vlaiv

I disagree on this one. If you use above formula and want to compare signal levels versus noise levels (their relationship is quadratic, for both poisson and gaussian distribution, noise is square root of signal) - I would suggest using 25 instead of recommended values as that is square root of 5. Noise adds like linearly independent vectors - square root of sum of squares. Let's examine all three cases - 5, 10 and 25 as "swamp" factor. This translate into sqrt(5), sqrt(10) and 5 higher noise level. Let's call this factor C. Let's your light pollution noise be LP. Then your light pollution and read noise combined will be: sqrt( (LP)^2 + (LP / C)^2 ) = sqrt( LP^2 + LP^2/C^2) = sqrt(LP^2 * ( 1 + 1/C^2)) = LP * sqrt(1+ 1/C^2) This means that resulting noise is the same as if we had higher LP noise for factor of sqrt(1+1/C^2). Let's calculate this factor for our example values of C. Swamp factor of 5 gives sqrt(1.2) = ~1.095 or 9.5% increase in LP noise. This corresponds to 20% increase in LP brightness. Swamp factor of 10 will give sqrt(1.1) = ~1.049 or 4.9% increase in LP noise. This corresponds to 10% increase in LP brightness. Swamp factor of 25 will give sqrt(1.04) = ~ 1.0198 = ~1.02, or 2% increase in LP noise. This corresponds to 4% increase in LP brightness. So you see - there is no maximum really - it is only how much you want to increase effective LP noise (or any dominant noise source - or total noise combined with read noise). With x5 difference in noise value (equivalent to 25 swamp factor above) you get 2% increase in noise which would be equivalent to 4% increase in LP (for LP dominated scenario). For some reason I find it more acceptable than above recommended values.

Indeed, once you cross to galactic scale - things get "normal" again John above wrote very interesting account on how stars are vast distances away from each other. Here is one of galactic scales. Milky way and Andromeda represented by hands. Take 20000Ly to be 1cm. Andromeda is 11000Ly so it will be around two inches. Take your left arm and make about two inches of distance between your index finger and your thumb. Milky way is around 53000Ly so it will be something like an inch - take your right arm and again space your index finger and thumb about an inch. Now spread your hands about 1.2meters (or rather just separate them as much as you can without fully extending them). This is Milky way and Andromeda system to scale. If you do that - edge of observable universe is about 4 miles away from you (13.8 Bly as the light travels) or about 14 miles away from you (46 Bly in comoving coordinates - if you had ruler long enough to extend the whole distance). Difference is of course due to universe expanding - on is "travel" distance (at the speed of light - or rather how much light traveled since it started) and other is physical length. Once you start thinking in galactic scales - observable universe is not that large at all - just a dozen or so miles away from you at most

Don't worry about it - it is after all just a case of cloud sickness. Have not seen clear sky in weeks and what else to do then to browse web for telescopes and contemplate setups and their usability

Depends on your setup. - simplest rule - as long as you can while still having tight stars. - complex rule - determine read noise for your camera (in electrons) and see if LP noise or thermal noise is higher for your conditions (will depend on ambient temperature and light pollution levels) and then make exposure such that read noise is 1/5 of higher of the two - thermal noise or light pollution noise (very similar to above given advice - except you should not do it in ADUs but rather electrons and you have to see if thermal noise (dark noise) is higher than LP noise). Or simply go with 4-5 minute exposures

Are all answers final? Can I change my answer? My inner Sherlock Holmes went ballistic with this one. Here is my final answer and explanation worthy of Sherlock It is hair that has been split near its end and is situated close to sensor I'll use diagram to explain it: Light has already been "separated" into different converging beams depending on direction that it is coming from (stars in different parts of frame). Single spikes are due to hair representing single edge where light cone hits it - slightly bending so different stars in different part of frame give slightly different angle. There is a place where hair splits into two at an angle - resembling Y arrangement on bahtinov mask: and this happens only for one star - because converging beam from only one star is falling on the split. And hereby I rest my case.

I agree with above assessment that source of spikes is something in light path - some sort of edge. I'd also like to point out that it is probably not straight edge and that it depends on light direction One star has 3 spikes - this happens when the light passes close to multiple edges that are at an angle (bahtinov mask). Other stars seem to have single or max two spikes (second one being much less pronounced) and have varying angle between them - single edge can't produce this. My vote goes to spider web.

You do understand that one of requirements is imaging? Or rather two of them - planetary imaging and DSO imaging. There are simply people that want it all in one scope and I'm always torn what to recommend. We often end up explaining that there is no scope to do it all, but above 6" F/5 on EQ5 class mount that could possibly turn into dual mode - both Eq and Az with appropriate wedge - would come extremely close to being able to do it all scope (and by that - I mean do it all very good). For purely visual - I could not agree more. Have 8" f/6 dob and looking to get 4" f/7 frac as well

I don't have much problems understanding concept of infinity, and yes, it is just a very big number - one that every other number is smaller of However in terms of size of universe - one should not think infinity. We live in very finite universe - as we currently understand it. We all live inside a horizon and for all intents and purposes, everything inside that horizon is reality - causally connected to us and everything "outside" that horizon is not in any way connected to us - it can't influence us in any way, it can't be seen since light from it will never reach us, nor any force from that region will ever reach us. Question is then - how can we test if it really exists? Answer is - for all intents and purposes - it does not exist. It exists no more than field of pink daisies with giant orange marshmellow half elephant - half rinos dancing around on it. We can prove neither.

That is F/5 scope? With 1000mm and 5µm pixel size - you are already at 1"/px - that is highest sampling rate you should realistically go with. Using barlow will give you 0.5"/px - that is too much. Yes you remember correctly - I have 8" F/8 RC and I use ASI1600 - which has 3.8µm pixels. That gives me 0.5"/px - and I ended up binning x2 data every single time (sometimes even x3 because of poor seeing). Unfortunately - scope has not seen starlight this whole year, but that is hopefully about to change in 3-4 months - at that time I'll be moving into a new house and possibly have new obsy finished as well Why do you want to barlow when you are already at max resolution? Going with higher resolution is not going to give you any more detail.

Given that God is creator of universe - I don't see him/her lacking much or having much trouble in life - so I'm inclined to believe in his/her good disposition. Since so many of man kind is interested in him/her - I really see no way of him/her not respond in kind.

That would certainly be cheapest option - but it would not be "do it all" option. Some people really have all around requirements - they want to look at both planets and DSOs and the want to image both planets and DSOs. We really have only couple of mounts that can do that in economy range: - something like AzGTI - that can work in both EQ mode and AZ mode and is already motorized. Problem of course being weight carrying capacity. - something like EQ3 - again weight carrying capacity, absence of AZ mode out of the box and poorer DSO imaging performance - something like EQ5 - now we are in range of decent mount capacity, and I would say EQ5 is really starter mount for all round imaging. Sure both Eq3 and AzGti class mounts can do imaging - but that is for wide field imaging. EQ5 can do both wide field and medium power imaging. For that reason and the fact that EQ5 can be upgraded with either single motor or dual motors that have guide port or full fledged goto system, or you can purchase goto version out of the box - makes it ideal do it all mount - except for missing AZ arrangement that would be beneficial for visual. I would say that another good option for the scope would be good 4" ED doublet - around F/7. Problem is of course that such telescope will be outperformed by 6" newtonian in almost all aspects except portability, widest of the fields of view and DSO imaging. In any case - both scopes are pushing 5-6Kg with additional equipment and that is overkill for AzGTI, and up there on the limit for EQ3. There are lighter scopes - but they will be just poorer choice in one aspect or another. That and the fact that 6" F/5 costs about x3-x4 less than good 4" ED doublet (x2 less than okayish 4" ED doublet with some residual CA) - really make it ideal choice. Scaling it down can be done "1 step" - that would be 130PDS and possibly EQ3 mount. 130mm of aperture is still capable of being all rounder - it will be good on planetary (although not many people do planetary images with 130PDS) and it is confirmed good performer on DSO - both visual (in its class) and imaging. There is option for further reduction in size apart from mentioned 4" ED - which is actually step up in weight and considerable step up in cost. There are 114mm and smaller newtonians - but they have 1.25" focuser and there are no coma correctors for that format. All fast doublet scopes below 4" are really not good planetary performers - there are no good planetary images below 4" in general. Also, less than 4" of aperture is going to limit DSO observing capability.

What sort of focal length are you hoping to achieve and what pixel size will you be using?

The way I see it, maybe cheapest option would be to get SkyTee II mount head and EQ5 mount - to share tripod. Then use EQ5 for imaging and SkyTee II for observing? Pity there is no cheap mount in this class (£600 or so) that can do both, have a goto and carry 150PDS or similar scope. Just thought of something else as well - DIY dob mount for the scope and visual? It does seem like a lot of possible options - except cheap "out of the box" - do it all one (that would be there with EQ5 that can be adjusted for 90° latitude).

It is possible after modding the mount - not something novice astronomers will likely do as it involves some "metal works" - filing away part of the mount. I think that much better solution would be to actually redesign wedge on EQ5 - making it more versatile mount. Alternative is to take SkyTee 2 and turn that into EQ mount by placing it on a wedge (since it shares parts with EQ5 and can be fitted with motor drives for EQ5). Not sure how cost effective solution that would be.

Impact is not negligible at all. More than altitude of the target - there is issue of transparency. In fact these two are combined - each air mass attributes to extinction based on AOD - aerosol optical depth as well. Here is good article on the subject: https://skyandtelescope.org/astronomy-resources/transparency-and-atmospheric-extinction/ AOD can range between less than 0.1 to something like 0.5. Conversion between the two is 1.086 - so AOD of 0.4 is equal to 0.4344 mag of difference. When everything adds up, there can be even whole magnitude of difference in brightness of the target - that is x2.512 less signal or about 40% of original signal. Yes - this means that for same conditions you can cut imaging time to at least 40% in great transparency over poor one (effects compound with presence of LP as LP does not seem to be much reduced because of poor transparency - most of it is in bottom layers and gets compounded with poor transparency). For good forecast of AOD - check here: https://atmosphere.copernicus.eu/charts/cams/aerosol-forecasts Red means - more than whole magnitude per air mass!

You are probably right. In any case - RASA8" seems like greater value - maybe only limited by the fact that camera needs to be in front and mono with filters would be awkward to use.

Never thought about it really - but that little finder / guider is really light - 287g. Also - I wanted nice background for the images - one that captures perpetual cloud coverage / fog

Except I used 22mm that are fully corrected according to Celestron and we have comparing spot diagram of. Allow for 10% in overlap with mosaic and that is roughly the same FOV - Tak and FF sensor vs RASA8" 2x2 4/3 sensor

Unfortunately - there is no such gizmo - at least not cheap option. There is whole section of SGL dedicated to this kind of observing - it is called EEVA (electronically enhanced visual astronomy). As already mentioned - you can get various 1.25" electronic eyepieces / cameras - but these tend to be best for planets due to small FOV. If you want to show them the Moon and planets - that is a good option. With your scope, something like ASI120 will give views like this: So it will show close up of the moon, you can even check out some YouTube videos to get the idea: https://www.youtube.com/watch?v=k15ErQGVt4M That one is also 4" scope - although a bit different than yours but I believe FOV should be roughly similar. Btw, surprisingly good results can be obtained with smart phone at eyepiece, check this out: https://www.youtube.com/watch?v=_Kxjb9wtTzE For deep sky objects, things are not as easy - you need to do something called live stacking. Here you need to take longer exposures (like 10-20 seconds) and target appears after a few minutes. Here - larger sensor is better. This is field of view with ASI120 and your scope on M42 - only central core is seen. You also need suitable software to do this and lap top of course.

Flats will fail if you don't use proper darks. Whether you will see that - depends on how much dark current your sensor produces. You also need to use flat darks. There are other reasons why flats can fail and general rule is - if there is some signal not removed (or removed when it should not have been removed) flat correction will fail. This is because flat correction works only on signal that came thru the telescope opening and was subject to vignetting dust. Any other signal that is there and not removed - will be corrected although it was not subject to vignetting / dust shadows and that will leave dust/shadows and vignetting mark on the image. Possible causes: - dark current not removed - dark flats not applied - light leak when shooting lights, darks or very rarely flats - flats tend to overpower any small leak

Ok, now we have to be careful. If you want to see if Tak will have the same pure resolving power - you need to scale their spot diagrams accordingly. Let's go and do that for better understanding. First - we need to scale diagrams in to same units: We need to enlarge Tak spot diagram by factor of 100/18 = 5.555 because of scale of things - it has size of square 100µm while Rasa one has 18µm. Then we have to reduce size of Tak spot diagram by factor of 530 / 400 if we want to adjust for focal lengths and convert to angles. We need to enlarge it to 419%. At 11mm it has about 20% larger red (~700nm) spot diagram: So yes, it has about the same resolving power - without impact of atmosphere and guiding. If you throw that into the mix - it will only even things out further. Just remember - these are design specifications, not manufacturing ones. Which one has the reputation for better manufacturing of optics (to a higher standard - closer to the actual specs?). Is it really slower than RASA? Depends on what you want to image. RASA8" will accept about 22-23mm imaging circle (I know it says up to 28 - but let's go with "sharp" field - for the sake of argument). Tak will illuminate 44mm diagonal. If you are imaging very wide target - the will have the same speed. Tak has about 1/4 of light collecting surface, but RASA will need 2x2 mosaic. If you match sampling rate - they will be roughly the same in "speed". Rasa will image each panel 1/4 of the time with x4 higher light gathering. If you image just narrower field of view provided by 22mm of RASA8" - then RASA is indeed about x4 faster.

I would throw some math on that problem For my location difference between IC342 being highest in the sky and being at "9 o'clock" (west/east of Polaris) is 0.3 magnitudes in extinction. It is about mag25 in surface brightness. Let's say that I'm imaging it at 1.3"/px with 8" aperture in mag20 skies. What would be the difference in SNR or imaging time for same SNR if we assume 0.3 difference in magnitude? In above conditions for two hours of exposure we get loss of 24% of SNR due to object position - or equivalent of additional about 1.5h of imaging - or 2h of imaging near meridian flip would be equivalent of 3.5h of imaging when target is at 6/9 o'clock with respect to Polaris. This is however - not general rule and things change depending on declination of the object and your latitude and brightness of the target as well as light pollution (and as you imagine - all other factors ).

30mm of aperture in terms of resolution - not in terms of light gathering power. That is really not that bad resolution when we talk about long exposure imaging. Sure, such telescope would not produce excellent images of Saturn or Jupiter - more something along this lines: This got me thinking - what is equivalent aperture of resolution when we add in the seeing? Left is actual perfect aperture expressed in mm in 2" seeing and 1" RMS guide error - right is equivalent aperture - pure resolving power (again in mm). As you can see, perfect 80mm scope in 2" seeing and 1" RMS guiding will resolve the same as 33.1mm of aperture with no atmosphere influence and with perfect tracking. 250mm scope won't resolve much more as with 2" seeing and 1" RMS guiding - we are entering domain that is seeing/guiding limited. With Tak apos - actual airy disks are about twice as big as airy disks of 200mm scopes - since they are only ~100mm in diameter. Same spot diagrams means that in case of Taks - they are twice "as condensed" with respect to perfect aperture. In order to fully compare spot diagrams of two scopes in terms of resolution - you need to scale them to same Airy disk size and express them in angular units. Longer focal length scope will have larger spot diagram in microns - not because it is of lesser quality - but because it magnifies more.

This really bugs me quite a bit - you know, what scope to choose that will do it all. Good for planetary observing, good for DSOs, good for planetary imaging, good for regular imaging. Cheap, or rather as affordable as possible. No scope can do it, right? Well, I have a candidate for it, and I just realized that equipment manufacturers hold us back My candidate is - 6" F/5 newtonian on EQ5 or rather EQ5 equivalent that "has AZ capability". When I said that manufacturers hold us back - I meant - EQ5 mount can't be converted to AZ mount easily. It requires modding. Why on earth didn't they create wedge that can be put at 90 degrees for EQ5? I don't particularly like newtonians on EQ mount due to eyepiece and finder positions and constant need for rotating the tube. Solution - obviously AZ mount for visual. With that in place and simple motor for tracking - we would have it all. 6" of aperture for planetary observing and imaging. Even with larger secondary that enables astrophotograpy - we could still keep central obstruction below 30%. Low enough focal length for wide field observing. Something like Aero ED35 - would fit whole M31 in FOV while still keeping exit pupil at 7mm. At F/5 it won't be too demanding for photography. 750mm of FL is flexible enough to capture both nebulae and small galaxies. 6" is just enough as entry to serious DSO observing. Alternative is AZ-EQ5 mount - but that is not really budget option, is it? Also, tracking motor is not upgrade for that mount - like on regular EQ5 that costs x3-x4 less than AZ-EQ in non powered version.

Do exact the same thing you are doing now, except choose fourth option - Create super pixel. This will perform debayerization completely but it wont use interpolation. Then do the same thing - open file in AIP4WIN split channels / save green and open in AstroImageJ