inFINNity Deck

I live in the Netherlands near the west coast. This year indeed has been less generous with clear nights: A clear night is when I can leave the observatory open for the whole night without the risk of rain or loosing a substantial amount of subs due to clouds (I live at a Bortle 5-6 location and the shutters are not automated). Being away from home I missed 4 clear nights at the beginning of this year, so there were 27 clear nights in total (summer months not included, I do not image from about 20 May until 20 July). The last deep-sky imaging night was on 14 September, after that I only had a go at asteroid 1998 HH49 for a few minutes on 17 October and a few evenings with the planets on 22 October and 25 November. Even my daily solar observations have almost ground to a halt, currently only recording sunspots every three days on average (this month only 9 observations). Nicolàs

My GM3000HPS is completely silent when tracking, not a sound to be heard. I need to check the hand-controller or virtual keypad to see if it is stopped or tracking. Slewing does produce some sound, but here too one can choose between various slewing speeds which should make slewing pretty silent. Nicolàs

Funny, I happened to ask this same question last Friday to an astronomer who gave a talk about exoplanets. He just smiled and said that this still was a point of discussion, but that with "clearing" it was meant that all objects not being in Lagrange-points had to be cleared. Even our rock already has two known Trojans: 2010 TK7 (300m diameter) and (614689) 2020 XL5 (1.2km diameter). Nicolàs

Hi Kostas, that is a great image! Can you tell a bit more about the way you captured it? Was it a single 20 minute sub, or did you do lucky imaging (if so, what kind of settings)? Which camera did you use, the ASI462MM? Did you use the Dobson at native focal ratio (I believe that is f/6)? Nicolàs

The plate-solving software ASTAP does the same using your deep-sky images. I have written a tool FITSalize that uses ASTAP to produce an ASCII table that can be imported into Excel (it allows for deformation measurements as well). Here is an example of a measurement I did last year: Care should be taken that the H17 or H18 database with Gaia blue magnitudes (BP) is used as reference. Nicolàs

One way to arrive at a factor of 5, is to calculate the optimal focal ratio using the spatial cut-off frequency for the shortest observable wavelength. If we consider that we record in our images wavelengths between 400nm (blue) and 700nm (red) and assume that the shortest wavelength would in theory be able to show the finest details, then the optimal focal ratio would become 2 x [pixel size] / 0.400 = 5 x [pixel size]. @vlaiv for instance uses 400nm in his calculations (as he wants to be properly sampling over the whole visible spectrum) and so arrives at 5 x [pixel size]. The issue I see with this approach is that blue light is most scattered by our atmosphere, so it is less likely that we will actually achieve the highest resolution at that wavelength, even in lucky-imaging. At the other end of the spectrum we have red light of 700nm, which would result in an optimal focal ratio of 2 x [pixel size] / 0.700 = 2.9 x [pixel size], but that of course would be the lower limit (provided we image under an ideal atmosphere, using an ideal scope). I always used green light of 540nm in my calculations (as can be seen in my papers), which results in the 3.7 x [pixel size] as a maximum useful focal ratio for planetary imaging mentioned before (one can now easily imagine that this is not entirely correct for a reddish planet like Mars). The reasons for using that wavelength in my calculations were that the Sun's continuum reaches its maximum around it, that the Baader Continuum filter has a bandpass around it (for obvious reasons), that the green filter is more or less centred around it, and that ZWO colour cameras have their maximum quantum efficiency around it (actually slightly before it). For the article on the effect of seeing on the visibility of sunspots we simulated various telescopes under various seeing. From those simulations I later deduced the relationship between seeing and oversampling factor as roughly 1.6 x [seeing]^0.74. If we create a set-up with a C11 or an 180mm aperture Maksutov that samples optimal at zero arc-seconds seeing (regardless how we define it) and use it at a best seeing of 0.6 arc-seconds, the oversampling-factor will only be about 1.1 (so 10% oversampling), while at 0.7 arc-seconds this is about 1.2, so the ideal focal ratio will only have to be adjusted by a few tens of percent. But if the best seeing during an imaging session is 2 arc-seconds, the image will be oversampled by almost a factor of two. Weather permitting I observe, sketch and image the Sun on a daily basis for two scientific programs and also measure the seeing during those sessions using a Solar Scintillation Seeing Monitor: I do not measure seeing at nighttime, but can imagine that it can be a bit better than at daytime due to the absence of the Sun. Still I do not expect to image diffraction limited very often (if ever) with the C11 here in the Netherlands (due to its aperture the C11 is seeing limited above a seeing of about 0.4-0.5 arc-seconds). I therefore firmly believe that it is for above reasons that I have yet to find a planetary image that was not oversampled (or at least not more than by 10% or 20%). But again: please post an image here that is not oversampled as I would be very interested to see it. Nicolàs

Hi Stuart, I would love to see those images, would be great to do some FFT-maths on them. You say that "Theory is all very well but in practise more detail is gained by using the longer fl". This is not true, see this article I published with a befriended nuclear physicist. Resolving power only depends in wavelength and aperture (see formula 2 in that article). You can have a 5000mm focal length scope, but if it is f/50 (aperture 100mm) you will never get the detail of a 200mm aperture scope, even if it is only f/10. But we are wandering off-track. Let us try answering as best we can Craig's questions. Nicolàs

Hi Craig, I never tried the ASI224MC and in general I am not too fond about OSC-cameras, but that is entirely personal (I do have a ASI290MC, but never got a decent image out of it while others have no issues with that). The ASI224 has 3.75 micron pixels, so basically the same as the ASI533MC Pro. Thus optimal imaging is done at f/11, so 2x barlow should suffice (I would recommend a telecentric one like the TeleVue PowerMate). Regarding the ADC: these work best at f/10 and slower telescopes, so 2x barlow should be fine (see here, although f/15 is mentioned a lower limit as well, and so is f/20). If we look at the Rolls-Royce of the ADCs, the ones by Gutekunst (you can buy a few dozen ZWOs for one Gutekunst), we learn that f/10 or f/12 and higher is recommended: "In order to get best performance from the ADC, we should use slow f/ratios, preferably above f/10 or f/12, so unless you use such a scope, a barlow should be used if we want optimum performance". Nicolàs

Hi Stuart and Bosun, yes, the factor 5 is well known to me, but so far no one managed to explain why it should be used (other than because others do so). The only reason I can think of, is that the planet shows up larger in the image, but you will not gain any additional detail when going above a factor 3. Only when imaging with a perfect scope (i.e. Strehl ratio 1), perfectly collimated (is it ever?) and from outside our atmosphere, you may find it useful to go to a factor 3.7. Above that physics does not allow to gain more detail, but you will have the downside of longer exposure times. I have discussed this before in this thread and shown what happens if we down-size and re-size the image to original size (spoiler: no detail is lost): If you take the time to read my articles and especially the second one (linked from the first one), you will understand that indeed going much above 3 x pixel-size is rather pointless. If you want a larger planet on screen, simply up-scale it by 200%. There will be no difference in detail, but less noise due to more data and perhaps even slightly more detail because of the shorter exposure times. Regarding the latter: suppose I am imaging using a ZWO ASI174MM with 5.9 micron pixels and do that with a C11 (my currently preferred planetary scope) at native focal length of f/10, I will achieve a theoretical resolution 0.43"/px. Today Jupiter is 49.4" is in diameter, so 115px across and thus (assuming a perfect sphere), a total surface of (115/2)^2 x pi() = 10387 pixels. If I go to f/20 I will get a total surface of 115^2 x pi() of 41548 pixels. Now changing the focal ratio does not change the amount of light entering the telescope from Jupiter. In other words: each pixel only gets a quarter of the photons when increasing the focal ratio by a factor of two (i.e. a quadratic relationship). So when going from a factor of 3 to a factor of 5, we have to raise the exposure time by a factor 5/3^2 = 2.8 to get the same amount of photons per pixel, while we do not gain any additional details. I am imaging at a factor 3.4, simply because my scope is f/10 and I use a 2x PowerMate and above camera. So my focal ratio is f/20 and camera has 5.9 micron pixels, thus factor 20/5.9 = 3.4. The optimum would have been f/17.7, but that is not easily achieved, so I stick with it, even though it comes at about 28% longer exposure times. If you like below images, there is no reason to go above a factor 3 (two of them are resized 200% , all where imaged at a 3.4 factor). Nicolàs

Hi Craig, first about your set-up: Your camera has 3.76micron pixels. Theoretically you will achieve optimal conditions at a focal-ratio 3x that figure (see my white-papers on that), so at 3.76 x 3 = f/11, so the 3x Barlow may be a bit too much, will cause oversampling and longer than necessary exposure times. It is beter to go with 2x or 2.5x Barlow and resize the planet 200% afterwards. I would add an ADC (Atmospheric Dispersion Corrector) to your equipment list (if not already present). Yes, that should work. I have no knowledge on AsiAir, so I will skip that... 😉 I always use a Bahtinov mask on a nearby star or, in case of Jupiter, on its moons. Nicolàs https://www.dehilster.info/astronomy/jupiter.php https://www.dehilster.info/astronomy/mars.php

This week I received files from another two IMX571 cameras as well as from the camera under discussion here, this time taken with EzCap (QHYCCD software). The other two cameras are another QHY268 (indicated with "MG" in below image) and a Touptek. The other QHY268 was used in High Gain 2CMS Read Mode and normal High Gain Read Mode. This time I compared the files using CCD Ciel, each section shows a section of the image at 300% with under it the statistics and below that the histogram: Although switching from Voyager to EzCap lowered the standard deviation, it still is significantly higher than for images from the other two cameras. Reason for that must be the three clear peaks in the histograms of this camera. Given all the tests I fear that the issue is in this particular camera, not in the model or IMX571 sensor. Nicolàs

Here are the histograms of William's subs: Nicolàs

Dear William, thanks a lot for these five images, what a difference these make! A quick inspection learned that there are not even a handful pixels that have ADU above 1000 (one thousand). I would be happy all day with that! 🙂 As I wrote in my previous post there indeed seems to be an issue with Voyager, at least in Low Quality mode. So the owner will try to produce subs in High Quality mode, but I also asked him to create a few subs with the QHYCCD software, that (I hope) should at least produce raw data. I will forward your post to him and ask him to replicate. Nicolàs

Yesterday I received three bias-frames and oddly enough there was a big difference between them (histograms by APP, pasted in the screen-dump of AvisFV): It appears that Voyager, the software used to capture the images, has a low quality mode at which the FITS can be stored. The owner switched to another mode which made the FITS looking much better. Later this week I hope to receive a few dark frames using a different setting in Voyager. Nicolàs

Thanks guys for your input, much appreciated! I just came back home and found a few new files captured in different ways with the camera I was offered. Will analyse them tomorrow and post the results here. What I can say, is that those high pixels are not hot pixels as they appear randomly in the images, so it seems to be more a kind of shot noise (random telegraph noise), but with strange high values. Maybe it is also good to explain why I am concerned: I plan to use the camera for spectroscopy with a 2400lpm grating (R around 17000). At this moment I am using a LHires III with a QHY163 mono, which is only 12bit. At this high resolving power the data of 600s capture of a mag 5.4 star only is 50% higher in ADU than the background (approximately 3200 ADU signal vs 2000 ADU background). Using these relative short exposure times we (an international group of 12 observers) are trying to find short period variability in Be-stars. So stacking is no option, while the low photon levels makes it difficult (if not pointless) to do multiple captures and stack them. The QHY163 does not show the same kind of noise in the images I get from this QHY268, nor is that the case for the two ZWO ASI1600MM Pro Cool cameras I use (same sensor as the QHY163), nor with the ZWO ASI174MM, ASI290MM and ASI290MC I use. Here are three histograms for comparison (all of darks, histograms by APP): Nicolàs

Hi Steve, thanks for that info. I have passed it on to the owner and asked him to take an image in that way. Nicolàs

Hi Steve, thanks for your reply and images. The header is indeed pretty poor. The -15°C image was taken at Gain 26 and Offset 16 in Readmode extended FullWell 2CMS. The other images in the mode Photographic DSO at Gain 56 and Offset 32. That is correct. That indeed looks much better... Nicolàs

Hi Gary, thanks for your reply. Could you do me a favour and create a dark using this same short exposure time, so that we can see that the exposure time causes it? We made the dark at daytime, so decided on a short exposure time to avoid light issues, but perhaps it was shorter than what the camera can handle (although I do take darks of milliseconds on my ZWO planetary cameras without issue to remove hot pixels). Nicolàs

Dear Stargazers, I was offered an occasion QHY268M that has barely been used. Before acquiring it I asked for some darks, which I took together with the owner, but those raise some questions. If I zoom in I see that some 1% of the pixels have an ADU of about 24800 on a background level that is only 1% of that (i.e. around 250-300), which seems a bit high to me: I have placed three FITS-files on my server: https://www.dehilster.info/astronomy/docs/TestShot__0.50s_-15.0C.fit https://www.dehilster.info/astronomy/docs/TestShot_-0.05s_-5.0C_avg16kADU.fit https://www.dehilster.info/astronomy/docs/TestShot_-0.05s_-5.0C_avg33kADU.fit The first is a dark at -15°C at 0.5s exposure time, the second and third are at -5°C and were allowed to collect some photons at 0.05 seconds each, resulting in an average ADU of about 16k and 33k. They were all taken without telescope attached. I would like to know whether this is typical for the IMX571 or QHY268M or is it perhaps due to the short exposure times? I would appreciate if there any IMX571 users on this forum that are willing to share some darks and/or lights with me. thanks in advance for your input! Nicolàs

I think it does. All movement may lead to vibration if you are (un)lucky enough to have something producing the pier's own frequency. Possible causes are walking (not in your case), traffic, tremors, meridian flip, image centring, etc. Nicolàs

There is less risk of vibrations if the pier widens towards the base. Here is my tubular pier: The tube is 250mm in diameter, 3.4m tall, weighs about 350kg and stands on a 0.80m diameter base-plate. It is held at this base by 16 M12 anchors, not bolted down, but clamped in between nuts below and above the base-plate to minimise stress on the chemical anchors. Nicolàs

A few weeks ago I replaced the fan on one of my ASI1600MMs, a very simple job. I got mine from Teleskop-Express (48 euro including shipment to the Netherlands). To replace it, simply unscrew the three outer screws on the rear of the camera, after which the back plate comes off, but still is connected by its lead. Unplug the lead and take care that the cylindrical housing does not fall off (I replaced the fan with the camera in situ). Then remove the four screws that hold that fan and mount the new fan with new rubbers, grating, and screws . Finally re-assemble the camera and test it. The whole operation takes only a few minutes. Nicolàs

Yes, you could use those to mount the observatory and fixate them to the bedrock using chemical anchors. If you use the M10 male to female versions, you could use M10 anchors, cut them to size and then mount the vibration dampers. I use these dampers for the motor that drives my dome. The motor now can turn several degrees on its base without affecting too much the observatory, while not transmitting any sound from the motor to it (see fig.2 in https://www.dehilster.info/astronomy/dome_automation.php). Nicolàs

I have seen quite decent images using mobile phones, their cameras are getting better by the year. looking forward to those results as well! Nicolàs

Hi John, you are right that smaller size is better, simply because the frame-rate will be (much) higher, but otherwise it is not much of an issue to use a larger size sensor (well, that is if we do not mind consuming a lot of disk-space 😉). Indeed quite a few imager do use 2x oversampling for the same reasons you mentioned. Looking forward to your results closer to the optimal focal ratio! clear skies! Nicolàs