not easy - but when you over do everything - well color starts to pop out. Maybe even too much color - there is blue rim on one side showing ED doublet is not color free. I did not sharpen - just color thingy in Gimp.

Correction does seem better - but not quite there yet. How do you take your darks? Is the camera on the scope? Is there any chance of light leak in your optical train?

They are needed in order for flat calibration to work. Dark current signal needs to be removed as it was not subject to vignetting and when you correct with flats - if there is still some signal left that was not subject to vignetting - you'll "counter" correct it - you'll make inverse of vignetting. You can see this in your image - center is dark and dust shadows in that part of the image are bright. Do follow proper calibration workflow in order to have good results, it is there for a reason.

Did you take other calibration files as well - darks and flat darks?

To me it is not reduction in imaging hassles, so I won't be giving it a try, but anyone else is of course free to do so.

With astroimageJ it is rather easy - you alt + click on any particular star and it will give you FWHM profile in new window. It won't do average on the image, but if you do that on few stars - you'll get pretty good idea of what average FWHM is. Another way is - just select and configure photometry tool and select multiple stars with it - you'll get results table containing FWHM - you can then copy/paste that in spread sheet and do average

I don't think regular C8 will be up for the job. From what I've seen - C8s always produce bloated stars for some reason. Could be the fact that SCTs have spherical aberration unless their mirrors are at exact prescribed distance? Since focusing is done with moving primary - there is actually only one position where telescope will behave the best - as soon as you remove diagonal and/or add elements to optical train and refocus - you'll change distance between mirrors and introduce some spherical in the mix. I've seen some good images produced with EdgeHD optics, so that is better bet - but also check spot diagrams on those as well, as some models are not quite diffraction limited.

I think that ZWO is again messing with drivers and trying to hide amp glow. They are probably removing constant offset or something based on their measurement of dark current. As far as calibration goes - I would just check if darks are now consistent. Something silly like removing constant offset can compromise darks if not done properly. We want darks to be repeatable and well behaved - this means that if you take 10 darks at some temperature and settings - they should all have same mean value and they should all calibrate each other out perfectly (that means - if you take any two darks and subtract them you should get 0 mean image with pure noise - do FFT on it and check for any anomalies - FFT should also be just noise image). If this is true - then you don't have to worry about silly median values you are getting, but it also means that you should do proper calibration - darks, flats and flat darks for best results.

I wonder how wide field eyepieces perform at say F/2.8 to F/3.2? Because if they perform ok - we have a winner then https://www.teleskop-express.de/shop/product_info.php/info/p11772_TS-Optics-200-mm-f-3-2-Parabolic-Newtonian-Astrograph---Carbon-Fibre-Tube.html

That is rather decent combination. As far as I can see, scope comes with rings and dovetail bar and that is all you need to be able to mount it to that mount. All is included to get you started.

No. IR/UV cut really does nothing for visual (except protect your eyes if you are looking at something high in UV). We don't see that part of spectrum anyway so cutting it off makes no difference to us. What you can do is try #56 and #58 - greens, on their own #56 will let more light in and be less aggressive in removing unwanted light, while #58 will in essence be the same as interference as far as what it passes - except it will be much darker because it passes less than 50% of light in that range compared to interference that passes more than 90% of light.

Are those values in pixels or in arc seconds? They need to be in arc seconds in order to be comparable.

Ah ok. So, factor of x1.6 relates to FWHM of final image and "optimum" sampling rate (optimum is quoted because it is really approximation so we can't really say optimum value). If you have 2.6" FWHM then you need to sample at 2.6" / 1.6 = 1.625"/px Since you have between 2.5 and 3.0 - then your sampling rate of 1.9"/px is spot on, as above would give sampling between ~1.56"/px and 1.875"/px and you are on upper bound of that. As far as getting expected FWHM, it goes like this: We take three major sources of blur and we approximate each one of those with Gaussian. These three convolve one another and produce final blur - also Gaussian (convolution of Gaussian by Gaussian is another Gaussian). Their variances add to produce final one - which means that standard deviations add in quadrature. - seeing FWHM - we divide FWHM value with 2.355 to get sigma of Gaussian - Airy disk - Radius is given by 1.22 x lambda, while sigma of Gaussian approximation is given by 0.42 x lambda (source: https://en.wikipedia.org/wiki/Airy_disk#Approximation_using_a_Gaussian_profile) - guide RMS is already sigma of the spread You take those three values and you take square root of sum of their squares and then you multiply resulting value with 2.355 to get FWHM.

I think it is still somewhat shy of 4° and costs an arm and a leg

I'm talking from diffraction limited point of view here - some scopes are not diffraction limited over their imaging field so things will be a bit different for them. Detail in the image that you capture depends on several components - seeing, guiding precision and aperture size - to name 3 main ones. They work together and no single component is main one, but important thing to note is that - aperture size plays a part. For example - with 80mm scope, you simply can't hope to realistically go below say 1.8"/px in long exposure astrophotography. In my view - you have rather good match with your current setup - Esprit 80/400 and ASI1600 if it samples at 1.9"/px. I don't really understand this part: HFR on good nights is 2.6 which implies seeing of 1.6". There is relationship between FWHM (which is the same for perfect Gaussian as HFR) of stars in the image and above mentioned 3 components. If you have FWHM of 2.6" in your image and you guide at 0.6" RMS, then seeing must be around 1.7" FWHM. If you guide with 0.8" then seeing needs to be 1.1" FWHM. In any case - larger scope will give you both a bit of edge on possible resolution and speed of capture. You can almost never achieve 1.5"/px image with 80mm scope - but you'll be able to do it with 6"-8" of aperture. On very good nights you might be able to go down to 1.2"/px (but this requires good skies, good mount and good focus - and of course diffraction limited telescope).

I don't think you get the game We want to maximize aperture while still giving 4° or more TFOV. There are many options for 100mm giving 4° or more of TFOV. But I do appreciate the idea - TeleVue NP127is with 660mm of FL would provide 127mm (of excellent aperture) and paired Pentax XW 40mm (46.5mm field stop?) would give 4° TFOV. I think that is by far the most expensive realization of large aperture idea though - and I'm not even sure Pentax XW 40mm is in production any more.

Not really. It is about ratio of colors - or if something is blue - you want it to remain blue. Our eyes don't see IR photons so their contribution to our color vision is always 0. Camera sees those photons unless we put UV/IR cut filter. Say you have "perfect blue" and "perfect green" both with some IR photons. Our eyes will see them as perfect blue and perfect green as we won't see those IR photons. But camera will record RGB ratios that are not perfect blue and perfect green - because cameras have some sensitivity for every component in IR part of spectrum. Blue wont be 0:0:1 any more and green won't be 0:1:0 any more - they will be something like 0.1, 0.1, 1.1 and 0.1, 1.1, 0.1 (scaled down or whatever). Problem is - that you can't undo this unless you know IR light intensity and exact QE for each R, G and B in that part of spectrum - and you usually don't unless you take same image with IR pass filter to subtract from original one.

I think wiki is more accurate. Check lightpollutionmap.info as well - it also gives both values (I think). In any case - Bortle scale is not really with hard limits as it is descriptive in nature, so there is bound to be some difference in interpretation unless we take some values to be standard - like Bortle 4 goes between SQM A and B.

Ok, so first - plate scale is term used for plate solving - and it represents ratio of physical distance on old film photography to angles on the sky - it can also be used for images, in that case we are talking about pixels rather than mm or whatever unit of length is used. Sampling rate - is again the same thing only coming from different field - it is how much sky is covered by single pixel - or rather what is the distance between two adjacent pixels (but since pixels are tightly packed - it is the same as size of pixel). Both tell you mapping in pixels versus angles in the sky, or how much sky angle each pixel contains. No such thing as slow scope, but that is whole another story so we won't be getting into that Well, that mount is going to be good for camera + lens astrophotography. Your scope is too heavy / big for that mount for astrophotograpy - it will be ok for visual (although eq5 would be better there as well). Do you know what the backlash is? You say you want to measure backlash, but that is going to be very hard with camera that you have. Backlash can be measured and is often measured with guiding equipment. Computer is automating the process, but you need to have it control both the telescope mount and guide camera in order to measure backlash. Backlash in a mount is "empty movement" of one of mount axis. Say you have DEC backlash. If you move mount in DEC using slow motion controls (knobs that you turn by hand and mount tracks in that axis) and then stop and change direction and turn the same knob in other direction. Mount might not respond immediately and it might take say 1/16th or 1/8th of a turn before mount stars moving in opposite direction. That turning of your DEC knob before mount actually starts turning is called backlash and it is expressed in arc seconds - which correspond to motion of the mount around that particular axis when you turn your knob for 1/16th or 1/8th (or whatever was empty motion) when mount does track. This is useful to know precisely in arc seconds - for computer software that operates your telescope mount. It needs to know how much to "add" to motion when changing direction - knowing that this "added" winding of knob will actually count for nothing in terms of mount motion. Say your computer moved mount in +DEC and then you tell it to go to 30 arc seconds in -DEC direction. You have 7 arc seconds of backlash in DEC axis. In this case, computer will turn DEC axis for total of 37 arc seconds worth of turn. 7" of those will be used up for clearing the backlash and rest 30" will be used to actually move mount 30" in -DEC direction. Makes sense? Not sure how do you think of recording backlash with camera since actual backlash does not move mount at all - it is "empty" motion of mount axis knob (or rather whole worm mechanism - but I used term knob as it is easier to relate to). As long as you are clearing backlash - you'll get the same images. Ok, so this is for your reference only as it is good thing to know. Coolpix L15 has 1.76µm pixel size. It also has 1/2.5" sensor (it is actually not 1/2.5" sized sensor - it is very strange unit for sensor size - it has nothing to do with inch - 1" sensor having something like 16mm diagonal - not even close to 25.4mm). This is needed to calculate actual focal length of the lens attached to camera. It says 35-105mm equivalent - so actual lens focal length is that much smaller as is 1/2.5" sensor is compared to 35mm sensor (or full frame). https://en.wikipedia.org/wiki/Image_sensor_format 1/2.5" sensor has crop factor of about 6 - so actual focal length of lens on camera will be x6 smaller than is quoted as "35mm equivalent". 35/6 - 105/6 = 5.83 - 17.5 now we can calculate (approximate, as we don't actually know exact focal length of the lens - only approximate or "35mm equivalent") pixel scale / plate scale / sampling rate / arc second per pixel / "/px (all terms used for same thing). formula is pixel_scale = pixel_size(in µm) * 206.3 / focal_length(in mm) Thus pixel scale ranges between 1.76µm * 206.3 / 5.83 - 1.76µm*206.3 / 17.5 = 62.28"/px - 20.75"/px So you see why your camera is not good option to record back lash, even if there was feasible way to do it with images (and I'm sure there is somehow but it is probably rather complex - like shining laser from the mount on the wall, turning one whole revolution of axis knob in one side, then in other, then back in first - taking images, measuring distance to wall - measuring laser dot displacement on the wall and so on ... ) - you have too coarse sampling rate. You cannot possibly measure angles smaller than ~20" as image would move less than one pixel - and there is really no good way to measure between (actually there is - you can determine position of a star down to fraction of pixel - but again, that is outside the scope of this answer ). Ok, astrometry.net is rather simple. Take two of your images, go to https://nova.astrometry.net/upload upload first image, hit "go" or whatever and wait for it to solve. See this video for details: https://www.youtube.com/watch?v=hz8poNTOtgw Notice that you'll get center of the image on results page: as well as pixel scale. Once you have your set of coordinates - you can then calculate distance between them: https://astronomy.stackexchange.com/questions/19287/angle-from-2-point-on-celestial-sphere/19292 To the first approximation - you subtract RA of first and second and DEC of first and second and do Pythagoras on those (square root of sum of squares - finding diagonal). This works for small distances - where curvature is close to 0 (good for values associated with backlash - like few to few dozen arc seconds). Second method in ImageJ goes like this: Open images in ImageJ and take first image. Adjust brightness and contrast until you can clearly see what is in the image: Take two prominent stars and mark with line segment between them: Then hit analyze / measure and look at length (result of measurement). In case you can't find length being displayed as result of measurement - check menu option Analyze / set measurements. Start stellarium (Check out stellarium.org to download it - it is free planetarium software) - find your target, select angle measure tool (one of the plugins if I'm not mistaken) and measure distance between those two stars: Now we have distance of about 6576 arc seconds there and 260 pixels - so our pixel scale in this example is 25.3"/px Now hit Images / stacks / images to stack and you should get your two images being joined into single window with slider on the bottom (this is called stack in imagej - or just collection of similar images). Next you can do Image / stack / z-project and choose max method: This will create new image where there will be stars from both images: Again set line marker joining two stars and measure distance: I roughly measured 5.2px of distance between these two frames. If we have 25.3"/px and 5.2px, then we have 25.3 * 5.2 = 131.56, or about 131 arc seconds of distance between these two frames.

Yes of course it is - provided that you have enough precision. You need to know plate scale / your sampling rate - it can also be measured or calculated. For calculation you need to know focal length of your lens and your camera pixel size, and this will give you arc seconds per pixel. Alternatively - take one image of the known part of the sky - identify two stars. Measure distance in pixels between two stars (in imageJ - line selection tool will give you distances. Measure distance between two stars in Stellarium using angle measure tool. Divide two values to get arc seconds / pixels measure. Another alternative is to upload image to astrometry.net and have it plate solved - this will give you pixel scale as well. Now that you have pixel scale - open both images in ImageJ and create small stack out of them (stack in imagej is just sequence of images). Z-project them using max - this will give you image where you'll have stars from both images visible. Identify same star that is duplicated and measure distance in pixels it moved. Take pixel scale and multiply with distance in pixels - this will give you arc seconds that image moved between shots. Alternative - upload both images to Astrometry.net, have them both plate solved - it will give you centers in RA/Dec coordinates - and just calculate distance in arc seconds between those two points in RA/DEC. By the way - welcome to SGL

It turns out that there are other "more affordable" solutions out there. Siebert Optics does custom made 3" eyepieces - there is 42mm 75° eyepiece. That can be paired with ES 3" diagonal and TS152 F/5.9 achromat scope (this part is affordable, first two - not so much). I can't find data on field stop of 42mm 75° eyepiece, but it comes also in 2.7" flavor - which means it is less than 68mm. According to AFOV/Fields top math - FS should be around 55mm. That is 3.5 degrees of sky at 152mm And here we go again - just a bit more aperture with just a little narrower TFOV - now you see why I wanted firm 4° limit. Bresser 127mm F/5 achromat with Aero ED 40 - and yes, this eyepiece is still available as Lancerta ED eyepiece: https://tavcso.hu/en/product/LA40ed That will keep things nice and low cost and give quite a bit of aperture. Only thing I can't really get past - I find Bresser refractors really ugly - white paint and that oversized dew shield and that plastic finder base and all of that . Oh nooo - it is 8mm exit pupil. See the problem? By the way - say we go for 6" newtonian. It would need to have at least 40% secondary to get illumination over whole 46mm field stop (I just came up with this figure - did not do the math, so it can be a bit more or a bit less - but I figure it's about right). Now if we take 94% per mirror, 4 element coma corrector which is x8 air glass at 99.5% we get - 126.6mm of clear aperture - or about same light gathering of 127mm scope (which itself should be corrected for diagonal and at least 4 air glass surfaces so it ends up being 125mm). F/4 newtonian would need 28mm eyepiece, so I really need to go with at least F/4.5, but then FL is 675mm and we can't get 4° out of that in 2" format.

C'mon, a man can dream when it's cloudy, right?

I think we have winner - except it has a bit less FOV than 4° https://www.teleskop-express.de/shop/product_info.php/info/p14000_TS-Optics-152-mm-f-5-ED-Rich-Field-Refractor-with-4--RAP-Focuser.html + https://www.teleskop-express.de/shop/product_info.php/info/p7392_Explore-Scientific-star-diagonal-3--with-99--reflection.html + https://www.teleskop-express.de/shop/product_info.php/info/p7379_Explore-Scientific-30mm-100--3--UWA-Eyepiece.html And roughly 6000e later - we have best 4° view money can buy right?