Using a camera on a telescope with eyepieces or reducers?

[dan_adi]

22 hours ago, sploo said:

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.

I have seen very short exposures on bright objects like the planets. I haven’t seen good pics of faint nebula and galaxies with such short exposures. Vlaiv can explain better than me about signal to noise ratio and optimum exposure length for faint fuzzies. I will look for dso images with very short exposures as you mention and see how they compare with “classic” long exposure photography. Thanks for the info!

[vlaiv]

7 minutes ago, dan_adi said:

I have seen very short exposures on bright objects like the planets. I haven’t seen good pics of faint nebula and galaxies with such short exposures. Vlaiv can explain better than me about signal to noise ratio and optimum exposure length for faint fuzzies. I will look for dso images with very short exposures as you mention and see how they compare with “classic” long exposure photography. Thanks for the info!

Have a look here:

https://www.astrokraai.nl/viewimages.php?t=y&category=7

As far as I can tell - most of the images are taken with 16" dob and 1s exposures. Detail is incredible for such a short exposure and number of subs (most are less than half an hour of imaging).

Thing with short exposures vs long exposures is just read noise. If we had a camera that has no read noise - exposure length would not matter at all (as long as we can get stars to align subs to).

In fact as long as read noise is not dominant component of the noise - there is very little difference between short and long subs of equal exposure length. Large scope will such in photons making both target and sky background brighter in shorter amount of time (this really depends on sampling rate for particular case, but I just want to explain certain point) - higher values for target and background - higher the shot noise from target and LP noise and less important read noise becomes - less difference between shorter and longer subs for same total imaging time.

[sploo]

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.

[sploo]

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?

[vlaiv]

23 minutes ago, sploo said:

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?

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:

[dan_adi]

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.

[sploo]

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!

[dan_adi]

2 hours ago, vlaiv said:

Have a look here:

https://www.astrokraai.nl/viewimages.php?t=y&category=7

As far as I can tell - most of the images are taken with 16" dob and 1s exposures. Detail is incredible for such a short exposure and number of subs (most are less than half an hour of imaging).

Thing with short exposures vs long exposures is just read noise. If we had a camera that has no read noise - exposure length would not matter at all (as long as we can get stars to align subs to).

In fact as long as read noise is not dominant component of the noise - there is very little difference between short and long subs of equal exposure length. Large scope will such in photons making both target and sky background brighter in shorter amount of time (this really depends on sampling rate for particular case, but I just want to explain certain point) - higher values for target and background - higher the shot noise from target and LP noise and less important read noise becomes - less difference between shorter and longer subs for same total imaging time.

I accidentally found the same site, and didn’t see your reply.  Very very interesting! Thanks for sharing. After browsing that site I can justify getting the new asi 2600 16 bit cmos camera :)))

[sploo]

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.

[vlaiv]

17 minutes ago, sploo said:

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.

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 ...).

[sploo]

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).

[sploo]

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).

[laser_jock99]

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.

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!

[sploo]

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.

[laser_jock99]

On 09/03/2020 at 11:38, sploo said:

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.

I only use DSLR's at the moment. With a DSLR you have a rectangular imaging area superimposed over an imaging circle. The trick is to get the pick off prism in the area of the long side of the DSL image rectangle so it doesn't intrude into the imaging area.

[sploo]

11 hours ago, laser_jock99 said:

I only use DSLR's at the moment. With a DSLR you have a rectangular imaging area superimposed over an imaging circle. The trick is to get the pick off prism in the area of the long side of the DSL image rectangle so it doesn't intrude into the imaging area.

That makes sense, thanks. I'm using a DSLR too.

I'm starting to build up some of the parts for the DIY racking rig so with luck I'll be able to start putting together some designs in the next few weeks. If I can get that to track for some tens of seconds then guiding will be the next step.