sploo

42 minutes ago, Stuart1971 said:

Yes that is exactly what I mean, but they don’t do one with 126mm fastening centres, just 120mm.. 

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.

30 minutes ago, teoria_del_big_bang said:

I do have a printer and can design a handle, although they are not as easy as you think to print a good comfortable handle because they either need a lot of support in middle of handle if printed correct way up or you print them upside down, or on their side but each of these gives some issues with making a comfortable one as you dont want it with square edges .  

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)?

On 06/03/2020 at 20:59, laser_jock99 said:

Thanks for endorsment!

I run a 12" F4 scope on an EQ6 (pushing the limits on weight loading....). Typically though I'm only imaging at 1200mm (or 870mm if using my focal reducer).

The only real wisdom I can offer is that off-axis guiding works best with this kind of set up.

I tend to limit my exposures to 10 minutes max, but more often three minutes to avoid too many dropped subs.

Good luck!

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.

2 hours ago, steppenwolf said:

‘The Chanctonbury Observatory’ you say? I heard that Steve bloke was a bit of a rogue and didn’t know his photons from his protons 😁

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

13 hours ago, steppenwolf said:

It works in stages. Below is a 3000:1 version from the original Mesu mount project:-

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 😉

12 minutes ago, Jonk said:

No, if it were to be opened, any dirt or debris could cause issues... not to mention bits flying out!

There is a spring system in there as it can be heard when uncoupling the motors and moving by hand.

I’d also love to see the inner workings.

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).

1 hour ago, tooth_dr said:

*(What does init mean?)

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).

13 minutes ago, vlaiv said:

Total integration time is related like aperture, provided that you match resolution. Actual SNR formula is rather complicated. You can get same SNR with smaller aperture as with larger if you change sampling rate - or arc seconds per pixel.

For example - you will get the same SNR from 100mm at 2"/px as 200mm at 1"/px in the same time.

You can still use 1s exposures - just means you need to get 3600 of them vs 2000 of them with smaller scope (that is just example number, actual number will depend on host of factors - QE of sensor, your sky background value, sampling rate, etc ...).

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).

7 minutes ago, dan_adi said:

I did found an interesting site http://www.astrokraai.nl/viewimages.php?t=y&category=7 

M51 with a 16 inch scope, 2000 frames at 1 sec exposure. Really interesting! I guess the bigger mirror helps collect a lot of photons even at 1 sec exposure and the new cmos sensors with low read noise are helpful too.

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.

1 hour ago, vlaiv said:

That depends on several factors. It is surface brightness that is most important I think. It also depends if you are dark adapted or not. More dark adaption you have - more you will loose ability to see the color.

Prime candidate for seeing color in telescope is M57 for example, as it has high surface brightness. Don't magnify your target too much as you will spread the light over large area and the photo sensitive cells in your eye will receive small number of photons - and fail to trigger response. Don't get fully dark adapted (I know this sounds counter intuitive - but if you want to see color in planets and DSOs - you don't want to loose your color sensitivity and go into full dark adaptation).

And yes, it takes rather large scope to see some color in DSOs

Btw - you won't see red / blue color in Orion nebula like in images - you will start by sensing a sort of greenish / teal color. This is because eyes are most sensitive in green part of spectrum - so OIII and to some extent Hb wavelengths. Eyes are fairly insensitive to Ha wavelengths.

This shows two different regimes and sensitivity to light - note that Scotopic vision almost goes away for wavelengths above 600nm. Once you switch to night vision - you don't really see Ha wavelengths - Photopic vision is responsible for that. This means if you want to see any sign of red color - don't get dark adapted. Similarly in low light conditions when you start switching to Scotopic vision - you are much more sensitive to green/blue part at around 500nm - this is why you have hint of greenish hue in Orion nebula.

Best for viewing color / nebulosity would probably be Mesopic vision - this is crossover when eye uses both cones and rods to see - but neither at its best:

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!

53 minutes ago, vlaiv said:

Large scope will such in photons making both target and sky background brighter in shorter amount of time 

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.

1 hour ago, vlaiv said:

No, that 500 rule is gross approximation. If you want to do proper calculation for this case - use sampling rate and sidereal rate and see what you get from the two.

Let's say that you are using modern DSLR sensor that has pixel size of about 4um or so. You are using 1500mm focal length. This gives you 0.55"/px, or each pixel is 0.55 arc seconds "long". Sidereal rate is about 15"/s. This means that in a single second - star will streak across 30 or so pixels. If you have FWHM of about 3" - that is 6 pixels, and you can start to see star elongation at 20% larger major radius. This means that elongation can be at most about 1.2px (6 pixels * 20% = 1.2px). That is 0.55 * 1.2 = 0.66 arc seconds.

With sidereal rate of 15"/s - this will give you 44ms exposure not to exceed 0.66 arc seconds elongation.

As you see - this figure is about x7.5 less than you estimated using 500 rule.

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)

On 28/02/2020 at 17:49, barkis said:

You could do some nice wide field Astro photography with your dslr and various lenses. 
Work towards a acquiring a manageable fast Newtonian or an Apo. chromatic  refractor if you can raise the 
the money.  A good equatorial mount can be bought second hand too, not everything needs to be new.
Getting what we desire  is not easy, but a lot of satisfaction can be got doing it all slowly, and with forethought 
This AP can murder your Bank Account, so proceed with caution🙂.
Ron.

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.

On 28/02/2020 at 17:26, dan_adi said:

Thanks for the correction. How long can you expose without a wedge or a derotator? Always good to learn something new

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).

On 28/02/2020 at 16:59, alacant said:

Hi

Phew. To get started in astro-photography with that telescope and stand a chance of getting results, I think you'd need to imitate one of these. OTOH, I'm almost certain @laser_jock99 may be able to help with alternative ideas.

Good to have others on board with big reflectors:)

Cheers and good luck.

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.

On 28/02/2020 at 16:45, dan_adi said:

I think you mean with no autoguiding. Every mount has to track the object beeing photographed. Indeed there are alt az mounts that have direct drives and absolute encoders. Those are very expensive and hard to make as diy. Another thing is an alt az mount need derotator or an equatorial wedge for photography.

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.

2 hours ago, carastro said:

Can I get an understanding of the back focus of this lens. Is this the distance from the base of the lens to the sensor?  (is it the very rear end of the camera lens) 

My Camera Atik460 = 13mm back focus and the EFW = 22mm = 35mm 

I think I read somewhere that the back focus for this camera is 44mm so I need a further 9mm is that correct?  

With the Geoptik adapter I can't get closer than about 46mm because the foot of the adapter fouls the EFW when trying to screw it on and can only fit it with a small adapter.  Have tried to remove the foot but it won't budge. 

Carole 

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.

6 minutes ago, dan_adi said:

And the 8. Big targets like nebulas require short focal length

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).

6 minutes ago, dan_adi said:

If you are just begging it would be recommended to do some reading about mounts, telescopes, focal reducers etc, before you spend money on expensive astro gear. Astrophotography has a steep learning curve. It is very rewarding and awesome when you learn how everything works .
For astrophoto the Precision of the mount is perhaps more important than the telescope. You will have faster success with shorter focal length telescopes because they demand less on the mount and autoguiding. Image scale is also very important for success. I think the recommended scale is between 1 and 2 arsec/ pixel. Below 1 arcsec/pixel you must have very clear skies and very good mount.

So first:

1. make sure your mount capacity is ok for your telescope

2. get an apropriate coma corrector/reducer

3. check your spacing requirements. All correctors have a required backfocus, so you will need additional spacings

4. get an appropriate camera as to get 1-2 arcsec per pixel scale

5. Off axis guiding is reccomended for long focal lengths. Another option is separate guidescope. A third option is on axis guiding but I’m not sure a newtonian has the 60 ish mm backfocus requirement. You have to know your telescope backfocus.

6.software. Some are free some are not. I like Prism because it is all in one solution.

7. When in doubt ask on the forum, there are a lot of helpful people here

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.