pointedstick

Tonight: seeing Mercury and the entire Pleiades in the same field of view, and then getting to share it with my father and daughter.

Most people have no understanding of the relationship between consumption of utilities and their cost. They will overconsume and then complain about the price rather than reducing their consumption. You have to have a good pre-existing relationship with someone to have a fruitful conversation on this topic. Of course, because their eyes never adapt to the dark! Looking out the window at night is like staring into the abyss.

Splitting Beta Monocerotis for the first time!

To close the loop on this, I've handed the telescope over to my father and he's very happy with it. We got to see a lot of things together and it was great fun. The PushTo smartphone app feature remains awesome and useful! Ultimately I ended up replacing the stock tripod with a smaller, lighter, more portable, and more stable carbon fiber model: https://store.sirui.com/collections/tripod-legs/products/sirui-am2-series-am-284-profilegs-carbon-fiber-big-tripod. That worked out very well. It is a very suitable tripod. I liked it so much that I eventually got one for myself to hold my own telescope, now that I've been bitten by the astronomy bug too! Views of Jupiter and Saturn are wonderful even with my little 80mm refractor.

Cool, I'll check out that list of targets once it's darker out. The smoke from Arizona's wildfires seems to have mostly passed by now. http://www.cleardarksky.com says that conditions are almost perfect! I might hike over to a nearby hill too, and see if the view is any better over there. I'm lucky enough to live within walking distance of a good tall one. And it seems like my itch for a Barlow is on the money. Baader makes an optically-matched screw-on 2.25x one for my Hyperion zoom, but it seems like it might be a pain in the butt to put on and take off compared to a regular old 1.25" one?

Let me provide a brief update: everything is working smoothly. The app is functioning very well on clear nights. The position it sends me to is not often 100% accurate, but it's in the ballpark and I'm not having any difficulty finding it in the region where it's directed me to. However I find myself using the app in a different way: once it's gotten its bearings, I point the telescope to things I can see in the sky and use the app to tell me what it is that I am looking at. Then I can zoom in on the screen and see what's nearby but not not visible to the naked eye, and see it with the telescope. This is a lot of fun, and I'm quickly learning the sky by sight. I can find Polaris, Arcturus, Spica, Antares, Vega, Sadr, Albiero (which is beautiful) and a few others on my own now. The Baader Hyperion zoom is really nice too. I'm using it exclusively and not touching the included 40mm and 10mm eyepieces. I managed see Jupiter in the early morning after trying and failing to see the super blood moon. That was an interesting experience. I watched the moon become eclipsed and disappear entirely! I think the cloud cover prevented enough light transmission. But while I was out, I saw Jupiter up in the sky and got a decent view of it in the telescope! That was cool. On the other hand, I haven't had amazing experiences looking at the Messier objects. The Hercules Globular Cluster is just a dim blurry smudge. Other globular clusters are not so thrilling either. Bode's Galaxy is practically invisible. I haven't been able to see any nebulas. I haven't stayed out much beyond midnight though. I'm wondering if things simply aren't high enough in the sky to get a good view at this time of year, or if this 150mm aperture f/10 telescope is just not up to the task. I think Celestron would make a bunch of money if they sold the tripod and mount separately. Once I hand the whole rig off to my dad in a few weeks, I am going to get my own telescope, but I feel like I want to keep using the app navigation feature.

Thanks again Vlaiv.

OK great, thanks! Actually this makes me realize I have a further related question related to how it interacts with a non-fully-illuminated image circle. If you have a telescope setup where the image circle is not fully illuminated, will a focal reducer crop out some or all of the non-fully-illuminated area, or will it simply scale down the whole thing so that you still end up with a non-fully-illuminated area, only smaller? Also is there a way to calculate the size of the illuminated area if the manufacturer doesn't provide that data?

OK I have one final question on this subject: Would it be largely pointless to put a focal reducer on a Mak because the extra theoretical field of view you'd gain would be exactly offset by a reduction in the image circle? For example this Alter MK 503 f/10 Mak has a focal length of 1270mm. If one added a 0.5x focal length reducer, it would double the field of view but halve the image circle and therefore you wouldn't actually be able to make use of the larger field of view, right? It would "see" the same thing? So in this case, am I correct that the only possible advantage of doing that would be if you have a small sensor that will still fit in the smaller image circle and want a lower focal ratio to reduce your exposure times?

Oh I get it. The more performance or precision you demand of a system, the more something else becomes the weakest link and prevents you from reaching the desired level. It's about stacking the variables in your favor: - Living in a place with generally good seeing and going out on a night that actually has good seeing (it's 100% cloudy tonight so I'm stuck indoors doing research instead of looking up at the sky) - Using an 8" aperture scope so you are less likely to be diffraction-limited by your level of seeing - Using a mount with a guiding error less than the other two - Achieving that guiding performance using a guide scope system, periodic error correction, encoders, etc. ...And so on. Definitely seems like it would be easier to start with cheaper equipment that tops out at an achievable resolution of 2 arc-seconds rather than chasing 1 immediately. So again I see why the small refractors are recommended for beginners. It seems like they make it easy to get pretty good results for large targets with entry-level mounts. I think you've helped me understand that trying to image small dim faraway objects with a small Mak is likely to be a frustrating and unsatisfying experience, at least at the beginning Anyway thanks for explaining all the math. I think I'm going to need to upgrade from a text file to a spreadsheet. Ultimately I think I will probably end up taking the conventional advice of "mount, mount, mount, camera, telescope" and get as good a mount as my budget allows with a small refractor. Upgrading the camera and telescope later is likely to be cheaper. Famous last words, right...

So I found a diffraction limit chart and used that to generate a table of arc-second resolution limits for various telescope apertures to reach the full visual spectrum: - With a 175mm telescope: resolution is limited to 1 arc second - With a 150mm telescope: resolution is limited to 1.4 arc-seconds - With a 125mm telescope: resolution is limited to 1.6 arc-seconds - With a 100mm telescope: resolution is limited to 1.9 arc-seconds - With a 90mm telescope: resolution is limited to 2 arc-seconds - With an 80mm telescope: resolution is limited to 2.5 arc-seconds - With a 70mm telescope: resolution is limited to 2.8 arc-seconds Basically, to actually achieve that atmospherically-limited 1 arc-second per pixel resolution across the visible spectrum, it seems like you need a minimum aperture of 175mm. Then I used that to generate a table of minimum camera pixel sizes for the achievable resolution in arc-seconds with various telescope designs/apertures/focal lengths. My impression is that it's fine to have smaller pixels than these, but any larger and you lose out on potential resolution. Refractor: - 70mm / 350mm: 1.69um * 2.8 arc-seconds diffraction limit = 4.75um max pixel size - 72mm / 420mm: 2.03um * 2.8 arc-seconds diffraction limit = 5.70um max pixel size - 80mm / 480mm: 2.32um * 2.5 arc-seconds diffraction limit = 5.81um max pixel size - 80mm / 560mm: 2.71um * 2.5 arc-seconds diffraction limit = 6.78um max pixel size - 100mm / 714mm: 3.46um * 1.9 arc-seconds diffraction limit = 6.57um max pixel size - 125mm / 975mm: 4.72um * 1.6 arc-seconds diffraction limit = 7.56um max pixel size - 150mm / 1216mm: 5.89um * 1.4 arc-seconds diffraction limit = 8.25 max pixel size Classical Newtonian: - 130mm / 650mm: 3.15um * 1.6 arc-seconds diffraction limit = 5.04um max pixel size - 150mm / 750mm: 3.63um * 1.4 arc-seconds diffraction limit = 5.08um max pixel size - 200mm / 1200mm: 5.81um * 1.0 arc-seconds diffraction limit = 5.81um max pixel size Astrophotography-focused Newtonian: - 150mm / 420mm: 2.03um * 1.4 arc-seconds diffraction limit = 2.85um max pixel size - 150mm / 610mm: 2.95um * 1.4 arc-seconds diffraction limit = 4.14um max pixel size - 178mm / 500mm: 2.42um * 1.0 arc-seconds diffraction limit = 2.42um max pixel size - 200mm / 800mm: 3.87um * 1.0 arc-seconds diffraction limit = 3.87um max pixel size Richey-Chretien: - 150mm / 1370mm: 6.64um * 1.4 arc-seconds diffraction limit = 9.29um max pixel size - 200mm / 1625mm: 7.87um * 1.0 arc-seconds diffraction limit = 7.87um max pixel size Classical Cassegrain: - 150mm / 1836mm: 8.90um * 1.4 arc-seconds diffraction limit = 12.46um max pixel size - 200mm / 2400mm: 11.63um * 1.0 arc-seconds diffraction limit = 11.63um max pixel size Schmidt-Cassegrain: - 150mm / 1500mm: 7.25um * 1.4 arc-seconds diffraction limit = 10.1um max pixel size - 200mm / 2000mm: 9.69um * 1.0 arc-seconds diffraction limit = 9.69um max pixel size Maksutov-Cassegrain: - 127mm / 1540mm: 7.46um * 1.6 arc-seconds diffraction limit = 11.94um max pixel size - 150mm / 1800mm: 8.74um * 1.4 arc-seconds diffraction limit = 12.20um max pixel size RASA: - 200mm / 400mm: 1.93um * 1.0 arc-seconds diffraction limit = 1.93um max pixel size Does this look broadly accurate to you? If not, can you tell me where I went wrong? If it is (broadly) correct, I also understand that field flatness and coma become important too because you want a clear image across the full field of the sensor. The larger the sensor, the flatter and more coma-free the image has to be, right?

Okay, new terms for me to learn: "airy disk" and "diffraction limited field". If it's possible for two f/6 scopes to have different "speed", what does the focal ratio mean? It is mostly useless for astrophotography?

Thank you everyone, this is super helpful. Let's see if I understand: Say I have a 600mm focal length f/6 telescope and I pair it with a camera with 3um pixels, for a value of 1.03 arc-seconds per pixel. If I also get a 1200mm focal length f/6 telescope and I pair it will a second 6um pixel camera, they will reach the same 1.03 arc-seconds per pixel. Am I correct that combination 1 will (theoretically) produce images of the exact same detail because its twice-as-long focal length is exactly cancelled out by its twice-as-large pixels, and the focal ratio is no better? And, since the telescope in combination 2 would need a much larger aperture to reach f6 at that focal length, wouldn't it already be worse since it would be larger and heavier and require a beefier mount? Or alternatively, if it kept the same aperture size but had a slower focal ratio of f/12 like the MCTs I was originally asking about, wouldn't it simply be worse in a different way, requiring 64 times longer exposures to catch the same amount of light? I think I'm starting to understand why people recommend refractors for astrophotography. It seems like you basically need a 3-4um pixel camera with the largest sensor/highest resolution you can afford, and a 600mm focal length telescope with the largest aperture you can afford that can be carried by your mount, keeping in mind the 1/2 weight rule. So can anyone tell me why this 4.35kg 610mm focal length f/4 classical Newtonian isn't the perfect do-it-all astrophotography telescope for cameras with pixel sizes of between 3 and 4um? Is it just things like build quality, collimation annoyances, and difficulty of balancing the optical train because of the Newtonian design? Or something else? Or is this really actually a great choice? Oh yes, I know that my skill level won't let me get those tiny targets anytime soon, and that starting with easier things is a better idea. I'm just doing my research now to try to understand the options and constraints so I can spec out a set of initial equipment that will all match and work well together. Knowing the physical limitations at play is useful so that I don't accidentally buy something that's too much or too little for things that can't be changed, such as atmospheric conditions. Camera pixel size will get smaller over time, but the atmosphere won't become more forgiving, so it makes sense to hold that as a constant and optimize around it. This seems like quite an expensive, equipment-driven hobby, so I'd like to avoid as many costly mistakes as possible. Besides, half the fun of a hobby for me is learning new technical information, so I'm already having fun for free!

Thanks for the information. I think I understand that the smaller the pixels in your imaging sensor, the harder the rest of the equipment has to work to actually make use of the high resolution. Do I have that right? However, I'm not sure I follow when you say that focal length is not really that important. Please correct me if I'm wrong, but if you stay within the 1-2 arc-seconds per pixel guideline and use a low focal length scope where the object is small in the image frame, won't the object occupy fewer pixels compared to using a high focal length scope where the object will be larger in the frame and occupy more pixels? FWIW dew is never going to be a problem where I live. The only time it ever gets wet enough for there to be condensation outdoors, the skies are cloudy so you can't do any astronomy anyway. What is "OAG"?

Lovely! Using that reducer to achieve 1300mm focal length at f/9.6, it seems like you basically turned your Mak into an SCT. I am starting to get the sense that an SCT or a Ritchey Chretien is going to be the superior choice for this due to its faster focal ratio. Is there any major reason why this would not be the case?