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Indeed - but I can't really understand how they are related either. It's not just simple size of things, I think that how contrast varies with space that is important. Here is a crop from above image - we see some variations in contrast in the middle of the image - but we start to loose it to the left and to the right. So it's related to size - but too much magnification hurts as well according to this - or at least if we equate feature size with wavelength in image. I'm not sure that is how it works - I think it has to do with size of transition rather than size of feature. How big is transition between light and dark area versus how big is actual object. This means that visibility of object will depend on how "soft" its edges are. To confirm this - there is another image on the same page I found previous one: Here instead of soft change - we have sudden change - so "change zone" does not depend on the size of feature - it is steep for thin as well as thick lines. Yellow line marks what should be seen - but we can clearly see contrast change past yellow line in this case.

You seem to be right in your estimation. I was intrigued by this question and did some more research - and I came to two very different conclusions. 1. Based on tube: Looking at this image and other images online - focuser seems to sit roughly at 1/3 from the beginning of the tube. Tube is 530mm long, so that would make distance from focuser to back side roughly 353mm. Given that primary is at least 15-20mm thick and that there is cell - it must be inside tube at least 40-50mm That still leaves primary to secondary distance of about 300mm - or about 1/2 of focal length. 2. Based on published data. TS quotes backfocus of about 98mm. Focuser is listed at about 75mm height, and tube thickness at 178. This would make secondary to focal plane distance of about 98 + 75 + 178/2 = 262mm Still quite a bit of room for vignetting, although 70mm is enough not to stop down scope if there is 262mm between secondary and focus position.

Interesting read - but here is important sentence: Since we don't understand why there is threshold contrast and why is it reduced in larger apertures - we did not answer anything really. And the answer might be this:

That is quite normal for single exposures. You need to stack multiple exposures and then process your images (histogram stretch) to reveal what is truly captured.

It's not going to be that much larger image, at least I think. Take for example two popular scopes F/6 8" and F/6 80mm refractor. Aside mirror reflectivity and central obstruction - let's imagine both are "refractor like". Difference in object size for same exit pupil will be what - x2.5? I'm sure that we can find two galaxies (now that galaxy season is upon us) that have same surface brightness - but one is x2.5 times larger than other. If large galaxy can be seen in 8" but can't be seen in 80mm - that would suggest that smaller galaxy can't be seen in 8". I find that hard to believe. Also - it is very easy to move galaxy so that about half of it (or rather 60% of it) is outside of the fov - that leaves only 1/2.5 of its size visible in 8" - according to size thing - it should disappear.

I find this hard to believe - this would mean that scope is effectively stopped down to below 140mm as secondary minor axis is only 70mm. Simple similarity of triangles shows that for single point of 100% illumination if secondary is placed half way between primary and focal plane - it must have half of diameter of primary. Secondary mirror with minor axis of 75mm sitting half way between primary and focal point would produce single point of 100% illumination (everything else would be vignetted).

While above is true - it does not explain why do we see more with larger scopes for same exit pupil - even if surface brightness is the same. Take two scopes of same F/ratio - one larger and one smaller aperture and use same eyepiece with both - you will find that you can see some objects in larger scope and not see them in smaller scope. Same eyepiece in both scopes will yield same exit pupil and hence same surface brightness (for both sky and target - contrast ratio remains the same) - we should be able to see same things in both scopes.

I'd go with 150PDS for number of reasons. It will be better for planetary with somewhat smaller secondary obstruction. x3 barlow and ASI224 will be excellent match with that scope. (ideally, if you want best planetary performance - I'd look at 6" F/8 newtonian for that role with x2 barlow - or getting a larger aperture scope - but that is probably not your primary interest). F/4 is very hard to collimate - and if instrument is not well tuned - it is hard to keep collimation. More than once I've heard that people need to mod their scopes in order to make them more rigid - less mirror shift and that sort of thing. It will need more expensive coma corrector to get good definition on APS-C sized cameras you seem to have in your signature. I have a feeling that 150PDS will be much more hassle free for intended purposes and will give you better results (F/4 can perform better on DSO but it will require much more skill, investment and care when handling in order to do so).

Larger the aperture - more detail you will possibly capture. For a given FOV - size of pixel determines how much detail you will potentially record. With planetary imaging - F/ratio, or ratio of aperture and focal length is related to pixel size - limit of what can be captured. This is summed up in simple formula: F/ratio = pixel_size * 2 / wavelength. If you don't want to bother with wavelength (and for lunar and white light it is ok to just plunk in 500nm in there) it simplifies to F/ratio = pixel_size * 4 There are couple of variables to consider - and they are not independent variables - they depend one on another. Aperture dictates level of detail, but focal length dictates zoom. Pixels dictate how much of fine detail in that zoom will be captured and sensor size is equal to number of pixels times pixel size and also dictates size of FOV. There is one thing that you can do - and one you should not do. You can choose to capture smaller resolution image but you should avoid capturing beyond what telescope can resolve. What do I mean by this? Say that you calculate F/14 to be optimum F/ratio for your pixel size - but you find that when using this F/ratio - there is too much zoom and you won't fit whole solar disk on your sensor. It is ok to use faster F/ratio - say F/7 as you it won't hurt your imaging and result - you will capture full disk with level of detail that your camera can provide. In different scenario - say you have DSLR and you find that good focal length to fit nicely whole solar disk is 1800mm - but you have 90mm of aperture. That translates into F/20 - but your limit is F/14. In this case - you should not use F/20, but instead limit yourself to F/14 and then simply crop image to produce nice FOV. If you use F/20 - it will hurt your image as there will be more noise then it needs to be (system is slower then it needs to be) and more noise prevents you from sharpening properly. Hope this all makes sense.

Well, yes - it appears that star FWHM is indeed very high. One explanation could be - local seeing. That one won't be picked up by forecast but can impact things greatly. This means - heating from local houses, or large concrete / asphalt structures that absorbed heat over the day if day was sunny. Anything that will affect high power planetary viewing will also increase FWHM. Another thing could be focus. I personally find Bahtinov masks to be very imprecise at focusing. Best thing to do is to monitor FWHM / HFR values in frame and focus exposures and choose focus that minimizes those. Better and more expensive way is to let the computer do that to you by using auto focusing with focus motor (something I plan to DIY add to my imaging scopes).

Don't confuse zoom with resolution. Long focal length and small pixels will give you more zoom. In daytime photography when you are not near the limits of resolution - this simply means that you'll be able to see a tree at a distance on a hill. With astrophotography where we are operating on the limits of what telescope can resolve - limited by either seeing (long exposure) or telescope aperture (planetary / lucky imaging) - resolution means sharpness / detail In my view - it is important to match "zoom" with resolution - or level of detail. For example: these two images show same lunar region. Although they are zoomed in roughly the same - first image has more resolution. Much more detail can be seen and detail is sharper. In fact - first image has proper matching of zoom and resolution. All that can be seen on that zoom level is actually seen in the image. Second image is simply blurry. By the way - these two images were shot with same aperture 100mm - but first image was shot using lucky imaging technique using planetary camera and processed carefully. Second image was taken with ST102 and DSLR - it is single frame. I think main difference is recording and processing technique. Lucky imaging was invented and is used for a reason. It simply allows for optics potential to be exploited almost to the limit if properly used.

Something is a bit off here. Stars in your image don't really look like they are 4" FWHM. Could you post raw linear stack in fits format for inspection. I want to see what sort of FWHM figure AstroImageJ will give. By the way, I expect FWHM to be around 2.8"-2.9" for conditions that you mention - 1.88" FWHM seeing and 0.85" RMS guiding with your scope

I recommend using Baader Solar Continuum filter for both imaging moon and sun. This filter is used in combination with other filters - like Baader AstroSolar Safety Film - or Herschel wedge. It this item: https://www.firstlightoptics.com/solar-filters/baader-solar-continuum-filter.html It must be used with regular solar filter for the telescope - not by itself. What does it do? It makes things sharper and adds contrast. It does this in couple of ways: 1. It isolates wavelength of light in white light solar that has good contrast of interesting white light features. This is how regular filters for planets work. 2. It reduces effects of seeing as different wavelengths of light bend differently in atmosphere (think prism and color separation) and this creates additional blur. With isolation of one wavelength (and its neighboring wavelengths) this effect is reduced - sharper image with more contrast is produced. This is feature of narrowband filters 3. It operates in center of the spectrum. Refractors have sharpest image / highest Strehl ratio in the center of the spectrum because they are optimized for visual use. In this part of the spectrum our eyes are most sensitive and it pays to be the sharpest in this part of the spectrum. This is particularly true for achromatic refractors as narrowband nature of the filters remove chromatic aberration and any spherochromatism is also removed. For this reason - I recommend having this filter for white light solar work anyway, but if you have this filter - you can use it for both solar and lunar work (some people use IR filters for lunar work or narrowband filters like nighttime Ha or OIII filters for the same reason) and when using this filter - you can use achromatic scopes as well without fear of chromatic aberration. Only drawback is that it will produce monochromatic images, but neither Solar white light or lunar produce much color anyway (there has been trend of boosted saturation "color/mineral" moon shots lately - but in reality moon is mostly grey and without any color). You probably want to use ND3.8 version of that filter as it is better suited for photography. ND5 version is for visual. https://www.firstlightoptics.com/solar-filters/astrosolar-photo-film-od-38.html Just be careful! ND5 is suited both for visual and photographic work, while ND 3.8 is only suited for photographic work. ND3.8 is a bit better for photos as it allows shorter exposures by letting more light in - but that makes it unsafe for visual work. If you plan on using it both visually and photographically - use ND5 version like you planned. I don't see why not. This is my first ever solar white light image (and although I have right gear, I haven't really imaged anything since): This was taken with camera that is equivalent to ASI120 (QHY5IILc) on 130mm newtonian with Baader solar film (ND5 version as we were observing solar eclipse at that time). This one was taken by my friend with a mobile phone at the eyepiece while observing eclipse: This was my attempt of capturing whole lunar limb - it is mosaic because sensor was too small - but Moon moved a bit between panels so it looks elongated a bit. My processing is also rudimentary so panel seams can be seen:

Well - no You can still do nice solar images with small refractor as well - just don't expect very high resolution images. What does that mean? Like I said - exploiting full resolving power of the telescope will make your full solar disk be 2400px in diameter (that is with 72mm scope). Larger telescope will simply let you make more detailed / larger image. It really depends on how detailed you want to go. Here are few comparisons to help put things into perspective. With Solar Ha telescopes - people often image with smaller apertures. 35, 40, 50 or 60mm are very common apertures as dedicated solar Ha scopes past 60mm become very expensive rapidly. Given that Ha wavelength is longer than most visible light - that also reduces level of detail. Full solar disk of 50mm scope for example is less than 1000px in diameter. On the other hand, Moon is same apparent size as the Sun in the sky, so it is pretty much the same target as far as imaging goes (if we consider size and level of detail). Most people will find 1000px moon image to be very small and most appreciate zoom in on individual features. Here is an example of full lunar disk image (right clicky / open in new tab thingy and zoom in to 100% for full detail): This was produced with 100mm Maksutov - and even reduced in size because seeing was not allowing for full resolution image. It all depends on how detailed you want to go and if you are going to use scope for something else as well. 72ed is nice little travel scope and small imaging scope and fun wide field scope. If you plan in using it in any/all of those roles next to solar imaging - then I say, get that one. You don't need ED scope for white light solar imaging. With Baader Solar Continuum filter - you can use simple achromatic refractor. Filter will remove any chromatic aberration and make views very sharp. I highly recommend that filter for solar white light imaging with any scope. In that sense - simple 80mm longer achromat will give better imaging result, so if budget is tight and any other intended purpose can be served with 80mm scope (just use at least F/7 one - don't use ST80). Mak100 is another cheap option that will serve solar imaging role - and you won't need barlow. Here are some formulae to help you decide: Optimum F/ratio of setup for lucky imaging: F/ratio = pixel_size * 2 / wavelength For example, for ASI120 with 3.75um pixel size and Baader Solar Continuum filter which is 540nm filter - formula goes like this F/ratio = 3.75um * 2 / 0.54um = 13.888 = ~F/14 (we convert 540nm to 0.54um because we need same length units) Sensor height in mm for particular focal length Sensor height >= 2 * tan(18 / 60) * focal_length (here I put that FOV is 36 arc minutes to have some space around the solar disk - as Sun's diameter is 30 arc minutes) At 1000mm - needed sensor height would be ~10.5mm to capture the whole disk.

Solar imaging is like other planetary imaging - best way to get good images is to use lucky imaging approach. If you already have DSO imaging rig - you probably have guide camera. That is what you should use. Any guide camera capable of fast raw output will be better choice than DSLR. Herschel wedge serves the same purpose as Baader solar film - solar white light, not solar Ha. But unlike solar film, it can only be used with refractors. Solar Ha is much more expensive. Anyway, back to solar white light imaging. Capturing full disk images, if that is what you want to do - will depend on combination of focal length and sensor size. Another important thing is - you need aperture for resolution. 72mm scope will have very little aperture. At sampling matched to resolving power of the telescope - max solar image that you will be able to make in white light with say Baader Solar Continuum filter - is Sun image of 2400px diameter (0.77"/px). Depending on pixel size of your camera - you will have to use some sort of barlow. For the same amount of money as 72ED - you can have 100 or more mm of aperture and get more detailed image of the sun. Here are some examples - 4" F/10 achromat with Baader Solar Continuum filter will be very good solar instrument. If you have 2.9um camera - you don't will not need barlow as optimum at 540nm (Baader solar continuum) for 2.9um pixel size is ~F/10.75. You can also use 4" F/11 achromat - a bit more expensive but optically a bit better (won't make much difference in image quality but images will be a bit easier to process). Mak127 will be good match for 3.75um pixel size.

My guess is that it is meteor. It can't be quite seen as image is significantly reduced, but to my eye - there are two parts to that trajectory - "bottom" part which is fainter and suggests faster motion and "upper" part that is thicker and somewhat wobbly - which suggests either slower speed and / or change in brightness - in either case - it is indicative of atmosphere interaction (which both slows down and increases temperature of falling object). Fact that it is highly curved - suggests very steep angle on actual trajectory, it is actually thing of perspective - something like this: From this angle - arch looks very curved Same bridge "head on": arch curve is much straighter.

I think that it's the focal length of instrument that is important here. You can easily see this if you examine what will happen if object and image distances are the same. This happens at twice focal length - regardless of the scope. In another words - for Mak127 - if you place object at 10 feet (twice the focal length if foot is ~300mm) focal plane will also be 10 feet to other side. For both 72ED and 90mm APO - this is closer to 3 feet to either side. Larger the ratio between object position and focal length of scope - less out focus is needed (if we extend object to infinity - zero out focus is needed - regardless of focal length of scope as focal length of the scope is always finite value).

Say they were 1500000mm away (1500m = 150000cm = 1500000mm), focus position shift goes like this: 1/focal_length = 1/object_distance + 1/image_distance (image_distance being effective focal length in our case). 1/1500 = 1/1500000 + 1/X 1/X = 1/1500 - 1/1500000 = 1000 / 1500000 - 1 / 1500000 = 999 / 1500000 X = 1500000 / 999 = 1501.5 So yes, change in focus position for something 1500m away is very small - only 1.5mm of out focus needed compared to infinity focus - this translates to about 5mm change in focal length. Not much, but still there. As a comparison - testing on target 100m away would yield focus position at 1522.84 or about 22.84 out focus or about 85.7mm of focal length change.

For imaging, if price is not parameter - LS60: 1. 60mm > 50mm so more resolution with 60mm obviously 2. SS60-DS is F/15.5 scope. For imaging Ha wavelength at F/15.5 - you need 5.084um pixel size for optimum sampling. With 60mm scope, critical sampling is at 1.128"/px. Sun is roughly 1800" so that translates into 1596px diameter of the sun. This means that you need about 8.2mm high sensor if you want to capture full solar disk. You will be hard pressed to find fast mono cmos that is about 5um pixel size and 8.2mm or more in height. Closest thing is mono ASI183mm with binned pixels - that is 4.8um binned pixel size and 8.8mm height Such camera will have 3.2e of read noise at best (by itself it has 1.6e read noise but you will need to bin x2 - and that doubles the read noise). Lunt 60 is F/7 scope. With F/7 scope, needed pixel size is 2.296um pixel size, so 2.4um pixel size fits nicely. With 1596px at 2.4um you need sensor that is 4mm in height to capture full solar disk. ASI178mm has needed specs, is much cheaper than ASI183, has lower read noise at 1.4e (compared to 3.2e of binned ASI183) - you avoid the need for software binning, and it has slightly faster fps than ASI183 at ROI size needed to capture full disk.

Yes, focal length depends on primary to secondary distance. I've seen people measure their scopes before and it is linear relationship. By the way - spherical correction also depends on this distance, so there is only really one focus point where scope works at optimum. Another thing to keep in mind - if you did measurements during daytime on target that is not at infinity - focus position changes for objects that are near by - adding to the problem as focus point moves further out - thus prolonging focal length as well (maybe that is why you got higher measurements then expected?).

For that sort of money one can get E.fric mount which is in another class (but is somewhat rudimentary / industrial even compared to Chinese mounts).

That makes sense - it would lessen it, but not remove it completely.

No one mentions this camera in similar price range: https://www.altairastro.com/altair-hypercam-269c-colour-camera---tec-cooled-1097-p.asp I wonder how it would stack up given that it is modern Sony 4/3 sensor for £899.00

I think you can - but it would be best to try out one night when you don't plan to image but it is still clear - like when the full moon is out or similar.

Yes, you can dither only in RA - sure, some mounts like StarAdventurer don't even have powered DEC - and those can only dither in RA, but if you dither in RA only - and guide in DEC - you will put all your offsets on a single line - RA line.