sploo

OK, so if you take an M6 bolt and started to screw it into the fixing holes in the scope how much thread do they take up before you can't screw it in any further? That's important to know in order to get the right length bolt and ensure it doesn't either bottom out on the hole (before holding the handle tight), or conversely, not have enough "bite" into the holes in order to hold the handle onto the scope safely.

That's what I suspected; a relatively large (but simple shape) such as a large handle is probably not the best application of a 3d printer.

Do you mean a handle like this: https://www.boneham.co.uk/Products/knobs-and-handles/bridge-handle ? If you can't find an off-the-shelf unit that fits then a 3D print would certainly be an option, but for a simple 'u' shaped handle something in wood would be easy to make. I could probably dig out some scraps, and it can be stained black. How deep into the scope do the M6 threads go (i.e. how much thread is needed for the scope body)? How heavy is the scope (and are you looking at trying to carry the scope using the handle)?

Thanks. I'd be happy just to get a 1 minute exposure! Because I currently only have DSLR camera gear I understand that long exposures aren't a good idea. With the off-axis adaptor, doesn't the prism/mirror intrude into the image being captured? I guess not, but it seems odd that it doesn't.

Many thanks both. If the only purpose of this thread is for the 1.25" eyepiece adapter then, because it couldn't be used when a DSLR is mounted onto the male T thread, I could buy a 2" eye piece to T thread adaptor for permanent attachment to the camera, and leave this original ring for eye piece use only. I just wanted to make sure there wasn't some really important photography related reason for having that female thread.

Well, from the few pages I've read so far it looks as though he knows what he's talking about. But, you think I might get my camera hit by protons? Are you positive? (I'll get my coat)

The attached image is the first item that slots into the focuser on my Skywatcher 300P. The right side is (unsurprisingly) a smooth tube with a 2" OD. On the left there's a male thread with an (approx) 42mm OD, approx 0.75mm pitch (i.e. a T thread for a camera adaptor, rather than a 1mm pitch "M42" thread). The female thread on the left side is something I can't identify. It's approx 35.5mm ID, and looks to have a 1mm pitch. Anyone know what it's for, and what this part is (i.e. I want to buy another, so I can leave one permanently screwed to a camera T adaptor)? PS I couldn't see anything obvious on https://agenaastro.com/articles/guides/miscellaneous/astronomy-threads-explained.html

Many thanks. Interesting to see the use of a long axle and double disc for the first reduction pairing. My current plan (waiting on parts) is a 100:1 harmonic drive reducer, followed by a DIY 10:1 friction reducer. Based on the above, it at least looks as though my 10:1 design should be feasible. BTW Steve... The Chanctonbury Observatory... I've just bought a book about photons by some bloke called Steve 😉

Makes sense. I just wondered if any part of it were visible. The claimed 2000:1 reduction ratio is significant, so I'm trying to work out how it's done (I'm assume multiple stages, as otherwise the main disc would have to be huge).

Initialise? BTW Can you see the components of the friction drive in this mount? With a desire to DIY build something similar I'd love to see a few seconds of video of it in action.

Last night I got chance to process some captures from my first attempt at connecting a DLSR directly to the telescope. The Canon 5D4 has a 4K video mode that (whilst criticised for various reasons) is well suited to getting a large number of frames of the moon. With the huge light collecting power of the 12" scope (vs my fastest camera lenses) I was easily able to cover the sensor with a good image, at ISO100 and with 1/125s exposures; fast enough to negate the issue of a lack of any tracking. Stacked through PIPP + AutoStakkert, and sharpened using Registax, the results are pretty good (by my standards anyway). I then tried the same technique on M42 (not really expecting it to work). With 1/25s exposures, at ISO 12800, I got a few tens of seconds of faint (and very noisy) video. To my surprise; running what was approximately a 44s total worth of exposures through Deep Sky Stacker did actually produce a not-completely-terrible image. There's clear pattern noise as I effectively only had light frames, but it does give me some confidence that a (planned) DIY tracking mount (that was only good enough for exposures of a few seconds) wouldn't be a waste of my time. Early days, but a positive start. I've also taken delivery of the Making Every Photon Count book (though haven't yet had time to take a look).

Makes sense; thanks. Aperture size vs exposure time is something I'm comfortable with from "terrestrial" DSLR photography; but in that world you don't usually think in terms of arc seconds per pixel, so that consideration is new to me (though I get the concept of 100mm at 2"/px vs 200mm at 1"/px).

Yea, that gives me some hope too. My 12" scope should be about 1.8x "slower", but that still means that 2s exposures may be acceptable.

Mind blown. I was aware you lose some colour vision when dark adapted but didn't realise it was to that extent. Teal is a perfect description for the colour I saw in Orion. I've definitely learned something today - thanks!

I experienced that (visually) for the first time last night (the first night since owning the 300P that I've seen clear skies). Being able to see Orion through the eyepiece is a pretty incredible experience. Unsurprisingly a 305mm aperture is collecting rather more light than the ~70mm of my longest camera lens. Interestingly, Orion appeared as a dark cloud (probably with a greenish hue) to the eye, but shakily holding my phone against the eyepiece and taking a snap resulted in the more familiar pink-with-blue centre I've seen when using the DSLR. I assume with a really wide aperture scope that colour might be visible visually?

As a Canon DSLR user, read noise has been a major topic of complaint (at least from about 2008 up until the release of the 1Dx2, 80D and 5D4). Fortunately their modern sensors have mostly solved those problems. They're not quite as ISO invariant as the best Sony sensors, but you can now push an exposure by a few stops in post without it falling apart. The question of exposure length is something that's interested me. I understand that if a pixel may get hit by 1 photon of light on average once per minute, and you're taking 30s exposures (with a few seconds' pause in between each shot to allow your sensor to cool) then there's a decent chance you'll miss many of those photons during the cooling gaps (vs taking a single long exposure). However, multiple short exposures are going to be less sensitive to tracking issues, and obviously there's less risk of any single exposure being rendered unusable by a gust of wind or a cloud rolling over. Indeed, having run 4K video captures of the moon on the 5D4 (for stacking) I do wonder what the result would be of running a long video capture on a DSO. I'm pretty certain the night mode on my phone basically does that (taking a quick burst of high ISO/short exposure images, then blending). I've ordered the "Making Every Photon Count" book; hopefully it will cover that subject.

I've done a little astrophotography over the years (DSLR + lens + Skywatcher Star Adventurer) but picked up my first scope a few weeks ago (starting nice and simple *ahem* with a Skywatcher 300P Dobsonian). Last night was the first chance I've had to actually see some sky, so I dragged the scope out across the patio (who cares about collimation eh?), gave it at least 30s for the mirror to cool down (that's 30s, not 30m), and... Orion is stunning. Never seen something like that "live" before. Shakily holding my phone against the eyepiece and getting a snap resulted in an image with at least as much colour as a multi-second exposure on the DSLR + lens (a large aperture is clearly king). Imagine what the results will be if I can use the scope with some competence... Anyway, good start. Here's to learning more.

Absolutely; I suspect it's an old rule of thumb that dates from the film days. Certainly even with a wide angle lens I wouldn't be exposing as long as 30s without tracking. I've found that 10s at 16mm focal length is just about acceptable from the point of view of a wide milky way image, but you can still see trailing if zooming into the image. (I probably should have made that clearer in my earlier post - there is the "500 rule", but I personally wouldn't stretch exposures that long with a modern high res sensor)

Indeed (bank account killing). I've taken a few shots using lenses and a Star Adventurer mount (anything from about 16mm up to 400mm). Obviously the 1500mm telescope is a different ball game.

In 35mm (full frame) DSLR photography terms, the "500 rule" is often used; that is, an exposure time no longer than 500s divided by your focal length. I.e. for very wideangle shots (16mm lens) you can expose for 500/16=31 seconds before star trailing becomes an issue. A 1500mm telescope used for prime focus would, I assume, only allow 500/1500=1/3s exposures. If using a Canon APS-C body then it's 1.6x shorter (as you get a field of view on the crop sensor that's approximately the same as a lens with a 1.6x longer focal length).

Cheap at 1/10th of the price 😉 I've just scored a 100:1 harmonic drive from a seller I've used before (already got one of the same type, but it's currently running my CNC machine's 4th axis). I've got some stock to have a go at creating a friction drive for the final stage (10:1) to give me 1000:1 for building a mount for use with a stepper. The torque specs of the mechanical parts look to be more than sufficient for the telescope mass; I guess the challenge is going to be getting enough stiffness in a DIY mount.

I'm pretty certain the thread was Alt-Az mounts with no tracking or guiding at all (not even goto allowed). Lots of examples of multiple 1s exposures! Surprisingly good results TBH.

Canon EF has a 44mm registration distance (https://en.m.wikipedia.org/wiki/Flange_focal_distance) so if this is a lens with an EF mount then the distance from the rear face of the lens flange surface to the sensor plane does indeed need to be 44mm.

Yea, dumb *ss here knows that now 😉 The stupid thing is that I've shot M42 with shorter focal length camera lenses, so there's no reason why I couldn't have worked out that 1500mm was a bit long beforehand! Still, I do hope to be able to see some of the planets at some point, and the one few brief moments (since buying the scope) that the moon appeared out of a cloud showed me that it can produce really impressive results (at least for visual work).

Thanks. I've got plenty of photography experience (including long exposure), and some astro (using a DSLR + camera lens + Star Adventurer). The 300P came up used for a very good price, so whilst I understand it's probably not the best first telescope for a noob, I wanted something significantly "better" than the lenses I have - as I can cook up anything from a 400mm f/5.6 to an 1100mm f/16 with my current lenses. I'll clearly need to look further info coma correctors.