Camera Labs

OK, I'll share one image but only to demonstrate what might be possible. The session was very much a spur of the moment decision and, predictably enough for the UK, after the first burst of six shots the clouds rolled in preventing a second attempt with a hopefully improved focus.

The telescope has a 70mm diameter objective with a focal length of 480mm (f/6.8) and I was using a 17mm focal length Plossl eyepiece yielding a modest magnification of 28x. I used ISO 1600 and from the EXIF data the camera's 50mm f/1.8 lens was running at f/3.2 and exposure length was 1/125th of a second. I was a bit surprised to see that the lens wasn't wide open (more finger trouble!) but on reflection as long as it was wide enough to capture the full exit pupil from the eyepiece no harm was done.

The only post-processing was to throw away the colour information (there was a slight blueish cast because the Hutech filter was still in the camera), auto-correct the brightness and resize and compress for display in this post.



The main purpose of this trial was to verify how easily you can fill the frame of the 400D cropped sensor with the Moon. To do this with the sensor at the telescope's prime focus I would need to add at least a 2x Barlow/Powermate™ lens to the train. The second part of the test was to try and see differences in "seeing" between the succession of images shot at 3 fps. That didn't work out because of the slight misfocussing so it will have to wait another night.

Hand-holding the camera was only a small challenge as the 400D/XTi with the 50mm lens is really compact and light. With a right-angle eyepiece adaptor on the telescope it was quite easy to initially position and steady the camera and set the shot up by using a little finger resting lightly against the eyepiece. That finger was removed shortly before pressing the shutter release.

As a way of capturing a quick and dirty image of the Moon or Sun with an appropriate solar filter (never look at the Sun directly through a telescope without the correct filters if you value your eyesight!) this technique certainly works. However, for the best results the camera should be mechanically held in place as part of the telescope's optical train. That would also allow frame stacking if desired which would be difficult to do with hand-held shots.

Addendum: A more successful session tonight insofar as I solved the ease of focussing issue by using the telescope's focus wheel rather than the camera's lens. Unfortunately the end result clearly shows the limitations of the 400D/XTi sensor when working at ISO 1600 with sensor noise destroying fine detail.

Conclusion: For decent results of the Moon with a 400D/XTi the ISO setting has to be brought way down but that precludes hand-holding the camera to the telescope's eyepiece unless you have a telescope aperture of 5 inches for a magnification of around 30x. However, with a telescope of that size, unless it has very fast optics, you may as well attach the camera at the telescope's prime focus as the Moon's disc will probably almost fill a cropped sensor anyway.

Bob.

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OM-D E-M5 + ED 12-50mm, H-PS14042E, H-F007014E, L-RS014150E, M.ZUIKO DIGITAL 45mm 1:1.8.
Gitzo GT1541T, Arca-Swiss Z1 DP ball-head.
Astrophotography: TEC 140 'scope, FLI ML16803 camera, ASA DDM60 Pro mount.

Hi Bob, was that a cropped image, or the whole frame?

It was the whole frame. That was using the 17mm eyepiece to give a magnification of 28x. With my 7mm Nagler eyepiece yielding a 68x magnification only around half of the Moon's disc is captured with the 50mm f/1.8 but even at ISO 1600 the necessary shutter speed is too slow for good results.

This experiment has certainly shown how good the human eye is at some things. With the 7mm Nagler I get an 80° plus apparent field of view, an awful lot of which is "Moon", and you get that wonderful three dimensional illusion near the terminator. If only our eyes had a limiting magnitude of 12 or better and a built in zoom how extraordinary the night sky would be. Maybe the Borg got some things right!

Bob.

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OM-D E-M5 + ED 12-50mm, H-PS14042E, H-F007014E, L-RS014150E, M.ZUIKO DIGITAL 45mm 1:1.8.
Gitzo GT1541T, Arca-Swiss Z1 DP ball-head.
Astrophotography: TEC 140 'scope, FLI ML16803 camera, ASA DDM60 Pro mount.

Bob, if you'll indulge me a slight off-topic: respect to your Nagler!

Televue makes some of the best eyepieces I've ever used and I'm the proud owner of the 4.8mm and the 20mm - the latter is quite simply awesome. I had an alignment problem with my Televue Genesis a few years back and took the opportunity during a trip to new York to visit the Televue HQ. I met Al Nagler and he was a really nice chap. Great work those guys are doing...

Sorry, just thought I'd share!

As for photographing the moon again, you'll be able to get away with much slower ISOs using prime focus. Just before the eclipse the other night, I was getting good exposures of the full moon at 1/250 and 100 ISO at f10.8...

Since I could grab a steady image at about half a second (with a poorly aligned mounting), there was also plenty of latitude for further magnification with a powermate or teleconverter...

.
For those reading this thread who may not be familiar with using telescopes with cameras attached, the TeleVue™ site has some very clear diagrams of how to couple a camera to a telescope's optical train here. The technique I have been using is called Afocal Imaging (or "Digiscoping").



You are right. It was an easy experiment to try with the 50mm lens to hand but I am glad I didn't spend any money just to try this technique out. Your exposure times and ISO setting are interesting. Even though your telescope has a bigger objective and the magnification was apparently a little less it makes me wonder whether I was capturing all the light leaving my telescope's eyepiece.

Anyway, getting back to the topic of using the prime focus, a 4x Powermate™ would probably do the trick together with my the telescope's motor driven mounting. Exposure times would still be short enough, even at low ISO, to occasionally capture a really crisp image during a moment of clear seeing.

Bob.

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OM-D E-M5 + ED 12-50mm, H-PS14042E, H-F007014E, L-RS014150E, M.ZUIKO DIGITAL 45mm 1:1.8.
Gitzo GT1541T, Arca-Swiss Z1 DP ball-head.
Astrophotography: TEC 140 'scope, FLI ML16803 camera, ASA DDM60 Pro mount.

Hello Bob, reading through your posts could you please point me in the right direction possibly ?
As someone who is a complete novice to the field of Digiscoping what would be required to begin with ? I have an EOS 30D with 24-105mm lens set-up as my normal everyday lens, but also have a fixed lens - namely the EF 60mm canon lens.
What do you recommend in the way of telescopes etc.. to get into the field of Digiscoping !

I take it you have had a look at the TeleVue Imaging Methods I linked to in my previous post. I'll split my answer into two parts.

Astronomical

If you want to photograph the Moon in the way I described above then any reasonable aperture telescope (70mm diameter plus refractor or 100mm diameter plus reflecting telescope) could be pressed into service even if it is on a simple fixed mounting. When buying a refracting telescope it is important to get one with the minimum of chromatic aberration so avoid the budget models.

The camera really needs to be mechanically fixed to the telescope to avoid the hand-holding problems I encountered. For afocal imaging ("digiscoping") anything fainter than the Moon you will need the telescope to be fixed to a motor-driven equatorial mount which will counteract the Earth's rotation. Planetary imaging will be a tough challenge requiring long exposures and high magnification and I would guess an absolute minimum would be a reflector of 200mm diameter: N.B. I am out of my comfort zone here so you will need to do your own research. Those long exposures will require an accurately aligned and driven mounting. I think you can see where I am going here. Astronomical imaging can make normal photography look cheap!

Terrestrial

If you want to use afocal imaging on terrestrial subjects during the day then obviously the requirements are a lot simpler, i.e. a good telescope on a really solid tripod. As you can see from the TeleVue article the focal length of your camera lens is increased by the magnification of the telescope. So when I used a 68x magnification with my 50mm f/1.8 I was effectively using a 3400mm telephoto! The downside, of course, is that with a 70mm objective that meant I was working at f/48 which might just (!) have been diffraction limited.

The moral here is to decide what your maximum desired effective zoom will be and then make sure the telescope's objective lens has a sufficient diameter to give you the shutter speed you need. If your desired field is terrestrial then there is absolutely no advantage in going for a telescope with a fast aperture other than portability. In order to get the image size up all a fast aperture means is that you pay more for the telescope and more for the short focal length eyepiece and end up with an image which is no brighter on the camera's sensor!

As an example of what this technique could do if you need extreme lenses, say you want to photograph birds (feathered) on the nest. I can't vouch for the quality but you can pick up a 120mm (4.75") Flourite ED Apochromatic f/7.5 refractor for £1500 including 45x and 180x eyepieces. You would have to buy a really solid tripod and some accessories but you could come away with change from £2500. If this telescope feeds a 50mm camera lens then with the 45x eyepiece you are getting a focal length of 2250mm at f/19. Alternatively, strapping the camera directly in front of the prime focus of this telescope you are getting a 900mm f/7.5 lens. OK, it's fixed aperture, manual focus and has no IS but compare the cost (£1500 for the telescope) with the Canon EF 600mm f/4.0L IS USM which retails for over £5300 in the UK. And the Canon can't be "extended" to 2250mm! OK, I know, it's an extreme example and a very specialised application.

Back to the real world! For a good selection of astronomical telescopes in the UK I will take the liberty pointing you to Venturescope here in the UK. There are other vendors who I am sure are just as good but I have personal experience with them and the owner, Ninian Boyle. He is more than happy to help and knows his stuff (but do your own research first).

Bob.

P.S. Thanks for the question. In composing my rather lengthy answer I learnt a lot myself and I hope I haven't made any mistakes in my analysis.

_________________
OM-D E-M5 + ED 12-50mm, H-PS14042E, H-F007014E, L-RS014150E, M.ZUIKO DIGITAL 45mm 1:1.8.
Gitzo GT1541T, Arca-Swiss Z1 DP ball-head.
Astrophotography: TEC 140 'scope, FLI ML16803 camera, ASA DDM60 Pro mount.

You are to be commended on your knowledge of Astonomical photography ! I think your reply more than answers my initial question Bob, thank you.
It looks very much like the topic of astro-photography is an extremely in depth and wide ranging subject in its own right. The detailed answer you have given is an excellent starting point, and I think I visited the suggested link you made from the earlier posts. I downloaded the guide to astrophotography presented by Ninian Boyle and studied it. My particular interest lies with the night sky, and attempting to capture the images which are easily found in it to start with !

Once again thanks for your prompt and definitive reply,

Simon

Hi Simon, in addition to what Bob's said, you can get some great results quite easily at 'prime focus' - ie, using the telescope as if it were a big camera lens. All you do is buy an adapter which has your camera's bayonet mount on one end and a tube on the other which fits into the telescope where the eyepiece would normally go. Then focus using the telescope controls...

That's how I took my Lunar Eclipse shots elsewhere in this forum...

Thanks Gordon. I did make a brief mention of the technique in my "Terrestrial" section but I should certainly have included it in the "Astronomical" section as well.

As you know, unless you have a long one the size can be disappointing. My little Pronto, with its 480mm focal length, only produces an image of the Moon about 4mm in diameter which is only just over a quarter of the height of the 400D sensor. As you mentioned in an earlier post, a Powermate™ or equivalent is needed.

I touched on the subject of diffraction earlier. With conventional camera lenses diffraction seems to start being noticed by reviewers such as yourself by around f/16. If my sums are right (never certain at this time of night!) to nearly fill the 15mm height of a 400D sensor with the Moon's image requires an effective focal length of around 1700mm. At f/16 that means the telescope's objective lens has to be very roughly 100mm in diameter. I guess that means that you can use a suitable Powermate™ with your TeleVue™ Genesis (101mm objective) without diffraction becoming an issue during those rare moments of perfect seeing.

From that argument anyone who is keen to do well at lunar photography should be looking towards a 100mm plus diameter objective lens and something quite a bit larger still for planetary photography.

In your experience has size been an issue or has seeing always spoilt the party?

Bob.

_________________
OM-D E-M5 + ED 12-50mm, H-PS14042E, H-F007014E, L-RS014150E, M.ZUIKO DIGITAL 45mm 1:1.8.
Gitzo GT1541T, Arca-Swiss Z1 DP ball-head.
Astrophotography: TEC 140 'scope, FLI ML16803 camera, ASA DDM60 Pro mount.

Hi Bob, seeing can certainly ruin the image, but I do know the Genesis can easily take a 2x converter at prime focus and still produce great results. My focal length is 540mm, so only a bit longer than yours. Those images were actually taken with a Tele plus MC7 2x teleconverter from Jessops - affordable and good quality. So sure my aperture is greater, but our focal lengths are very similar...

I'd give it a shot and see what you think...

Hi Gordon and Bob

Interesting subject here.

Can you guys post a pic of your astro-gear ?
It would benefit those with little imagination.

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Nikon D7000 and a bunch of Nikkors

Hi Davis,

Have a look at the first picture in my thread Big Sky Astrophotography. Obviously the camera was no longer attached to the telescope for the afocal imaging session and I replaced the featured 24-105 lens with a much smaller, cheaper and lighter 50mm f/1.8.

Bob.

_________________
OM-D E-M5 + ED 12-50mm, H-PS14042E, H-F007014E, L-RS014150E, M.ZUIKO DIGITAL 45mm 1:1.8.
Gitzo GT1541T, Arca-Swiss Z1 DP ball-head.
Astrophotography: TEC 140 'scope, FLI ML16803 camera, ASA DDM60 Pro mount.

Thanks for the advice. One 2" 4x Powermate™ has just been ordered. Ninian, at Venturescope, even hopes to get it in the post this afternoon.

That means the Pronto will be running at an equivalent of 1920mm and f/27.2 which will be interesting and probably only feasible as it's mounting is motor driven. The image of the Moon will slightly exceed the 400D's sensor height but that would only be a problem if I wanted to take a picture of the full Moon which, as you know, is photographically not very interesting.

I'll try and get some shots with it done before the waning Moon disappears completely, weather permitting, and report back.

Bob.

_________________
OM-D E-M5 + ED 12-50mm, H-PS14042E, H-F007014E, L-RS014150E, M.ZUIKO DIGITAL 45mm 1:1.8.
Gitzo GT1541T, Arca-Swiss Z1 DP ball-head.
Astrophotography: TEC 140 'scope, FLI ML16803 camera, ASA DDM60 Pro mount.

I look forward to seeing the results Bob! I got a 5x 1.25in powermate for use with the webcam, but haven't tried it yet...

David, I'll take a photo of the setup next time it's out!