sploo

Indeed. I found a thread (on this forum I think) of people doing DSO imaging using Alt-Az mounts with no tracking. Good fun to see that it's possible. I will experiment - as soon as it stops raining where I am!

Lots more useful info - thanks. To answer the mount question: you've already helped me with that in another thread (I'm looking at making a DIY, probably fork, mount). I have some ideas under way for making a friction drive for the required reduction. I currently have both full frame and APS-C DLSR cameras; though I have concerns about hanging a large DSLR off the telescope focuser. I've also got some smaller sensor boards (e.g. Raspberry Pi camera), but I'd be open to buying one of the many astro camera modules such as ZWO models. I think I understand what you're saying re clear aperture and being able to project a field large enough to cover the sensor. Ignoring the question of coma, is it possible then to take a long focal length telescope (e.g. this 1500mm scope) and project a wide field of view onto a relatively small area? E.g. project about 2 degrees (I think) for M42, but inside an image circle of about 15mm diameter; such that you could capture it with a ~11mmx11mm sensor like the ZWO ASI533MC?

Another plus for that book then (I've seen it recommended a few times). I'll have to get hold of a copy - thanks.

As a reasonably competent "technical" photographer, but with no telescope experience, I'm trying to get started with a telescope and astrophotography. Since purchasing my first telescope there (of course) has been a healthy mixture of cloud, rain, rain, snow, cloud, gales, frogs, locust, and a bit more rain; so if any of the following questions seem dumb it's because I'm working from zero practical experience. My scope is a Skywatcher 300P (1500mm focal length, f/5). I understand that when used for prime focus; I get just that - a 1500mm f/5 lens (though I haven't yet got my head around the size of the image circle that would be projected, or what field of view that would give when a full frame DSLR is mounted to the focuser). Experimenting with https://astronomy.tools/calculators/field_of_view/ I see that M42 would be too "large" for this configuration, but good with a 0.5x reducer (yellow and green on the attached image). I have a 1.25" adaptor to attach a Canon (EF mount) camera to the focuser, but there doesn't appear to be any way you'd mount an eye piece or reducer in the pipeline. How is this done, and what reducers would be recommended? I see that for visual work, a long eye piece can result in an exit pupil (EP) that's too large for comfortable use (e.g. 40mm on an f/5 gives an 8mm EP). If using an eye piece for imaging, how do you calculate what would be seen by the sensor? E.g. with a full frame (36x24mm) sensor, and a 25mm eyepiece with a 40 degree field of view (5mm EP and 0.67 degree TFOV on my scope) what would I actually see? Or is the answer "whatever the astronomy.tools field of view calculator shows you"?

Vlaiv gave me some useful info in this post; which may be helpful for calculating required gearing and microsteps: stargazerslounge.com/topic/348545-motor-for-ra-axis-on-modified-dobsonian-mount/?tab=comments#comment-3791469

Late to the day, but I am a camera/photography lens guy who's trying to understand telescopes, so... for this lens: Stopping down is reducing the physical size of the aperture (which means going to a larger f-stop number). The f-stop is the ratio of the focal length to the physical aperture size. E.g. 135mm focal length at f/2 is 135/2=67.5mm aperture. Stopping down to f/4 would result in the lens having a 135/4=33.75mm aperture. At (or near) infinity focus, the depth of field should be pretty large, so it shouldn't be too hard to focus on stars even at f/2 (anything from a few hundred meters away should be in focus). Good camera lenses generally get a bit better when stopped down a little from their maximum aperture (e.g. maybe f/2.8 for this lens). Having said that, the extra light collection of f/2 may be more valuable to you than the extra sharpness (depending on your preferences). Wide open you'll also likely get some vignetting (darkening of the corners). The area of the aperture is inversely proportional to the square of the square of the f-stop; in simple terms, if the f-stop is 1.41x smaller (the square root of 2) then you get twice the light collection. I.e. f/2 has twice the light collecting area of f/2.8. Each time you stop down by ~1.41x, (f/2->f/2.8->f/4->f/5.6->f/8) your required exposure time will double. Manual focus lenses usually had a hard stop at infinity, but autofocus lenses usually focus a bit past infinity (to allow the focus motor to overshoot without causing damage). That said, some modern manual focus lenses may also focus just beyond infinity too. I find that electrical insulation tape is great for preventing the focus ring from being accidentally bumped once set. The depth of field scale is indicating what near and far distances should (just) still be in focus at f/11, f/16 and f/22; but at infinity focus I wouldn't worry too much. It's only when focusing closer that you'll really see the shallow depth of field from a large aperture (small f-stop number) come into play. If you're putting the lens in front of a crop sensor camera (one that's smaller than the "full frame" 36x24mm) then you'll only see a smaller image circle projected by the lens. In my (still limited) understanding of eyepieces, this would be like having the same focal length eyepiece, but with a smaller field of view. Photographers often talk about the "reach" of smaller sensors, because ultimately if you're displaying the resulting image at the same print size it appears that the smaller sensor has "zoomed" in further (but it's only because you're taking a crop from the middle of the lens projection, and printing it at the same size). In simple terms, a Canon crop sensor DSLR (i.e. anything that isn't a 5D or 1D) has a 1.6x smaller sensor than full frame, and will give a you field of view similar to a 135x1.6=216mm camera lens on a full frame camera.

I'm limited at the moment in terms of exposure time due to the DSLR (I've been advised anything between 30s and 2 minutes max per exposure), but certainly I won't be able to attempt multi-minute single exposures, so it may be ok. Maybe...

I thought about one of those (a sort of Poncet Platform I believe) but I really like the idea of making a fork mount. I'm probably making life hard for myself aren't I 😉

Interesting - thanks again. I've done lucky imaging of the moon (though only currently with a DSLR + lens + SkyWatcher Star Adventurer mount), and some limited DSO shooting with the same (30 second exposures max). The little Star Adventurer mount is obviously not going to carry the 300P (somewhere in the 20kg region I think), and commercial EQ mounts capable of tracking such a tube tend to be expensive. I have some basic engineering gear (lathe), plus a CNC machine (though the latter would not handle steel), so DIY is attractive.

Interesting; thanks. I probably wouldn't try to make a full goto system, but that's an interesting site. It may well be that some of his existing PCB parts would be suitable, and given the low cost, probably more sensible than making one myself.

Understood - many thanks. At ultra low rpm I was thinking that the stepper movement would not be sufficiently smooth, but with a 1012.5:1 reduction that still means (I think) somewhere in the region of 150 microsteps being done per second (with 200 steps per circle and 64 micro steps per step). The large diameter of the Dob style mount had occurred to me (from the point of view of being able to fit in a big worm wheel. That should also result in plenty of tooth engagement, and therefore reduced risk of stripping the teeth. Photography: initial planetary, and hopefully some DSO. I currently only have DSLR gear, so exposure times would be limited by the sensors. PS A quick Google didn't provide the meaning of "EEVA", so I'm afraid that one's lost of me.

I've recently picked up a used Skywatcher 300P; the truss tube version on a non-GOTO Dobsonian mount. For astrophotography I'm considering building an equatorial fork mount - in this instance, quite simply a Dobsonian mount, but tilted at an angle; such that the Az axis now becomes RA. I'd leave the Alt (now Dec) axis manually operated, but I'd need to motorise the RA axis. Given that it would need to rotate at sidereal rate (once every 86164s) that's going to need some hefty gearing reduction. I see that worm gears (with a large worm wheel) are a common solution; but even with a very large worm wheel (several hundred to one reduction ratio) that's still nowhere near enough, even for a slowly spinning stepper motor. E.g. One rotation every 86164s is one rotation every 1436.066 minutes (0.0006963 rpm) A stepper running at 200rpm needs a 200 / 0.0006963 = 287213:1 reduction Even with two stages (two sets of worm gears and wheels) that'd require 535:1 reduction per gearing pair; still pretty large. Three or four stages (66:1 or 23:1) would be more practical in terms of worm wheel sizes, but then I'd worry that the smaller gears wouldn't be strong enough to support the 300P tube (risk of stripping teeth). What are others using for this sort of application? Perhaps an off the shelf heavily geared (e.g. planetary) stepper motor, plus a couple of smaller worm wheel stages, or maybe a combination of smaller and larger worm wheels (with the larger wheel at the output to carry the torque)?

A rather crazy combo of a 400mm f/5.6 lens, with stacked 1.4x and 2x teleconverters (1120mm f/16) works pretty well for me with a Canon 5D4 in 4K video mode. The 5D4 has a 1.74x crop in 4K video mode (for an effective 1950mm focal length), so fills the sensor quite nicely with the moon. 1/50s exposures (at 25fps video) seems to work pretty well; especially when on the tracking mount. For DSOs I've only really tried M42. I haven't motorised the mount, but I was pretty impressed with the design of the Dob mount on the 300P; simple, and does the job. I'd considered trying to make a lighter version, possibly with castors. However, looking more into tracking mounts I see there's an equatorial mount called an "Open fork". It's actually just an altazimuth, but where the azimuth axis is tipped to point at the pole. I think that could be an interesting possibility for a tilted Dob mount, with the az axis motorised.

Yep. My background is photography rather than astro (so I had to get my head around what "magnification" meant in telescope terms). I've had reasonable results with 30s exposures at 400mm on a full frame DSLR with the Star Adventurer; but that was a camera lens, so relatively small and light in telescope terms. The 300P is 1500mm if used as a prime lens, so that would be much more of a problem. Maybe OK for a quick burst of images of the moon, as the "500 rule" means that I might get away with 500/1500 = 1/3s exposures. No good for DSOs though.

Just the info I was after - thanks Rob. It does occur to me that with a camera mount that itself could rotate, a combination of alt, azimuth, and camera rotation, would probably allow long tracking. Obviously it's much more complex (physically and logically) than having a single equatorial axis, but for larger scopes I do wonder if it would be more cost effective than the amount of bulk required to create an EQ mount with a similar load capacity. That said, probably a Poncet platform would make more sense - although they can only usually track for some tens of minutes before needing to be reset.

Grand. Many thanks. Of course, I could try putting the 300P tube on the Star Adventurer 😆

Interesting, thanks. So... my take home is that short exposures (10s-30s) may be feasible with only tracking on the altitude and azimuth axes. I assume that multi-minute exposures would still be out of the question though?

Having done a bit of simple astrophotography using a DSLR+camera lens+Skywatcher Star Adventurer I've picked up a nice simple noob starter scope... a 300P flextube Dobsonian (yea, I know). Despite my complete lack of scope experience, I'm so far enjoying the views through the scope (and probably should be asking lots of questions about that); but my mind is turning to the photography question. In terms of tracking, I assume the altazimuth GOTO mounts would be able to find (and maybe keep) a particular star or galaxy centred in the field of view, but that they would not be able to correct equatorial rotation - such that you'd continue to look at the right object, but it would slowly rotate in the view? My 300P doesn't have the GOTO system but I'm mulling over ideas for adding motors - either altazimuth, or maybe a Poncet Platform for the whole scope+mount to sit on.