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Maybe above image is a crop? Do you have single unedited sub from your camera? Coma is symmetric aberration - and it should be minimal in center of the image for well collimated telescope. It is not central in above image - if you take lines going thru the stars and their tail (coma tail) - they converge to roughly the same point in the image and it is not center of the image: Either your scope needs to be collimated properly or you have cropped the image. In second case - it will help that you put target in the center of the FOV and then crop to it. Don't let it be to one side because it will lead to coma being asymmetric in the final image. Of course, way around this is to use Coma corrector - that way you will get good definition across whole field, but like I mentioned - there is no coma corrector in 1.25" format commercially available. You can DIY one - do a search for 1.25" coma corrector and you will find instructions in ATM sections of astronomy forums for DIY 1.25" two element coma corrector - you can try that. If you can't be bothered with that, maybe best thing to do is move to a bit larger but scope that is actually suited for AP - 130PDS. That one has 2" focuser and properly positioned mirrors so you can reach focus with camera.

I also vote coma. Scope needs collimation after mirror adjustment. Unfortunately there is no Coma corrector in 1.25" format.

I sense we are getting there - could you say the word and original meaning (and why does it originally mean what it means )

What sort of light pollution do you have? If you have stronger light pollution then you can actually use shorter exposures without loosing much in the end result. Only difference between short and long exposures for same total imaging time is in read noise, and only when read noise is significant component. You are using DSLR without cooling so you already have dark current noise that can be as much as read noise in short exposure. If you add light pollution noise to that, you could be able to achieve similar results with only 30s exposures. It's certainly worth a try - just remember to do same total integration time and not same number of subs. If you imaged for example for one hour with 2 minute exposures and made 30 of them. If you go for 30s exposures - do one hour again and do 120 exposures.

Ok, I'm ultimately confused here. I do understand that people use term - telescope "speed" to specify ratio of aperture to focal length and in that sense - there is only one understanding of the term "speed" or better call it F/ratio and it is in fact significant in many aspects of operation of telescope - for visual and for photography. Only problem that I see with this is using term speed, faster, slower as that implies something that is simply not true, and in that sense yes, many people will confuse it with scope's ability to put photons on the sensor. But if we agree that we don't call those terms - speed, faster and slower and instead use terms F/ratio, lower and higher, that don't have other significant meaning in this context - then I'm perfectly fine. Are we on the same page here and we talk about F/ratio and related terms lower and higher and we are not discussing fast telescopes in terms of imaging speed (speed to reach certain SNR at given target resolution) but rather just discussing the fact that there are scopes which F/ratio is low in number - like F/5 and such scopes have certain properties and there are scopes that have F/ratio in for example 6-9 range and there are scopes that have F/15 - focal ratio. These scopes behave differently optically so yes, there is real distinction - just lets not call them fast and slow (I can't really think of reason to call F/5 scope fast - other than in photography context - what, fast to setup? Fast to show aberrations at low cost eyepiece? Fast as fast spending of money on more expensive eyepieces to get decent field ) @Mr Spock - what use of term speed did you have in mind?

I would say that we can divide factors into two distinct categories: - ones that you can influence - ones that you have no control over Then concentrate on first group and understand what parameters are the best and how to obtain them.

I sort of agree with you, but given that it is called speed - a term that, as far as I know, originated from world of photography, and had meaning "speed at which image is formed on a film during image capture" - I think it would be good to point out that although we are talking about F/ratio and we use term "faster" instead of lower or higher - one ought not use it to compare speed at which image can be acquired with said telescopes.

Not necessarily. You can try without the IR/UV filter first and if there is bloat - then put filter in. Meniscus corrector should not produce much of secondary spectrum on its own - rest are mirrors. Depending on the camera - you don't need T2 nose piece if you don't have one. My ASI models come with 2" nose piece - you can use that with 2" focuser - just insert camera with that attachment and it should fit focuser. Inner thread is T2 but outer diameter is 2" - same as 2" eyepieces.

I had my doubts about stability of that tripod, so I went for this instead : That one is rock stable (I had HEQ5 class tripod leftover from upgrade to berlebach planet and I decided not to go for regular tripod sold with AzGTI). If you plan to get AzGTI combo with Skymax 102 - be careful. I went for OTA + mount separately as version of Skymax that comes bundled with AzGTI seems to be different - it lacks back side collimation screws for example.

Yes, you can operate it both with laptop and smartphone. There are a few ways of configuring it - it can create it's own access point and you can use smartphone or laptop to connect to it and then operate it or you can configure it to connect to already existing access point - and then you can use your device connected to same AP to operate the mount. I'm not sure what the maximum distance is, but I suspect at least 10-20m with line of sight. I operated it at least 3-4 meters away via wifi connection.

There is no such thing as back focus for cameras - you probably think of adapters that you used to otherwise reach 55mm required by some field flatteners or coma correctors. You don't need any of that for MN190 - just attach camera at prime focus with 2" to T2 nose piece and you are set to go. You should be able to reach focus like that - if not, you might need a bit of extension. No need for any special distance since MN190 is corrected by meniscus disk at aperture.

Ok, I get it now M54 is used for very large cameras and is one of "standard" threads. Best thing to do is to get yourself M48 Nikon lens adapter like this one, provided it adds only 8.5mm of optical path: https://www.rothervalleyoptics.co.uk/rother-valley-optics-m48-t-mount.html But do check first with them if it has required optical path. If not, you can purchase ultra short Nikon adapter and then use M48 extension rings to get proper distance. Or get this one: https://www.teleskop-express.de/shop/product_info.php/language/en/info/p3629_TS-Optics-T-Ring-from-M48-to-NIKON-Bajonet.html as it has proper optical length: (btw what is show on image looks exactly the same as adapter at RVO so there is a good chance that one being 8.5mm also).

But maybe it would still shine thru the clouds?

In fact - look at this recent discussion:

This is common misconception about black holes, that somehow same amount of matter (or even less) that was in star that produced black hole now suddenly starts attracting things more for some reason. It just does not happen. Things that fall into black hole and get sucked into it - would definitively also fall into original star - difference being that star radius is many orders of magnitude larger than event horizon and you simply cannot come close enough to star center without colliding with the star first. Once star is collapsed and all matter that was once star - gets into very small radius and has very large density - then funny stuff starts happening when you are close enough - but for things that are far enough - they just continue the same as before - when all that matter was still a star.

Ok, now I'm confused again - you attached two pictures - first one - showing what I believed to be M48 thread and unscrewed bit - this one: and then this image - with that attachment screwed in place: I was suggesting to try screwing that possible T2 thread (which is obviously smaller in diameter than M48) into your DSLR adapter and to see if it will screw in.

Black hole that close will change nothing. If the closest star to us became a black hole - we could only tell that it went dark and nothing more. Only real danger might come from gamma ray burst - if those are indeed produced by supernovas, and if it is directed towards us (not sure but I suspect GRBs emanate from poles of rotation of collapsing star - might be wrong at this).

Are you sure? Same adapter you used to attach your camera to 8" dob?. I was probably not clear again sorry - let me try one more time - this time I won't try to explain it with words. Try to screw this thread: in here: Both should be 42mm wide, and only reason can be thread pitch - it will start to screw in but will stop after a half a turn to a turn (if that happen don't force it, you can damage the thread).

I think I was not clear enough. I meant that you try little thing that you removed not M48 thread. I was hoping you will figure out if smaller bit has T2 or M42 thread on it. If it has T2 thread - then it should fit without any issues. Don't try field flattener on DSLR adapter - but rather than little thing that screws into field flattener.

Ah, ok. From what I can see on FLO website for that FF/FR - it has M48 on both sides - scope side (2" filter thread is M48 - so you can connect it to scope via threaded connection if you focuser supports it - that is good to avoid tilt), and it has M48 on camera side - together with "M42 tread" which could be T2 thread actually rather than M42 thread. From your image I see that adapter included - screwed in its place in second image. Check if that is indeed T2 thread - take it off and screw just it to T2/Nikon adapter that you already have. Be very gentle and don't force it - if thread has different pitch you should not be able to screw it in more than half a turn to a turn. If you can screw it in multiple turns and it screws in easily - then it is matching thread and it is in fact T2 adapter - then you should be all set. At least I think so - you would be for Canon camera as it has lens mount that requires 55mm of distance and canon adapters are made like that - and it matches perfectly to distance required by this FF/FR. As far as I can see - Nikon F-mount has this distance at 46.5mm and if your adapter adds 8.5mm of optical path - you should be ok to exactly 55mm - unfortunately I can't see info on the link you provided, it does not say optical length of that adapter.

I'm a bit confused what is your problem. First let's figure out what your connections are and then we can see what sort of adapters do you need. Also bear in mind that M42 usually refers to 42mm x 1mm metric thread which is different than T2 thread which is 42mm x 0.75mm - pitch is different. Most of astronomical gear uses in fact T2 mount while some older photography lenses use M42 mount - don't confuse the two. Focuser has (possibly) a thread - what is it? Flattener will have front - scope facing thread and rear - camera facing thread - which ones are they? Your camera will have thread on it - which one is it? (most likely it is T2 thread unless you have very large camera). Do you use any other accessories like filter wheel, OAG or anything like that? Since reducer/flattener operate at certain distance - it is also important to know this distance and to know sensor distance from camera thread. What sort of camera are we talking about?

You do know that there is no correct and simple answer to that question? I can give you simple answer. Use gain 139 and offset 56 (or even higher - I use 64) for both LRGB and NB. Use 1-2 minute exposures for LRGB and 4-5 minute exposures for NB. Use at least 64+ calibration subs. Use darks, flats, flat darks. No need to use bias. If you get clipping (and you will on bright targets) - use filler exposures of 10 or so seconds (calibrate those with same master flat - but their own master dark of same duration). Are above recommendations best for you? That highly depends on quality of your guiding and level of your LP. If your guiding can handle it and you have low light pollution levels - feel free to go with longer subs for both LRGB and NB then above given

Actually - I recommend leaving some headroom and completely avoiding any minimum values in multiple subs. Your advice is sound - looking at the histogram and making sure it is not clipped is what it is all about, but with CMOS sensors, you really need some head room just to be safe. If you want to be safe - determine minimum value that your camera outputs. It is often either 16 or 4 for 12 or 14 bit ADC cameras - in case of ASI1600 it is 16 for example. Take number of bias subs, stack them with minimum method (instead of average - just plain minimum stacking no sigma stuff) and inspect that in software like Fits Liberator. In fact - Fits liberator will give you stats panel and besides checking histogram out - you can check stats panel and observe minimum pixel value - it needs to be higher than min value camera is producing (only way to be certain there is no clipping). If you have bad pixels - you should remove those somehow (set to 1000 or something via pixel math) - but I think these are rare. Why do I recommend higher offset than otherwise sound advice given above? - to make sure histogram is not clipping. Many CMOS sensors suffer from what is called telegraph type noise that happens because electrons "leak" - either out of pixel wells or into pixel wells or between pixel wells - and that creates either higher or lower values in such pixels than would be expected by statistics of bias sub. Take a look at this example: It is animated gif made out of 10 darks - 300s darks from new ZWO camera (6200 model). You can clearly see dark and bright pixels popping in and out "of existence" (I love that quantum mechanics jargon). These pixels can get clipped even if your histogram is fine as they can be much lower than surrounding mean value. With ASI1600 this feature is really prominent - it happens on adjacent pairs of pixels - look at std dev stack of darks from my ASI1600: With this stack - brighter the pixel - more variation there in set of dark subs. It is interesting that higher values clump together in particular way - two adjacent pixels diagonally ordered seem to share "defect" - and leak electrons between themselves. We can further observe this phenomena like this: Here is histogram of single pixel values across 64 dark subs: This is from a well behaved pixel that does not display telegraph type noise. It is as we would expect - resembling bell shaped curve that read noise produces. Now look at what histogram of Telegraph type noise affected pixel looks like: It has not one but multiple bell shaped peaks - it has base read noise but also depending if pixel leaked electrons or electrons leaked into pixel - it has other bell shaped curves offset from primary one. This means that even if we capture primary bell shape with our offset - which is in my case around 62-63e (because I use offset 64 and unity gain) - there is chance that some pixels in some subs will be clipped. I know that this is very low number of pixels - roughly less than fraction of one percent of pixels is displaying such behavior - but this is advice for those that really want to take care of even tiny sources of noise.

I advocate use of ASCOM drivers over native for long exposure imaging. If you use ASCOM drivers, you should be fine and not affected by this change. If not - make an effort and switch to ASCOM drivers. You will need to redo your calibration subs (at least darks - you should be fine with existing flats if you have permanent setup and reuse flats). For this model, it is actually exposed and needs to be set properly. From what I've seen - it is exposed for other ZWO camera models as well. Fact that you have offset setting and that it was set to 21 at one point by default - meant that there was some clipping with darks and as a consequence there was more noise in subs and sometimes flat calibration failed because of wrong offset settings. This happened for people with low offset setting (quite a bit of clipping and changed mean dark sub value) that were imaging in higher LP (LP acted as offset from lights and there was no clipping for those - and when you subtracted darks that clipped - there was wrong mean signal level that caused issues with flats).

Here is suggestion for additional background removal technique - do "upper" sigma reject couple of times and do linear fit on remaining pixels to do background removal. Simply do few iterations of pixel selection - calculate average and sigma, and discard all pixels above some multiplier of sigma - for example all pixels with value higher than 3 sigma. Do linear fit on remaining pixels and remove it from the image. You can do couple of rounds of this removal. If you do color - use same settings and you can use "AND" operator on pixel mask for each channel - that improves chance that you have background selected. This method works very good as long as you have pure background framing the object - I suspect it will not work good on images where you have nebulosity in whole frame (there is no background which you can try to isolate in that case).