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I think that main question you have to ask yourself - what is typical range of magnifications that you'll be using. Lowest magnification that Maksutov 90mm can provide is about x40. If you need lower magnification than that - go with short refractor. If not and you don't have very high budget - go with Maksutov.

Interesting that ASCOM driver does not let you set offset value. I have 3 ASI cameras and use older version of driver at the moment (haven't updated in a while) - but every version of driver so far let me set offset on every camera. In any case, when that thing with driver gets resolved - I've found that ASCOM driver is better / more stable for long exposure imaging, while native driver is preferable option for planetary / high frame rate imaging.

ASI224 ADC (atmospheric dispersion corrector). x1.5 barlow element (any x2 barlow element will do if placed at correct distance to sensor. Closer you place it - smaller magnification factor, further away - larger magnification factor. Cheap option Baader classic Q x2.25, expensive / best option Baader VIP) Electronic focuser as option (I don't use it - but many people prefer to use electronic focuser). You don't need auto focuser - one controller by computer, you just need way to focus without disturbing OTA and introducing vibrations. I just deal with vibrations - touch up focus, wait second / two - judge focus from live image, repeat if needed ... USB3.0 capable laptop with SSD and enough disk space (you'll need around 50 or so GB free - more you have, longer videos and more interesting project you'll be able to do - like animations of shadow / moon transits on Jupiter and so on). Don't use diagonal mirror for imaging - use simple extension. SCTs are designed to work at certain primary / secondary distance. If you change that distance too much (and you do when focusing) - you introduce spherical aberration. It is best to use it as you would for visual - with supplied diagonal. It was probably optimized for such usage. This means that if you have it with 1.25" diagonal - it's best to insure that camera sensor is about 80mm away from rear port of telescope (100mm if you use barlow as barlow moves out focal plane).

It is quite a complex topic, but here is some basic info to get you started. You are quite right - arc seconds per pixel tells you the scale of the image - or how many pixels across does your target have. It depends on focal length and pixel size and formula is: sampling_rate = pixel_size * 206.3 / focal_length where pixel size is in micrometers and focal length in millimeters. Another important point is that while you can sample at any resolution - it is not wise to do so. After some point - you will be over sampling, or using too many pixels per arc second - too many pixels per target. This is because there is limit to what can be captured in terms of detail. This limit depends on context. For planetary imaging it is set by telescope aperture alone (and wavelength used), but for deep sky imaging it is set by telescope aperture, seeing influence and mount performance (tracking / guiding precision). It is rather complex matter how these three interact to produce level of blur, but for all intents and purposes - you can measure star FWHM as pretty good indicator of sharpness you achieved. Good sampling rate for image with given FWHM stars is FWHM / 1.6 If your image has 3.2" FWHM stars - then you need to sample at 2"/px. Using lower sampling rate (higher numerical value of arc second per pixel is lower resolution / lower sampling rate / coarser image) is called under sampling and is perfectly fine (some people complain about blocky stars - but that is consequence of image viewing software and not capture process), while using higher sampling rate is called over sampling and is generally a bad thing because you spread light too much thus lowering SNR without any benefit of capturing additional detail - as there is none, it is blurred away by combination of seeing, aperture and mount performance. Smaller telescope apertures are suitable for lower sampling rates, for example - 80mm and smaller telescopes can deliver 2"/px or coarser sampling (like 3"/px, or 4"/px). 100-120mm usually gets you ~1.8"/px 150mm can produce 1.5-1.6"/px and 200mm can produce around 1.2-1.5"/px Many people use 1"/px or higher sampling - but realistically - you need 8" or larger, perfect steady sky and excellent mount to achieve that. My recommendation would be to aim for 1.8-2"/px if you choose 150PDS and ~1.5"/px with 200PDS.

Make sure you have it installed and then select this option: If you have more than one camera - it will give you "selector" when you first click - so you choose the right camera and after you can set it up via ASCOM options dialog.

Just point your telescope to nearby star and observe star pattern at high power. Compare the view with above list of different star images. Image won't be stationary but rapidly changing, like shown on this page: http://www.damianpeach.com/pickering.htm (Damian has nice set of animations for each of above point on Pickering's scale). When observing planets and the Moon - you'll notice softness that changes - as if looking thru the fire. It can be on different scales (depending on how bad is the seeing) You can actually focus on seeing and focus on planet - two different focus positions. If you focus on seeing - you'll see something similar to jet wash Or maybe this youtube video: https://www.youtube.com/watch?v=GqVjD3nBSQg (0:50 - most right engine) That is usually most visible when observing the moon and you slightly change focus (focus at our atmosphere rather than moon). Effect is readily visible when there is jet stream above or if you happen to observe over house with chimney that is burning some sort of fuel for heating (as you track planet - it can pass right thru chimney jet wash).

I agree, although sometimes it is hard to tell that planet is clipping even from histogram (histogram shows spread of pixel values and if only handful of pixels is clipping - it might not show on histogram). If I'm not mistaken, SharpCap has aid to show clipped pixels? Maybe that is better option?

Free version is sufficient. Here are results from ASCOM driver and native driver for ASI185 - bias file (0.125ms exposure): Quite different results for both mean / median and stddev

Try adding SharpCap to the mix of apps to test with. ASI provides two types of drivers - native drivers and ASCOM drivers. SharpCap can utilize both type of drivers so you can take bias with each and compare with above results. Something tells me that this has to do with driver used. I know that darks are not compatible between drivers - could be the same with bias subs.

It does exist, but is faint and most people don't manage to capture it. If you do a quick search on google - you'll see quite a few examples that managed to capture it:

Autostakkert!3 deals perfectly well with small amounts of rotation. Individual subs can have alignment points further away than rotation in about 2-3 minutes. You don't need to use derotation feature on movies up to few minutes long (that really depends on your working resolution). I do agree that longer video is needed - I often capture 40000 subs in ~4 minute videos (about 160fps) and stack only small fraction of best (often around 10% or less). That is actually not bad image. What alignment point size did you use? Try using something like 30px, and maybe post raw stack as wavelets are also a bit of an art (you need practice to get the most out of the image).

You are missing the third component in that equation - that is pixel size / imaging resolution. Two same F/ratios only have the same speed if they are used with same camera / pixel size. If you want to compare two systems, best measure is aperture at resolution. This means - you fix your working resolution and then compare apertures. Sampling resolution - or arc seconds per pixel is very important metric. What camera do you plan to use with either of these?

Before interference filters - people used absorption filters. Even now, bayer matrix on color sensor is made up out of absorption filters rather than interference ones. If you have mono camera - you can use red, green and blue filters or even some other combination to capture color. You only need to calibrate your data properly. Only drawback is lower quantum efficiency of these filters. Interference filters often pass 95-98% (some claim even 99%) of their respective band, while these filters - pass less - some even below 60%. You can find their curves online. For example #80A (light blue) has following transmission curve: #58A green: #25 Red has this one: And so on ... While interference filters have steep cut offs in their transmission curves - don't think that sloped curves are somehow inferior. In fact - they often help with color reproduction if properly calibrated.

It is really hard to saturate when you are at the critical sampling rate, even with high gain. I think that with critical sampling, you'll get something like 80000 photons per second per pixel. Multiply that with few milliseconds of exposure and QE of sensor - and you get couple of hundred of electrons under ideal conditions. Really short exposures won't saturate even the highest gain settings.

I agree that you should lower exposure length - but I disagree about the histogram - just don't look at it at all - it is useless and misleading for planetary. Exposure length should be at about 5-6ms and not longer. Gain should be used to control read noise. As you can see - gain 350 has the lowest read noise - so use that. Don't worry about possibility of clipping due to high gain - if you use short enough exposure to freeze the seeing - it can't happen. Capture will be dim - but don't worry about that either - that is something to be adjusted in processing. Drop the barlow, Mak180 is F/15 instrument and you are using camera with 3.75µm pixel size - that is perfect match without barlow. With 6ms exposure length you have chance of doing 167FPS (1000ms / 6ms = 166.666....). In order to utilize that - you'll need to use ROI: I outlined combinations that won't limit FPS. Just remember, if you go for shorter exposure length - like 5ms - you'll be able to achieve 200fps, while with 4ms - 250fps. Choosing ROI to support that is good thing. Using USB3.0 connection and computer that is capable of recording at such high FPS is another important bit.

I can say that I see a slight improvement there

Indeed, no need for coma corrector if you are going for planetary. That small central part of the sensor is well within diffraction limited portion of the field.

I had a brief look, and I really like the format in which stars are presented. I think it will be very easy to navigate to all stars presented in the book. Well done!

You should raise your gain to control the read noise. Read noise depends somewhat on gain setting chosen - it usually drops with increased gain. ZWO has graphs on their site related to each camera: According to this, best gain is around 64, but you can also use 90-100 range. This only affects brightness of image in capture - which is really not important. Stacking makes all the difference here. If you use AutoStakkert!3 for stacking (and you should), then you have this option: which will give you properly bright stack in the end. Boost your gain and lower your exposure time to something like 6ms to start with. Not necessarily. Barlow is a versatile addition - both for imaging and observing. You might not use it with your current cameras and scope, but I'm sure it will come in handy in future. There is no need to use it with your current setup for imaging. Any camera that has: Fast readout, small read noise and high quantum efficiency is good planetary imaging camera. I'm inclined to think that ASI183mm is better planetary camera when handled properly. It has lower read noise, higher QE, and will also have faster readout if you connect it to USB3.0 port and choose appropriate ROI. According to ZWO website it is well capable of supporting 6ms exposure to the full extent: (1000ms / 6ms = 166.666FPS and at ROI 640x480 it can capture up to 169.92FPS so 166.66 should be doable). Only thing that you'll need is filters and filter wheel, because it is mono camera. I prefer OSC for planetary as it is much simpler to work with, but in principle Mono+Filters will have small edge if handled properly (derotation if needed and so on). No - it would do even worse. Certain pixel size dictates maximum F/ratio that should be used. Going with higher F/ratio - just dims the image (lower SNR) and does not record any additional detail - as there are none - that is maximum useful magnification. ASI183 has 2.4µm pixel size - which has something like F/11 as critical f/ratio ASI120 has 3.75µm pixel size which has something like F/15 as critical F/ratio (when I say something like - that is because this depends on wavelength used and in my view, it is best to use wavelength around 500-550nm, although technically if you want to capture everything - you should go with 400nm, but blue part of spectrum is the most affected by seeing and it will be blurred away anyway, so I just go with middle of the spectrum - and peak of human visual sensitivity). That is what ROI is used for! If you capture just central 640x480 (smaller capture file size and faster frame rate): then you get FOV like this:

What were exposure lengths in these two recordings? One of important aspects of planetary imaging is to get exposure length right without looking at the histogram (looking at the histogram and trying to get image bright enough with exposure length is wrong way to do it). If you were going by histogram (often recommended for other types of photography) - then with barlow, image will be x4 dimmer as light is spread over x4 larger surface - which will then lead to x4 longer exposure to get the same histogram. That is detrimental to lucky imaging approach - where you aim to freeze the seeing effects instead of them having motion blur effect if exposure is too long. Ideal exposure length is about 5-6ms, but if you go to say 20ms or 30ms - you'll simply get motion blur in most cases except in the best seeing conditions. My guess is that exposure length was to blame, and having no barlow is actually closer to critical sampling than having x2 barlow since your Mak is already at F/12 and critical sampling rate for ASI120 (3.75µm pixel size) is F/15. Going over F/15 simply does not make much sense and lowers SNR of recording (and that in turn causes host of other issues - like less precise stacking and less room for sharpening).

I probably just had the tiniest session to date Not doing much observing lately (well, that is understatement - I don't do any really), so I just felt like grabbing whatever was lying around and having a look at Jupiter and Saturn. I'm planning a proper planetary session tomorrow evening, but for now it was up to 32mm Finder / guider scope to deliver! Scope is little 32mm F/4 achromat - so 132mm of focal length. ES62 5.5mm was at hand. That is planetary EP, so it should serve well. I had total of less than 5 minutes of observing - but I'm happy. Managed to see all four Jupiter's moons at x24. Europa and Ganymede were mere 15" apart - but it was clean split. Two bands were visible on the planet. Surprisingly scope was optimized for blue part of spectrum so there was no characteristic purple halo but rather red ring of fire around Jupiter. Same was visible around Saturn but to lesser extent. Saturn showed rings but no other features were discerned (neither on ring system nor on planet). In the end, I turned scope toward the teapot (I knew that I'll be able to find something easily there) and I immediately spotted M22. Very nice smudge for 32mm of aperture in Bortle 8. Sky is very transparent tonight. Did a bit of sweeping around and spotted a threshold nebulosity - but have no idea what it was. It had cluster associated with it (saw maybe 3-4 tiny stars in concentration) but could not recognize what it was - neither by looks nor by position as I was hand held sweeping at the time (probably M17?).

Larger prism surely helps. Here is simple guideline - beam is not vignetted in single point (center of camera) - if you place OAG away at such distance that it matches F/speed of your scope. Say you have 12mm prism and F/5 scope. At 60mm you'll have threshold - greater than that between OAG and focal plane and you'll have effective aperture stop (not only vignetting but reduced aperture). Closer than that and you won't have aperture stop but there will be some vignetting - just how much can be solved with a bit of trigonometry. 8mm prism will bring this distance down to 40mm for F/5 scope, while F/8 will have 64mm before aperture stop point (that is why it is more important to place OAG close for fast systems - slow ones have more "room").

Larger sensor with OAG won't help much. Even smaller sensors can be under utilized if distance of OAG is not optimized on faster systems. ASI185 has 8.6mm diagonal (7.3mm*4.6mm) and it vignettes even at F/8:

First hand experience is really all that matters, but I do have a question, if you don't mind. Did you use both cameras on the same patch of the sky? I've found (with same camera) that if I point the scope at part of the sky along galactic plane - I get many guide stars even with OAG, but if I point to the part of the sky that is away from plane of our galaxy - it is far more sparser and I often have only 2-3 guide stars in field of view. Maybe above discrepancy between cameras might have something to do with that?

I'm rather surprised with that. With difference in read noise being due to quoting highest read noise for ASI120 and lowest read noise for ASI290 Highest read noise for ASI290 is 3.25e so slightly better than 4e of ASI120, but read noise is really not that important for guiding where usual SNR is 50 or more per exposure. QE is the same. Not sure why would ASI120 be x10 worse than ASI290? I must say that I did not compare the two, but I did guide with QHY5LIIc which has same sensor as ASI120 color and I guide now with ASI185 and I did not notice any improvement in guiding due to camera alone.