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Let's try it this way: - re sampling resolution - depends on what scope you are using, quality of your mount and how wide you want to go. 1"/px is very high resolution and in most cases unattainable. You need mount that guides at least at 0.5" RMS or lower. You need very steady skies and you need at least 8" of aperture to get there. For telescope of 80-100mm of aperture, realistic maximum sampling rate is at about 2"/px. This does not mean that you have to go that high - if looking for wide field setup - simple fact is that you have limited size of sensor / corrected field and you won't be able to fit that many pixels. 3.5"/px is fine sampling rate for wide field. If we want to be specific about max sampling rate and Nyquist - there is simple rule to follow - measure your FWHM and go with sampling rate that is about 1.6 less than that. This means that one needs 1.6" FWHM stars in order to fully exploit 1"/px sampling rate. - re guiding resolution. Well depends on the mount you have and what is realistically achievable in terms of guide RMS. My advice would be to sample at about x3 per best possible RMS in terms of centroid accuracy. Centroid accuracy is about 1/16 - 1/20 of single pixel. To give a bit better explanation, here is how to calculate guider resolution. Let's say that you have mount that is capable of 0.5" RMS guiding under best circumstances. You want your centroid accuracy to be about 0.5/3 = 0.167". That will be 1/16 to 1/20 of a pixel so pixel size needs to be 0.167 * 16 to 0.167 * 20 = 2.67"/px to 3.34"/px You have guide scope that is 60mm F/5.9 or about 350mm of FL. You need a camera that has less than 6um pixel size to use as a guiding camera. With most common pixel size of 3.75um and 350mm of FL, you'll get 2.21"/px - which is slightly better than you need. With 290mm camera, which is a good choice for guide camera, you'll have 1.71"/px and if you choose that camera, then use x2 bin to further improve it's sensitivity as it will still provide you with 3.42"/px - close enough for above criteria. Hope this helps?

I pushed my 8" to ridiculous powers like x500+ just to see that moment when image "falls apart" - could not understand why people call it that, and I still can't. For me, image did no fall apart - it just became magnified / larger with the same level of blurriness (and darker of course). Best views of Jupiter to date were with 8" at x200, so again - no crazy powers required.

I like when image is sharp rather than magnified. x200 with my 8" scope and about x100 with 4" scope. I was observing two days ago after quite a long time - had my 4" Mak setup on balcony and took a glance at Moon, Jupiter and Mars. I found that ES82 11mm gave most pleasing views, although I also used ES82 6.7mm and BCO 6mm. On Moon I preferred 6.7mm but on Jupiter and Saturn - 11mm was more pleasing. That gave something like x118 magnification with 11mm and x194 with 6.7mm. Just to clarify - when I observed Jupiter - it was Europa transit at that moment and I caught second part of it. Seeing could have been better but it was relatively good. I could see Europa shadow with both 11mm and 6.7mm and the moment it emerged from in front of Jupiter - I could clearly see Europa itself - but actual disk was only clear to me with 6.7mm. However at that magnification and with 4" scope - you can't really be sure if you actually resolved the disk - it just looks different than star, a bit fatter but with airy rings around it - which is a clear sign that we are entering too much magnification territory.

It actually depends on your eyesight. For 20/20 person - you don't really need to go very high to be able to see everything there is to be seen. For 102mm of aperture we are talking about resolving features the size of 1.26". 20/20 vision person will see 1' detail - that is 60". In principle you will be able to see it all with about x47.62. Any higher magnification will not show you more - just make it easier to see what is already there. People often use x2-x3 that magnification because it just makes things easier to see (you don't have to work that hard like reading last line in eye doctor's office ) but after that, things just become too soft (not because optics is poor - rather things are naturally blurry because there is no additional detail).

Well, you can try two different approaches here. With Starnet++, this is how I would suggest that you try: - take all channels and stretch slightly and save as 16 bit images. Remove stars with Starnet++ - take Ha channel and subtract Starnet++ version from regular 16bit version. This should give you stars. If you did initial stretch right - this is about as much as you should stretch your stars or maybe just a tad more. - Now you can either stretch all three channels to your liking and do RGB combine or you could try LRGB approach (I'll explain that one as second approach without Starnet++). Once you have your image - you simply layer stars on top of it and do some blend mode (like lighten or maybe add layer mask with stars and normal mode - whatever puts stars on top of your image). That way you should have nice nebulosity with good color and white stars (no annoying purple halos and such). Second approach is probably a bit more complex but it does not involve Starnet++ (which could be advantage in some cases). You need to do LRGB composition with RGB in linear stage and synthetic L. Creating synthetic L is rather difficult to do, but in most cases Ha or Ha+OIII will be enough. SII rarely exists on its own and much more frequently it is in the same place as Ha. This lets you use Ha as luminance. In any case, once you have luminance - stretch it like a mono image until you are happy with what you have. Now you need to do RGB combine by applying RGB ratio method. But you need to assign sensible weights to each channel. Usually Ha will have some small weight like 1/16 or so of linear (it will still be linear data only scaled to bring it to same value as OIII and SII). Similarly you might need to scale OIII just a bit to bring it down to SII levels. Here you are not really stretching your data - just making it "color" compatible. Otherwise it will all be red with slight hue variations (because strength of Ha is so dominating). RGB ratio is rather straight forward: Resulting_R = Stretched_L * R / max(R,G,B) Resulting_G = Stretched_L * G / max(R,G,B) Resulting_B = Stretched_L * B / max(R,G,B) not sure how you are going to do that in either PI or PS or whatever software you are using (should be doable in both of those listed but using different approaches - either pixel math in PI or layers in PS).

In the end, I guess that is proper explanation, as for Maksutovs (and other Cats) - it is magnifying secondary that acts the same, right?

Yep, I've got some very strange astigmatism in my right eye so I amuse myself by trying to figure out PSF of street lamp (I see three moons / street lamps with my right eye). This is more like grease smudge - that sort of ghosting / scatter - it does not move with my eye but rather stays put with respect to eyepiece. My first reaction was that it is grease from eyelashes / eye - since it is in center of the eyepiece and it looks like that, but cotton swab with cleaning fluid made no difference - I expect at least to spread it around if not clean it completely. It is also present in various eyepieces (although that means nothing since I could have contaminated eye lens in all of them - but effect changes with eyepiece / scope combination).

I just realized that Ruud's explanation given in the thread that I linked is much more likely than mine above. Mak has secondary mirror that magnifies - that makes rays look like they come from much shorter focal length - so I guess that is the reason behind longer eye relief in Mak? In any case, I made one wrong assumption - smaller exit pupil will not necessarily touch edge of eye lens and whole thing with exit pupil can easily be tested - one just needs aperture mask to create smaller exit pupil and leave everything else the same (focal length and eyepiece). Adding such aperture mask should change eye relief if it is related to this. On completely different note, @Louis D, very interesting thread you linked and your images seem to show something that has been troubling me. I get this dark spot in the center of the view sometimes and I don't understand why it is there. It is not round and it causes blur / scatter and some light fall off. It resembles the most to images of GSO and Orion plossls 32mm. When I move my eye - this dark spot "counter moves" - or rather it's position relative to eyepiece stays the same. I can often "look behind it" (sort of). If I place it directly over bright source than everything turns nasty (like placing Jupiter behind it) - so much scatter appears in FOV of eyepiece. This is not scope related nor barlow related nor eyepiece related since I experienced this with different eyepieces / scopes / barlow or no barlow situations - but it does not happen always. For example in ES82 11mm and Mak102 - it was there but very subtle, more like just a bit of "misting" in that place rather than shadow. With ES82 6.7mm it was very obvious and distracting. With BCO 6mm it was evident but somewhere between other two in intensity. What could that be and why does it happen?

Barlow indeed produces higher F/ratio for any given scope and if above is correct - that would be explanation, but I don't remember that being offered as an explanation in previous discussion. Maybe it would be beneficial if I find actual discussion I was referring to and explanation given there. Ok, here is original discussion: And here is interesting point:

That one is rather simple You set your temperature to -16 and said camera to start cooling and went about your business. Few minutes later you returned and was very pleased to see it at -16C with 74% of power being used. Then you started taking exposures and things changed. Taking exposures makes sensor electronics work something - it is not the same as having sensor sit there idly being cooled. Work that sensor does means increased power consumption of the sensor itself and as a result - increased heat. Same thing really that happens with car engine. If you put it in neutral and sit there on a parking lot - it's not going to heat up much and ventilator will not even start turning. Take that car up hill and it will soon start to heat properly and ventilator will kick in. Rev that engine very hard and go up very steep hill and there is even a chance that you will over heat it. Working sensor produces more heat and it is harder to cool than idle sensor.

We know that barlow extends eye relief of eyepieces and some time ago I asked why would that be and was given an answer that I did not really understand well. Yesterday I had a brief session from my balcony - just a bit of planetary / lunar with Mak102 - very rewarding session indeed although seeing was not perfect (I have not been observing in ages). I had my BCO 6mm on my desk as I was meaning to sell it for quite some time - and for some reason I decided to try it in Mak. I was surprised by how much eye relief there is in this eyepiece. I remember it being very tight on my F/6 8" dob. Then I realized that it must be the same effect as with barlow. I'm now using it with F/13 scope and it feels quite comfortable. In any case, I do have one plausible explanation that I'll try to sum up in a diagram, but I'm not 100% sure it is solely down to that. Ok, I'm pretty sure I made a mess with above diagram, but let's try to decipher what I draw. It is about exit pupil size. Large exit pupil will have tighter eye relief than smaller exit pupil. In red we have rays of larger exit pupil. If we mark intersection of all of them - we get eye relief position - that is dark red vertical line. If we now observe smaller exit pupil - marked in blue - we can see that full intersection of those lines actually moved further away from eye lens. Here in diagram I'm making one assumption that stands to reason, but I'm not completely sure it is true - edge of the field rays that are still at full illumination will emerge at extreme ends on eye lens - like it has been drawn and not closer to center. Or to put it in another way - edge of the field "pencil" will touch edge of eye lens. We know that exit pupil is determined by F/ratio of scope and focal length of eyepiece (when we divide the two). 6mm BCO in F/6 scope will give 1mm exit pupil while same eyepiece in F/13 scope will give 0.46mm exit pupil. This effect is more noticeable with narrower field eyepieces because it depends on max angle exit pencil will make. With simple designs that angle is often 50 degrees or less (or rather half of that - because angle is observed with respect to optical axis). Does this make sense and could it be explanation for why barlows make eye relief longer and why eye relief varies with telescope (f/ratio) used? Maybe this is the reason why people used Orthos in the first place - they had decent eye relief on F/15 scopes

I think it is a "stretch" thing more than anything else. SII signal is bound to be much weaker than Ha and in order to see it more clearly - you need to stretch that channel harder than Ha. Since star profiles are close to Gaussian shape - this happens: If you do mild stretch on same Gaussian profile, you will get small diameter saturated circle - here presented as short cross section. On strong stretch you get much wider saturated segment. This makes stars in the image look "fatter" and also shows more of them or rather they tend to look denser because of their saturation point and number of pixels they occupy. What you could do as exercise to show if this effect is real or not is to take your Ha data and your SII data still linear and copy half of Ha image (left half for example) onto SII image thus creating sort of "split screen" in linear data. Then proceed to stretch that image until you are either happy with Ha half - which will leave SII half almost dark due to SII being very faint, or you are happy with SII side - that will in all likelihood overexpose Ha side. In either case, stars should look more or less the same in both halves (there could be some small differences and those would be due to seeing or filter band pass or optical quality).

Nice images, but I wonder - have you resized them? If I take Jupiter one to be baseline, Mars is now about 14" in diameter - which would make it about 1/3 or a bit less of Jupiter at the moment (which is about 47" in diameter). Saturn on the other hand should be just a bit larger than Mars in angular diameter (for planet) and ring system should be slightly smaller than Jupiter as it is 43" wide. In your image Saturn is almost as large as Jupiter and ring system extends almost twice the Jupiter diameter.

Sorry to hear about your wife but good thing she is well now. I've been hearing about some people having problems a month or more after having it. Very nasty virus indeed! Thank you all once again for your kind wishes!

Thank you all.

Well, I was supposed to have moved to a new house by now together with obsy under dark(er) skies and all - but you know, this year has been tough on all I guess. Build of my new house not started yet due to all this covid stuff - just some ground preparation works done. I took another project to be able to fund all of that so there is also lack of time for astronomy ... On top of all of that, I'm now in my 10th day of covid infection - luckily both wife and I have rather mild cases, still plenty of rest is needed (and self imposed quarantine as well).

I've adopted strategy (not that it's doing me any good - I've not managed a single imaging session this year) - of using summer and winter darks. Former are at -15C and later are at -20C, because most of the time, those are figures I'm relatively certain I can reach with ASI1600 in my ambient conditions. DeltaT of 45C is quite decent value really. I've seen cameras with less than that regularly. Most Atik models are between 35C and 25C deltaT for example. QHY model with this sensor - QHY 163 has deltaT at 40C for example. In fact, now that I did a quick survey of cooling performance across different manufacturers and models - only CCDs from a few (rather expensive) manufacturers offer more than 45C deltaT cooling solutions.

On my very limited experience, over here with 8" F/6 aperture things are like this: About 1/3 to 1/4 sessions will be acceptable for high power observation. Out of those about 1/10 will have moments of great clarity and these happen for a brief period of time about a few seconds and happen about 2-3 times in half an hour. Other 9/10 times things will be a bit different - there will be no moments of exceptional clarity but good and average moments will alternate with frequency of about minute or so? These are of course very rough estimates based on memory and not on actual recordings.

It turns out that ASI1600 also has 2 stage Peltier cooler according to their website: I guess they just used smaller / less powerful Peltier elements or maybe sensor is running hotter than CCD - since CMOS sensors have A/D stage at every pixel unlike CCD that has A/D in separate electronics. More transistors - more heat? In any case, I think that cooling system on ASI1600 is far from useless. It is very good indeed for what it's supposed to do. As you've seen even in 300s exposure if you cool it at -15C rather than -20C - noise associated with dark current will still be lower than read noise of that camera and much much less than light pollution noise for most of us (except lucky few that image from Bortle 1/2 skies). Camera cooling nowadays is meant to provide stable temperature for subs so calibration can work without too much hassle. Thermal noise is by far smallest noise component and usually not an issue.

Indeed - STT8300 has deltaT of 55C so it will reach 55C below ambient temperature. It also costs x3 compared to ASI1600 - some of the cost I guess went for better cooling - maybe two stage Peltier instead of one stage. On the other hand Kaf8300 in STT8300 has 0.15e/px/s at 0C. If we take that doubling temperature is 6C - that means something like 0.015e/px/s or about double that of ASI1600 at 0.0062e/px/s (both at -20C for comparison). Double dark current means that you need to go with 6C lower temperature in order to have same dark current as ASI1600 - it needs better cooling in the first place!

No, you are not reading it correctly. It was 25C ambient temperature. You set your camera to -20C. There is total of 45C between those two points of temperature scale. There is 25C from 25C down to 0C and another 20C when going from 0C down to -20C. That is total of 45C difference between 25C and -20C. You set your temperature as absolute value (software has no idea what is your ambient temperature and does not care) - in your case -20C and camera cooling system will happily reach it if ambient is less than 25C - for example 23C. But if ambient is 25C or 26C - it will struggle to go all the way down to -20C as it can only lower temperature by 45C max - if ambient temperature is 26C it will reach -19C at best (because difference between the two is 45C). Using mismatched darks can cause problems with flat calibration and show slight amp glow if camera suffers from it. Sensor dark current doubling temperature is about 6C, so using 2C different darks will cause about x1.3 x1.26 stronger dark current. This may be very little of dark current change or it can be a lot - depends on sensor. ASI1600 has about 0.0062e/px/s dark current at -20C This means that it will have total of 1.86e of dark current for 300s exposure. At -18C that number will be 2.3436e or difference of 0.4836e - less than one electron. You won't see this if your sky background is higher than that, and I suspect that your sky background is significantly higher than that. If you are imaging from SQM 21.5 skies (Bortle 2/3 skies - so rather dark), in single 300s exposure with your setup, sky levels will be around 50e - so about x100 stronger than difference in respective dark currents (one at -20C and one at -18C) - you won't be able to see that on the image as difference in brightness. Where it will show however - it will show in numbers, it will show if you have amp glow and you remove background and stretch your data, or it will show if you have significant vignetting / dust - it can lead to slight problems with flat correction. Less dark current camera has in the first place and with shorter exposures - temperature difference between lights and darks will cause less issues for image.

No - it is not able to reach -45C, it is able to reach at max 45C below ambient temperature. Cooling specs are never given in absolute temperature but rather temperature difference - how much cooler than ambient it can achieve. On a cold winter night when ambient temperature is -5C it will cool down to -50C but on a warm summer night with ambient temperature of 26C it will only go as low as -19C. This is of course maximum but in reality it will be 1-2C less than that due to various factors like relative humidity, how dusty heat sink is and if ventilator is well lubricated (viscosity of lubricant changes with temperature as well so fan won't work at 100% in all conditions).

Well it does say that max delta T is 45C so you can't really expect it to stably hold -20C if outside temperature is 25C. If you want stable cooling - go with 40C delta T - or subtract 40C from outside temperature and go for that one.

Quite right! You might not be able to achieve focus unless you go for model with low profile focuser or similar. I only managed to achieve focus with mine because I had QHY camera at the time that was 1.25" form factor - so I was able to sink it in focuser together with 1.25" reducer! I have another crazy idea It will fit your budget and you will get insanely fast setup - maybe lacking a bit in resolution ... Initially you were ok with 72mm aperture, right? How about 67.5mm? Would that work for you? https://www.samyanglensglobal.com/en/product/product-view.php?seq=323 And pair it with 224 sensor. That lens is F/2 - so very fast. Resolution will be lacking at 5.73"/px but it will provide you with enough FOV to show larger objects nicely: Or fit Markarian's Chain into FOV Btw, that lens is very good for wide field AP and will have good resale value because of that.

178 offers only marginal advantage over 224 for EEVA - if you use mono model. I would personally look into F/5 newtonian, 0.5" focal reducer and 224 as EEVA choice. 130PDS + https://www.teleskop-express.de/shop/product_info.php/info/p676_TS-Optics-Optics-TSRED051-Focal-reducer-0-5x---1-25-inch-filter-thread.html (this one is out of stock, but there is 2" version as well - not sure if there will be any advantage in using 2" version) and 224 camera. That should give you about 2.4"/px sampling rate with 130mm of aperture - not bad. Only drawback would be a bit of coma in the corners perhaps? Here is what F/6 in this arrangement looks like (this was very big scope - 8" F/6 so FOV is smaller): There is almost no coma in corners. At F/5 there is probably going to be a bit. Here is same FOV that you tested with this setup: