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My current imaging workhorse is an NP101. Decent focal length and speed with 540mm at f/5.4. I live in NE Ohio, though, and it's VERY cloudy here in the colder months. I see a bunch of people saying things like, "yeah, I don't really call it good until I've got 20 hours on the target", and honestly just want to scream out of frustration. Between my schedule and my atrocious weather, I'm doing good if I can string together 5 hours. Simple solution -- faster optics. RASA 8 seems like the knee-jerk solution for my HEQ5 mount, but it's only 400mm focal length. I'd prefer 500mm+, but the RASA 11 is far too much for my HEQ5 mount. If Celestron made an 8" f/3 RASA version (600mm FL, but without the cost and weight of the 11"), I'd gladly buy it. One idea I've floated around is saving up for a Tak Epsilon. As fast as the 180E is, the 160 is cheaper, lighter and has a bit more focal length. F/3.3 is still WAY faster than f/5.4; just 3 hours at f/3.3 is equivalent to 8 hours at f/5.4. If I got 5 hours at f/3.3, I'd probably call it good on anything I was shooting, at least if it's under dark skies or using a dual-band OSC filter. There's also other more inexpensive Newts that might work on the HEQ5. And as a longtime visual observer before taking up imaging, Newtonian collimation doesn't really scare me. But there's one burning question I have as someone who's never imaged with anything other than refractors -- what do y'all do with the diffraction spikes when you're processing in PixInsight? How does Starnet++ handle these? Do they end up on the stars-only or starless image? Clear Skies, Phil |
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| Diffraction spikes get extracted with the star layer. StarX does this very well. Sometimes extremely bright stars can be a pain, but that's just how it goes. |
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Hi, I'm Sorry to Say that, having a "fast" focal ratio, does not means You collect much photons on your CCD/CMOS sensor... This is a common misunderestanding on how linear acquisition devices works. Low f-number just means wider field of view, in fact the term "fast" was true only for film emulsions. The Total energy emitted per area of the Sky Is fixed and depends on flux photometry of that considered area. Of You have the same area of the sky for each pixel, doesn't matter of You have F8 or f4, the photo flux is fixed and You Will get faster accumulation of signal buy also of skynoise. The ratio between the two Will remain the same. The only way to get a greater signal Is ti have a bigger diameter... |
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Phil Creed: If you use SN++ V.2 it would typically leave a vary faint trace of the star's diffraction pattern in the linear phase and a somehwat stronger one in the non-linear phase in proportion of the amount of stretching you have applied, if you process the stars together with the subject. If you split star from the subject before stretching, then see my previous point about SN++ and linear images. If you use SXT then everything about the stars will be removed (and possibly something else as well). |
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I have had no issues with SXT and my E160 images. Diffraction spikes are the reason I bought the Epsilon. Since the Epsilon has about the same focal length as your Televue, you will see improved imaging times moving to an Epsilon. Dan |
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Hello Phil, Boy I hear you about the winter season weather, I’m in Michigan and have the same problem! I’m lucky to get even a couple hours in a night and haven’t even had that now for 2 months. I for the most part shoot with Newts myself and don’t really have any issue as most above have mentioned. I use StarX when I do the pulling of the stars. Maybe an exception would be Alnitak around the HH but I think thats an extreme case. Dale |
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Giovanni Paglioli: Sorry Giovanni, I need to correct some of your statements. You wrote that a low f-number means a wider field of view - that is wrong. The field of view is linked to the focal length (assuming same FOV of the used camera) adn not to the f-number. The f-number is a value indicating the "speed" of the optics, more clearly stated, the aperture of the optical system. Usually that is closely related to the opening diameter of the front lens (not always, but most often) - and that relates to the amount of photons, you can collect in a certain time. The wider open, the more photons can enter the optics train. What happens later, how many of them are used to create a signal is linked to the quantum efficiency of the receiving device. What you stated very right is that with a wider open (=lower f number) optics, the amount of non-target related photons increase as well - and yes, the relation/ratio between wanted signal and unwanted signal remains constant and is not depending on the f-number. If you like to dig deeper into this matter, please look at the very detailed explanations at https://telescope-optics.net rgds Georg |
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To add to the above, while it is technically correct that a "faster" focal ratio on the same size optics doesn't mean it collects more light. Practically speaking a "faster" optic is more useful compared to much more costly larger camera which is required to take full advantage of the "slower" focal ratio optic. So for nearly everyone reading this, you're better off with modest sized camera and a "faster" scope. I digress. Modern star removal techniques don't have any issue with diffraction spikes. The algorithms are trained on images with and without spikes. |
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To add to the above, while it is technically correct that a "faster" focal ratio on the same size optics doesn't mean it collects more light. Practically speaking a "faster" optic is more useful compared to much more costly larger camera which is required to take full advantage of the "slower" focal ratio optic. This whole thing is blowing my mind right now. I work in the photography business but I've never thought about that the light gathering is the same if the aperture stays the same and only the focal lenght changes. You could basically say that in theory, if I were to take a photo with hyperstar and a mosaic with the native focal lenght of the telescope both with the same exposure time you'd get the same bright image? of course one would be more detailed. This really changes the way I'm thinking about aperture and focal lenght, it's really mind blowing to me right now haha. |
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Jens: Not exactly. The amount of signal captured per panel will be several times lower for the same amount of exposure time. And in fact the amount of imaging time is several times higher still because you have to image every panel individually. The image will effectively be higher detailed but have a lot more noise. This is why in practicality you're better off with a "faster" optic. The only true way to take advantage of the bigger FOV is to have a larger camera sensor with either larger pixels or some type of binning/down sampling. Not to mention the issue of seeing in most geographic areas limiting the amount of detail you can capture at higher focal lengths anyway. |
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The OP asked about processing diffraction spikes and I think that question was mostly answered; however, the evolution of the thread into a discussion of signal strength is interesting and Giovanni got it mostly right. First, it is important to give up on the idea of how many photon the telescope collects. Instead, it is better to understand that the most important thing is the irradiance (either in W/m^2 or in N photos/m^2 collected during the exposure) produced in the image plane by the optical system. The image plane irradiance describes how optical power is distributed over the image area and is determined by basic radiometry. For an extended source, which is any object that creates an image larger than the Air Disk, the so called "camera equation" gives the irradiance (W/m^2) in the focal plane as shown below. In this case, the focal ratio is the only first-order optical parameter that goes into determining the output. Irradiance is not the same as signal strength. To determine the signal strength at the sensor, you then have to integrate ("add-up") the irradiance over the pixel as shown below. (R is the responsivity of the sensor.)  Remember that for an extended object, the focal ratio is all that determines image plane irradiance and signal strength is determined by how you sample that irradiance pattern. For equal diameter telescopes, sampling at the same rate in object space (i.e. in the sky) gives the same signal regardless of the focal length. That means that for a given diameter, the focal ratio doesn't matter! In that case, the only way to increase the signal, is to make the diameter bigger (as Giovanni said.) When you do that, you are simply decreasing the focal ratio, while making the field of view larger. For equal sampling rates in image space (i.e. in the focal plane...using the same sensor), a lower focal ratio (a faster system) will always give a higher signal than a higher focal ratio (i.e a slower system). Here's a quick example for three different focal ratios with a fixed diameter.  Just remember that the sensor size also determines the MTF of the system which is related to the ultimate "sharpness" of the image it can produce. Big sensors produce more signal (and less noise) and small sensors produce "sharper" images. The challenge is finding the right balance, but I'll stop there. John |
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Georg N. Nyman:Giovanni Paglioli: Hi! I think I was been too fast in my reply... Georg N. Nyman: I simply mean that, having the same size optics with a lower f-number means that You have a bigger field of view. Professional astronomers defines optics in relationship to the corrected field of view they can achieve, so the optic is not considered fast but wide field. In the useful link You provided there is a statement that says: "Light-gathering power of a telescope mainly depends on its aperture diameter. However, it is the system light transmission that determines how much of the light that entered the telescope actually arrives at the final focus. Transmission losses occur due to reflection, scattering and absorption of light, as well as due to obstructions and diaphragms in the light path." The f-ratio does not determine the light gathering capabilities of a telescope. Georg N. Nyman: The F number is just the relationship of the diameter of the optic respect to the focal length this means You have a narrow cone in high f-numbers and a "wider" angle ina lower f-number. In the end the discussion is more to be seen on the real and practical side I think. With digital acquisition device like CCD's and CMOS we are talking very different from emulsion film. With old emulsions the "reciprocity" law that states the invariability of the density achieved on the film having aperture/time/ISO just falls of becouse of the Schwarzschild effect also known as reciprocity failure. This simply means that if the exposure was too short, the chemical reactions doesn't happens and if the time was too long the density could not be augumented becouse of the chemical reaction nature. It is still correct to say that we still take "pictures" in digital world? We astrophotographers in particular could start to think in a more adequate manner. We are, de facto, taking measurements! What we call "noise" refferring to the "graininess" effect that we see just like old photography is, in reality, uncertanty of the measurement. We choose to visualize the measure we've done but, as digital sequence of numeric datas, we could choose to play them as "music" just like CD's or Mp3... We could not underestand them as they will sound really unpleasand but they still are exactly our collected datas. In the end, we are just measuring the photometry of an angle of sky in which we have choose to put a matrix that determines the minimum angle measured (the single pixel area in the array of pixel in the sensor) that is the sampling of the sistem. In the sky, even in the void outside the atmosphere, there is only a certain quantity of energy per angle considered thus a photon flux to be captured! There is no magic to alter that flux! The photon flux is not linear but poisson distributed so, no matter how fast You fill the pit of the pixel and how perfect is the sensor, in a given time You can only collect that flux of photons at the best... Anyway we can, thanks to their linearity, split the exposure and make statistics, yes becouse that's what we made to "enhance" the signal. We need SNR and we can choose how much uncertanty to tolerate or, as the professionals says "the confidence degree of datas". 5:1 means that, for the angle considered the signal is at least 5 times the uncertanty of the measure. Collecting light "faster" in Your sensor does not alter this SNR relationship since our worst enemy is by far the skynoise factor. Well, with an f4 system you fill the pixel full well in half of the time of an f5.6 but, what you put inside that pixel, has a lower SNR than of a longer exposure becouse of the fixed photon flux from the source respect of the random flux of the skynoise. Hope this is a bit clearer now... I underestand that this is quite counterintuitive but that's how it really works, we can only achieve a better SNR in the same exposure time of the same considered angle having a bigger light gathering ability that means bigger diameter. |
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| Anyway I have a Takahashi Epsilon 210mm that produces incredible spikes and, using star removing tool like starXterminator or Starnet++ just sometimes lives strange haloes or artifact. In general they perform really well even on big spikes and they are perfectly usable in general... |
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Giovanni Paglioli:Georg N. Nyman:Giovanni Paglioli: you stated ....I simply mean that, having the same size optics with a lower f-number means that You have a bigger field of view... Just again - no, that is simply wrong. The FOV (field of view) is well defined but if you think it is different, well think so. you stated......The f-ratio does not determine the light gathering capabilities of a telescope.... Just again, this also wrong. The light transmission capability of a system is depending on the optical layout, the number of surfaces, the type glasses used, the kind of coatings, the absorption coefficient of the used glass, the reflectivity of mirrors (if there are any) etc. But, and this is crucial - the f-number is the primary factor to determine how much light can enter the optics train at the beginning of the whole light path. I give up. It does not make much sense to keep explaining something which the other end understands totally different. |
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Georg N. Nyman: Georg, Be careful. In order to decrease the focal ratio for any given diameter, you have to decrease the focal length, which (for a given sensor) also produces a wider field of view. That's what Giovanni is saying and he is correct. If you look at the camera equation that I posted, it addresses how radiance (in W/st-m^s) is transmitted through an optical system. The focal ratio (along with the optical transmission factor) is what determines the irradiance (in W/m^2) in the image plane. The focal ratio is NOT "the primary factor to determine how much light can enter the optics train at the beginning of the whole light path." The diameter of the entrance pupil of the telescope determines how much light enters the optical system. John |
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John Hayes:Georg N. Nyman: John, yes that is correct, but the difference is the wording - your wording is correct but Giovanni did not say it like you did. I know that the diameter of the entrance pupil is the determining factor, but I thought, he is not aware of what the entrance pupil (and the exit pupil) of an optical system describes or exactly means, so I just simplified it a lot. Georg |
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Georg N. Nyman: I'm really sorry You take my words somewhat offensive, I really didn't mean that. I was referring to the link You gave me and what is stated in that link. I agree on what @John Hayes says that is, in fact, the optical truth. I simply tryied to enlarge the discussion on the whole system of "taking an image" not just stopping at the optical side of things. In the end You have a system that includes various elements, from the optics to the electronics and, every component introduces an "error" for the final measurement which, in digital world is defined as noise. You can do whathever You want but, as an end result, You will produce a measure of a photon flux for each angle considsered by the sampling factor of the system. Our struggle is to get the best possible SNR and that ratio is not dependant of the f-number due to the phisycs. I mean that, despite the system used for acquisition there is a physical limitation due to the energy distribution to be measured and we simply can't change this reality. Hope I was been much clear in exposing my thoughts and experience as I was been involved professional astronomy world for more than 40 years, helping in the making of many scientific projects for NASA, JPL, MIT, Mount Palomar ecc... This doesn't mean I'm god nor that I'm right, I just wanted to share my experience and knowledge and I'm still happy to learn from others. Thanks and I apologize if I was been offensive in any way. Giovanni Paglioli |
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This thread sure took a left turn. Don't worry about diffraction spikes. Some think they're cool. Of course it sometimes takes months to complete an image. 20-30 hours is not uncommon at all. In fact it's pretty normal, depending upon the target. Less for bright stuff. More for the really faint, especially when shooting with a OSC. Suggest getting comfortable with this notion of patience versus instant gratification. That's the hobby. Cheers! |
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Giovanni Paglioli:Georg N. Nyman: Giovanni, absolutely no need to apologize for anything - rather than you, I need to apologize for my hash critics which was primarily based upon misunderstanding of what you wrote and meant. I just read your contribution word by word and did not agree - without thinking about what you wanted to express. Sorry for that. I am also not the god of optics and I am also learning every single day - from others and due to my mistakes, I made. Thanks for your understanding and kind regards Georg |
