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| I'm new to all of this and for the life to me I can't seem to understand apeture! I have a DSLR T6I with a 85 MM rokinon f 1.4 lens, I understand that 1.4 let's in more light so get more light/detail in the pictures, so can someone please help me out, why would I or anyone wanna shoot in a different apeture then? Let's say like F4 if that's letting in less light? Does a a apeture like F4 make an imagine look better then let's say f 1.4?.. for example I recently shot the NA nebula at 15 sec exspures an 300 photos .. at f 1.4 what would happen if I shot it at again let's say F 4? |
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You can find a basic eplanation on below link: https://www.exposureguide.com/focusing-basics/ |
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| Sorry for the mistake - "explanation" |
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| When stopping the lens down (using a smaller aperture) you are avoiding optical aberrations (non-round stars especially at the corners of the image ) and vignetting (reduction of brightness toward the periphery). Optical aberrations can't be corrected with software. Vignetting can be corrected by using flat frames (and I strongly advise to always use flat frames). Mind that flat correction won't recover the lost signal due to vignetting. |
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Hi, When you have the widest aperture you let in more light, but you pay a price in image quality. It is not so easy to build an optical system that is aberration free at very wide apertures, especially aberration free by astrophotography standards. This is why a good f/1.4 lens will always cost more than a similar f/4 lens and a good f/4 telescope will cost more than an f/6 telescope of the same type and focal length. As Die Launische Diva said, when you increase aperture will have more vignetting and less perfectly shaped stars. Also if I may add, there will be an overall decrease in sharpness, plus chromatic aberrations. All those things can be difficult or even impossible to correct with calibration/post processing. An important detail to consider IMO is that aberrations are always worse far from the center and that the situation is highly lens-specific. While it is true that aberration increases with aperture for the same lens, actual performance varies. Lenses such as the Samyang/Rokinon 135 f/2 work almost perfectly when wide open, other lenses work well when stopped down at least 5 stops and the maximum aperture is just a marketing claim. Putting this all together may find that at f/4 your image is almost perfectly sharp and aberration free all across the sensor but at f/1.4 your image has very strong vingetting and the stars starts looking less circular at 30% from the edges and in fact appear like little X's right at the edges. Of course the f/1.4 image will be much brighter than the f/4 image. Basically, you should balance the following:
With all these in mind, you should go for the largest possible aperture that you can. It could very well be f/1.4. I am just answering why people would use a smaller aperture in the context of astrophotograghy and low light photography in general. In the context of normal light photography, aperture is a different matter altogether because it controls depth of field. Cheers, D. |
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| Also, with f/1.4 your depth of field will be very shallow and focusing will be harder. I'm coming from a regular photography background so if this comment is off base for deep space photography, please let me know. My telescope is an f/4 Newtonian and attaining perfect focus can be tricky. |
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If you frame an earthly subject in your astrophotography image, e.g. a tree next to the milky way then yes DoF can be an issue and narrower aperture might make things easier. You can of course just walk to your lens' hyperfocal distance from the tree whatever that distance happens to be (the galaxy remains at infinity) but it might not be practical or desirable for all compositions. So yeah in those cases a slower lens might be preferable. For pure deep space photos DoF doesn't matter because everything in the field of view is equally far, at infinity. Cheers, Dimitris |
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| Great information, thanks. |
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If you frame an earthly subject in your astrophotography image, e.g. a tree next to the milky way then yes DoF can be an issue and narrower aperture might make things easier. You can of course just walk to your lens' hyperfocal distance from the tree whatever that distance happens to be (the galaxy remains at infinity) but it might not be practical or desirable for all compositions. So yeah in those cases a slower lens might be preferable. Actually, it does matter. The faster the imaging system is (lens, telescope, shallow pool of rotating mercury, you name it...) the smaller is the depth of the in-focus zone (let's say equal to 2 times the nominal resolution of the lens), by the square of the focal ratio. The fact that the subjects are at the same distance does still leave the problem of bringing them to focus which is harder with faster optical systems. |
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| The fastest the optical system, the more unforgiving in focusing errors it is. The critical focus zone (CFZ, unit: length) is proportional to the square of the focal ratio, CFZ ~ r², where r is the focal ratio. For example, r = 2 for an f/2 system . Let's compare an f/2 system with an f/10 system. An f/2 system will have (2/10)^2 = 4/100 of the CFZ of the f/10 system, thus requiring a much more precise focusing system and much more care during focusing. |
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" I understand that 1.4 let's in more light so get more light/detail in the pictures" This is actually a common misperception for people coming into astrophotography from regular photography. The amount of light from a subject (let's say M31) entering a lens depends only on the diameter of the lens, not its f number. Two 70mm diameter lenses will let in the same amount of light from M31 regardless of whether they have a focal length of 200mm or 400mm. The light entering the lens doesn't "know" what the focal length is! The lens with the focal length of 200mm will concentrate that light on smaller area of your sensor, and therefore require you to use shorter exposure times to avoid saturating the sensor. It will also give you a wider field of view. If your camera had no digital noise, and you compared two images with same total exposure constructed with these two lenses at the same image scale and looking at identical fields of view, they'd have identical signal to noise ratios, i.e., look identically good. In astrophotography, you'd select which lens you want to use based on what you want to shoot. Knowing what field you want to shoot, you'd choose a focal length such that it fits on your sensor. Given a chosen focal length, a lens with larger diameter will allow you to collect more light, hence build signal to noise ratio faster, i.e., get to the image quality you want faster. It will also be more expensive. Other than that, the points made above are quite valid. Large aperture lenses for a given focal length tend to be expensive, difficult to construct, and have optical aberrations that require you to stop them down to get acceptable performance. Aberrations are particularly easy to see in stars which are point sources of light and among the most unforgiving objects to shoot! |
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andrea tasselli:If you frame an earthly subject in your astrophotography image, e.g. a tree next to the milky way then yes DoF can be an issue and narrower aperture might make things easier. You can of course just walk to your lens' hyperfocal distance from the tree whatever that distance happens to be (the galaxy remains at infinity) but it might not be practical or desirable for all compositions. So yeah in those cases a slower lens might be preferable. True. I was reading about it a couple of days ago because I was once again tempted to go for a fast Newt or a RASA. Even if you solve the focusing problem with an automated focuser (which is what most people do), you will probably still have to refocus more frequently because of thermal changes. So you win in terms of total exposure time required because of the faster system but you also lose available time to shoot because you spend it on focusing. |
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| Well, I'm not sure I'd go as far as to say that what you gain in terms of reduced exposure time is lost due to additional focus time. Quite the contrary! I've got a RASA 11, and while I do spend additional time focusing (early in the night), that's only a minute or two with a motorized focuser and V curve software. I'm taking 30 second exposures, and an hour's time on a target is sufficient for a reasonably good image (see my gallery to see whether you buy my argument). Compare to a similar F10 11" Edge HD's ~19x exposure time and I think we'd all agree that f/2.2 is to today's AP what goto visual telescopes were to star-hoppers! ;-) IMO, it's just really hard to beat the efficiency & quality of new fast setups like RASA (of course, at some cost$$ ). |
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I have always tried to put as much aperture as possible on the selected image scale. This makes for very fast systems. More than a decade ago I designed, manufactured and fielded an 8" F/2.6 astrograph that was diffraction limited over a full frame sensor and passively athermal. Thus the portable system required a focus check at the begining of the night rarely needing adjustment between imaging sessions and after 100's of miles of transport.. The point is such systems exist and have for quite sometime. Methods of design and construction are widely published. Unlike a cameras lens where focal length is fixed and aperture is adjusted, or a zoom where aperture is fixed and focal length is adjusted, I decide on an image scale and design the largest aperture possible instrument, this is my interest, imaging is a side line. There are these days a lot of very fine fast systems available, RASA is certainly one, the RH systems, and Peter Ceravolo systems are great. Decide the FOV you want and purchase or build the largest apreture you can aford. Make sure your image scale is supported by a lot of nights where seeing is stable enough for your image scale or you'll become frustrated quickly and lament the wasted money. Regards, Dave |
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| @Doug Summers@hbastro You are both right, there I guess is an implicit "in suburban conditions with moderate seeing, weighing at most 4kg and costing around $1000 max" in everything I say  I didn't know the RASA was athermal, I must say. Definitely on my whish list for when I move somewhere darker. Cheers, Dimitris |
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I am not sure the RASA or other the other instruments mentioned are athermal. Certainly a temp compensated focuser will help, there are many systems available. There is some range of temperatures over which critical focus is maintained. Below is a graph of temperatures for a night at my site. The yellow curve shows the outside temperature for an observing session. With data like this and the addition of focus offset data for your instrument you could design a passive compensator using those data, and differential CTE of materials. Then place the compensating device between telescope and imager. This is what I have done for my instruments.  |
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Still a bit off topic but relevant I think. This is kind of typical analysis when designing an instrument, and followed with actual measurements as a final tweek... Here are results of a comparative design study done by Joe for his Melior design. These results are for thermal soak conditions (no thermal gradients) and all spaces are assumed to be filled with aluminum. Joe ran the analysis for an apo triplet and a Petzval (both are his best guess designs) to make comparisons against. Here re the results graphically:There is thermal drift in the Melior design, but it appears to be less than half of the triplet and Petzval. No refocus required for an imaging session! Note from the graph that the -5C temp swing is a hand full of microns focus shift.  |
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I definitely didn't mean to suggest the RASA is athermal. Here is a graph of 300 focus runs at night showing the correlation of temperature to focuser position (for a RASA 11 + ASI EAF). The residuals of the trend line have a correlation to elevation (graph known but not shown). Together, a simple focus compensation (temperature and el) strategy can be implemented in software. Cheers, Doug |
