vlaiv

Don't be. I think almost everyone wiggled a cable or two at some point

It turns out that you really should avoid darks with Canon DSLRs. They apparently perform internal dark calibration on each sub (which is good as it avoids temperature issues). Use bias and flats. Bias should be used instead both darks and dark flats.

Not necessarily. If you don't guide - it is likely that you'll have "natural" dither. As long as you don't have perfect mount and it moves pixel-two every few subs - it's effectively dithering.

For a period of time I advised that one tries dark optimization for cameras that don't have set point cooling and have bias that works. Recently I experimented with Canon darks and bias files and I noticed that they can't be made to work with above algorithm for some reason. In fact they behave rather oddly. I suspected that Canon does something "behind the curtains" but I never bothered to dig deeper into that. It turns out that Canon does their own "dark calibration". On one hand - that is bad, as it changes the data without letting you know in which way. On the other hand - it does it (according to some sources on the net) - in rather good way as far as dark current is concerned. You don't have to worry about temperature mismatch because this dark current offset removal is done at exact same temperature camera was on when taking light sub. However this method does not remove bias signal, so there is benefit in using master bias. There is also one additional down side to this method Canon uses - it introduces horizontal banding in each sub. because each row is "dark calibrated" on its own. In any case - for DSLR, don't use dark optimization, don't use darks, use bias as both darks and flat darks and dither between subs

It is actually very good with exception of: 1. stars - that is due to lens and sensor size. Edge stars are really astigmatic 2. lack of flats If you start to stretch it - you start to get glimpse of Orion molecular cloud complex: With proper flats - this image can be properly stretched and will reveal much.

To be honest, it's few bits and bobs around the net. Let me see if I can find those links again. In the mean time, here is what I think is happening with Canon subs. There is part of sensor that is covered / shielded from light - like couple of dozen of pixels on one side of sensor (and probably on the bottom of it as well). After exposure is taken - for each row average value of those first 20-30 pixels is taken and subtracted from the rest of the row. This is like "row by row" dark subtraction, so there is no single dark offset that is removed from whole image, but each row has calculated dark offset that is removed. Problem is - I haven't managed to extract actual full raw data by any software to try to do dark optimization algorithm and to check bias. Anything I tried produces similar "calibrated" result. DCRaw does it, DNG converter from Adobe - btw. DNG file format supports this by row dark offset as well as one of its options, so check it there as well. Only thing I have not tried is APT and recording raws directly on computer. I don't know if Canon DSLR SDK reads out true raw or this "calibrated" raw as well. For example - some of information as well comes from this post: https://openphotographyforums.com/forums/threads/offset-black-level-in-canon-raw-files.12280/ (pixels are referred to as sensels - like "sensing pixels" or something like that) @RemcoDutch Attached tifs seem to be processed images - but have huge resolution - second one is 800mb and 12000x9000 or something like that. This suggests that you either used drizzle for some reason, or you resized image in PS? Either is unnecessary and former hurts SNR a bit. Could you post linear (not processed) fits / tiff 32bit stack straight from DSS?

Try stacking without darks. @alacant has been pointing out for some time (and I did not listen), and I've recently read some documentation on Canon DSLRs - they internally modify subs so that darks are useless (they perform a sort of dark removal on long exposure subs). Calibrate with bias as darks and do dither. 1 second is just simply too short. You have total of less than 5 minutes of exposure. Good astrophotographs require much more exposure than that - think hours instead of minutes (even if it is half to an hour of total exposure). If you can - do longer single exposures as well. If you want to beat read noise you need longer single exposures. Longer exposures cause issues with tracking, but if you can - go for at least 30s per exposure. I'm going to download your linear fits to see what I can do with it ... will post results back.

If you want the best - then get Riccardi x0.75 FF/FR This one would suit your scope: https://www.teleskop-express.de/shop/product_info.php/info/p11122_Riccardi-0-75x-APO-Reducer-and-Flattener-with-M63x1-Thread.html but is out of stock (maybe you can find it somewhere in stock) There is larger version (and more expensive), that you don't really need over smaller one - but is in stock: https://www.teleskop-express.de/shop/product_info.php/info/p5125_APM-Riccardi-APO-Reducer-and-Corrector-0-75x---M82-connection.html

Usually I would recommend T2, but my only concern is that two T2 rings I found available don't have good specs. One is all shiny which I would avoid and second does not have specified optical length. Maybe someone who owns Nikon F mount T2 ring can advise on optical length of the item?

I always assumed that PHD2 RMS is root mean square of guide star displacement - as in (expected position - measured position), but this seems not to be so. Here is quote from PHD2 manual: First part of quote is what I expect - graphs are actually difference between expected and measured star position, but then (my emphasis) sentence goes to explain that RMS value is relation to motions of the mount rather than star position. It looks like RMS is RMS of corrections rather than star position? Another piece of data to corroborate this is screen shot of my guiding that I have: So calculated RMS is 0.36", but if we look at this graph: It appears that 0.5" mark divides hits in 1:2 (around 50 and something inside 0.5 and 27 outside) - or ~67% of fall within 0.5" and not 0.36". Single RMS contains ~68% samples (68.27% to be more precise). This would mean that 68.27 hits should be inside 0.36" diameter - but there is much less (if only 67% is within 0.5). Why is this important? Well, because RMS can't then be used as accurate measure of guide performance. It shows just how much you made corrections - but if your guide settings are such that you don't make corrections when you should - it will look like good guiding rather than bad. Take for example min mo parameter. Set it too high and mount will not make corrections at all - guide RMS will be 0 but hits (expected vs measured star position) will be all over the place

Since you'll be using body of Nikon camera with Nikon F mount, you need to account for how "deeply" embedded sensor is inside the body. That is flange focal distance: Different cameras have different this distance. Coma corrector needs sensor at some distance from its thread - in case of Baader MPCC and M48 thread that is 58mm, so image looks like this: Blue line is total distance from MPCC to sensor and it is 58mm (for M48 - it is 55mm for T2 connection). Flange focal distance is red - it is 46.5 for Nikon F. You have one or two more bits in between - T/M48 ring (depends which one you choose) and any extender. These two need to be of combined length that is equal to 58 - 46.5 if you opt for M48 connection or 55 - 46.5 if you opt for T2 connection. In first case (M48 connection) you need 11.5mm of distance and in later (T2 connection) you need 8.5mm distance. M48 ring has optical length of 8.5mm (for some reason), while there is no data for regular T2 ring (could ask TS or other supplier for that info, depending where you want to purchase), shiny T2 ring has optical path of 1mm so you need additional 7.5mm of extension to reach 8.5mm needed for T2 connection. Makes sense?

You mention percent of distortion. Can you elaborate a bit on that? I'm not quite familiar of what that could mean. I understand two edge cases - AMD and rectilinear. Those two can easily be explained with a bit of geometry. Zero rectilinear is first image - each angle is mapped to its "projection" on field stop. Same angles near the center are smaller on focal plane than angles near the edge (see segments on straight line in first image). Second image is zero AMD. Each angle is mapped onto equal part on focal plane and focal plane is actually made up of joined segments along the arc (drawn in the image). This keeps all angles the same (zero angular magnification) - but bends straight lines. When you say 10% distortion that gives 41.6° while max that I can imagine is case of zero AMD - and it is ~39.25° - what the mapping looks like in that case?

@powerlord I did some more research on the topic we discussed here, and I need to stand a bit corrected. Several times I maintained that lower precision in star centroid will lead to (false) lower RMS - but that seems not to be true. I ran some simulations and lower precision actually increases RMS in star position. Why does lower guide precision then tend to report smaller RMS then? I found answer in PHD user guide: It appears that RMS represents not error in star position but rather RMS of issued corrections (motion of mount in each axis). In that case - lower RMS is directly related to Min Mo parameter. This one is, like I mentioned earlier, given in pixels rather than arc seconds. This means that same default value will have different meaning on guide scope with small FL and for example OAG with large focal length (or in another words - it's related to guide resolution). 0.2 of pixel at two different resolutions means that corrections won't be issued the same - with coarse guide resolution they will be issued much less than with fine guide resolution (where relative pixel size is larger).

It does not actually mean it will be rounded to nearest value - it was just example to show that precision will have impact on final result. Actual corrections will depend on Min mo parameter as well. Graph can look rather "calm" when you have low guiding resolution and high Min mo. Min mo parameter should be set at low value - say 0.1 at most. Min mo parameter is only one related to pixel size and not to arc seconds. If you have it set to say 0.2 - that means PHD2 will make correction only if detected deviation is larger than 0.88" (which is 4.4"/px * 0.2px = 0.88").

I'd say that is normal. First - drop in measured RMS. By entering correct focal length - you changed unit of measure. Different unit of measure gives different numerical value. I weigh 120 in Kg, but if I measure myself in lbs - that is 264.5. Does that mean that I'm suddenly heavier? No - I weigh the same, it just different units. Guiding at 0.3" RMS, does that mean you have excellent mount? I guess not. At 175mm of guider FL - odds are that you have too coarse measuring device to precisely measure RMS value. Say you are using camera with 3.75µm pixel size to guide (I see both ASI244 and ASI120 in your signature). That gives guide resolution of about 4.4". Centroid precision is 1/16 to 1/20 of a single pixel - so you can precisely measure star position in range 4.4" / 16 = 0.275 to 4.4" / 20 = 0.22" Let's take value of 0.25" to be your guide star position precision measurement. RMS measure is measure of how different / dispersed measured values are and say your real RMS is 0.6" (how much values "oscillate" around true value). However, you are trying to measure it with something that has precision of almost half of that. You simply can't precisely measure and express this value correctly. Imagine that you have half meter stick and you are trying to measure mean height of a group of people and how much it varies over that group. Your measurements will be 1m, 1.5m 2m - not much variation in that, right?

I would not qualify it as either good or bad. To me it just shows that we can't explain less CA on smaller aperture with same F/ratio by geometric optics alone. It is in my view inherently wave phenomena. Your initial argument actually supports this. I did not realize it at first but I do now. I don't know if you were aware of that or not, but the fact is that you compared where rays land with size of airy disk which is purely wave phenomena (nothing in geometric optics with rays explains airy disk / pattern) and as such is related to wavelength and not geometry of telescope. Larger aperture means smaller airy disk because we leave wavelength the same (we can't change wavelength of light by changing the size of scope) - and relation of the two define size of airy disk. In contrast, if we observe only rays cast in geometric optics - we arrive to conclusion that both scopes give defocus blur of same angular size, and at same magnification when looking at the star - both halos would look the same in both scopes - that is to the contrary of experience and what is really going on - where smaller scope throws up smaller residual color defocus blur.

What I actually wanted to point out was this: Angular size of say violet halo will be the same between two scopes. I confused myself with airy disk and the fact that it is smaller with larger aperture (angular size). At focal plane defocus disk from say red color will be twice as large for larger scope, but since focal length is also twice as larger - defocus disk is has same angular size in both scopes. If you look at unresolved star and notice violet halo - it will be of same size in both scopes at the same magnification.

Yes, you are actually right.

Hi and welcome to SGL. Well, you'll be able to observe: Moon, Mercury (rather difficult as it is always just before sunrise or right after sunset), Venus (also usually in the morning or in the evening - never during night), Mars in about 2 years, Jupiter and Saturn next summer. Uranus and Neptune will be no more than simple dot (one bluish and one greenish). If that is all you are interested, or these object are your primary interest and all the rest is secondary - then get scope like this one: https://www.firstlightoptics.com/maksutov/sky-watcher-starquest-102mc-f127-maksutov-cassegrain-telescope.html It is a bit above your budget, but will provide you with very nice high power views. If you absolutely must stay in £100-£200 range, then maybe take a look at this: https://www.firstlightoptics.com/celestron-astromaster-series/celestron-astromaster-80eq-md-refractor-telescope-with-motor-drive-smartphone-adapter.html

I must express my doubt in your need for x4 powermate for ASI224. If you need x4 powermate then it appears that your scope is F/3.75 or faster? In any case, with barlow lens amplifiers, amplification factor depends on distance of sensor to lens and removing 1.25" nosepiece and going for T2 will vary that distance and change magnification. Powermates and other telecentric amplifiers should be immune to this - and should offer same amplification regardless of the distance, but that is not always the case. Luckily I think that Televue publishes data on their powermates and amplification factor depending on distance. Let me see if I can find that for you. See this page: https://www.televue.com/engine/TV3b_page.asp?id=53&Tab=_app and in particular this graph: It looks like x4 powermate is quite uniform as far as amplification goes (unlike their x5 model), but it does increase with distance, and in fact with 0mm distance and below around 20mm it is less than x4. Interestingly enough x2.5 powermate has negative slope and decreases magnification with distance - not something I would expect, but telecentrics are "special" in this regard as you can see.

Just to see if we are on the same page here, do the following (draw diagrams): Say you have 50mm F/6 scope and 100mm F/6 scope. 50mm scope has red color being 1mm short in focus, so it has focal length of 299mm instead of 300mm, with scaled up scope what do you think will happen? If we scale up shift in focal length of red color - to 2 mm instead of one as everything is x2 in size (aperture, focal length) - and is 598mm, then if you draw the diagram red "hits Airy disk" in exactly the same place with 50mm scope as does with 100mm scope If we assume that focal length is changed by same amount instead of doubling - then on 100mm scope it will have 599 will 'hit Airy disk" closer to center. It needs to change more than doubling in order for "geometric" explanation to be able to explain things, and I don't see how can that happen.

What sort of distortion? They are both distortions of some sort. You can't map sphere onto flat plane without some type of distortion. AMD vs Rectilinear distortion. Two edge cases are: y = f * tan(angle) and y = f * angle where y is distance from center of the field to field stop and f is focal length of EP. If we take 27.4mm, half of that is 13.7 so we have: 13.7/40 = tan(angle) => angle = ~18.9 or AFOV = 37.8° for zero rectilinear distortion 13.7/40 = angle = ~19.6238 or AFOV of ~39.25° for zero AMD

Can you explain this bit?

Again - not important really. Each time you stack two subs - no matter how many bits subs themselves have - you get x2 more bit depth in the data. Stacking couple of hundred subs which we often do in planetary (say 5% of 20000 subs will be 400 subs stacked) will increase bit depth by say 8ish bits (log base two number of subs stacked). Even if you have 10bit or 12bit depth, what sort of gain will you be using? High gain to reduce read noise? If you use very high gain and you have 12bit depth - you'll effectively record less than say 8bit "resolution" in reality. Here are graphs for say ASI224: Most people will use gain of say 350 to reduce read noise to max. At that gain setting e/ADU will be around 0.125 e/ADU (for gain of 318 and less for gain of 350). That is x8 or "three bits" multiplier. Say you capture some number that needs 12bits to record - like 3000ADU - in reality number of electrons that you recorded (and remember you can't record fraction of electron really - anything that is "fraction" is just noise as there is always integer number of electrons) 3000ADU * 0.125e/ADU = 375e - which is really 9bit number (12-3) There is no "per channel" in capture time - each pixel records single color. Full three channel color information is restored after debayering. Regular debayering is just "making things up" really (interpolation) so for planetary it is better to do bayer drizzle approach (which again keeps original resolution from pixel size unlike interpolation that looses resolution) which restores color from stacking (and stacking increases bit depth). In any case - bit depth of camera is not something that will lead to posterization if everything is done properly. Using higher bit depth has some benefits in some special cases, but for planetary, for most part it is not necessary. 8bit capture is enough.

Compound scopes with moving mirrors like SCTs and MCTs probably have largest focus range out of all scopes. They can easily focus with any sort of accessories attached and focus quite near and far as well. Try here: https://www.scopereviews.com/list.html Down the page there is section on catadioptric scopes and you'll find reviews of C5, C6 and C8 for example. These are older reviews and focus more on optics and less on mounts supplied with scopes.