Use of a telephoto lens as a small telescope

On the site of an Oman amateur astronomer called Samir Kharusi I found I nice idea to replicate.
He made a small adapter from a 1.25" eyepiece to fit on a rear lens cap of a Canon lens. Actually, a telephoto lens is only slightly different from a telescope. It is a light- collecting objective device, designed to be mounted on a camera body to take photos as it just projects the image on the focal plane which is the sensor of the camera. Replace that camera body by an eyepiece and... there is a telescope.

However, there are a few 'but's.

First, the image is upside down. A camera does not notice that as it is upside down on the sensor as well, but camera software inverts it to show it erect in live view / electronic view finder and the optical viewfinder also inverts it by the pentaprism built in DSLRs with mirror box.

For astronomical use one gets used to it quickly even when using the device handheld. All Newtons and Dobsons also render an upside down image. Unlike mirrored images with refractors and SCTs with a diagonal, the image can easily be compared with a star chart by turning the chart around. When erect image is really needed, there is a workaround. Then one needs an Amici diagonal and a Barlow. An Amici diagonal is a star diagonal rendering upright images with a refractor without mirroring. For lower powers (<100x) the stripes effect of the Amici do not compromise on image quality. The stripe effect is due to the Amici design. However, due to the mandatory Barlow as otherwise there is not enough back focus, trhe power is at least two times as much and handheld use results in too much shake. In my case, the 70-300 with the Panoptic 24 and the Powermate 2.5x results in 30x power, which requires a tripod, but has an excellent image quality.

The adapter can be made by simple hardware store stuff (superglue, a thumb screw, best is M4 thread and a 32mm I.D. PVC plumbing tube socket) and a rear lens cap.
Make a 25mm I.D. hole in the center of the rear lens cap and paste the PVC socket to it with superglue. Let this set (see the instructions of the superglue) and drill a 3mm hole in the socket and tap a M4 thread in it. It may be possible that the eyepiece is too loose in the socket (31.75 => 32mm), try to fill this gap with a piece of duct tape inside the socket. Attaching the adpater to the lens and the eyepiece to the adapter makes a nice handheld scope. When the telephoto lenses have a tripod ring (usually optional with Canon), one can mount it on a tripod as well. When an Amici diagonal + barlow (in my case a Televue Powermate 2.5x) is mounted as well, then you have a convenient view like a 'real' telescope.

NOTICE: Some lenses have a rear lens (like the 70-300L) which can touch the eyepiece barrel when put too far in, particularly when unzoomed. The 70-300L does not have this problem when fully zoomed in and focused close to infinity. Touching the rear lens by eyepiece barrel might damage the coating, so this should be avoided in any case. The 200mm f/2.8L does not have this problem at all.

Image quality. This appeared to be very good, despite these lenses are not designed for visual use. Sharp to the edges with most eyepieces, better than most binoculars, unless you spend at least two grands for a Swarovski EL 10x50. Due to the fact that lenses contain 8-12 separate elements instead of two or at least threee elements in an astronomical telescope, there might be some more light loss, but modern lenses have so good coatings, that that is not a big deal. Modern Canon L or Nikon lenses have a transmission of at least 80% (measured by T stop values), not far less that telescopes.

It turned out that the 200mm has some color fringing when magnifying 20x or more, despite its excellent fringeless and sharp photo quality at full aperture. Here is indeed a result that these lenses are not designed for visual use but at lower powers (15x or less) this is not an issue. Strangely, the 70-300 (zoomed in at 300) does not have any focus softening or color fringing artefacts at 30x, it is really very sharp with visual use. Maybe because it is a zoom lens ??

Comparison with binoculars. As these devices are not too heavy, they can be easily used handheld. A major disadvantage is the upside down image in terrestial viewing, but, as stated earlier, in astronomical use this is not a big deal. But the other issue is the single-eye view which misses the more relaxing twin eyed view with binoculars. In the latter case it is true with high quality well-collimated binoculars which have no individual focus drift, i.e. focus of left and right shifts out on one of the two barrels during prolonged use. For me that is more fatiguing than using single eyed viewing. Light transmission is in most cases even better than binoculars, as the porro or roof prism design in, particularly cheaper binos is responsible for considerable light loss. Shaking is the same as binoculars with the same power. So a 15x monocular can be held as steady as 15x70 binos.
But use of filters is far easier: screw-in UHC or OIII filters is much easier than binoculars. Viewing emission nebulas with a bright 200 or 300mm tele with widefield eyepieces with filters is really amazing.

Tested equipment Canon 70-300mm f/4-5.6L, measured aperture 55mm with Panoptic 24 (12x, FOV 5.5°), Canon 200mm f/2.8L, measured aperture 70mm with Nagler 13T6 (15x FOV 5.5°).

I tested the Canon 300mm f/4-5.6L zoom and the Canon 200mm f/2.8L. Both lenses produce razor sharp images even at full aperture and for that reason I spent the extra money to get these quality lenses. In astrophotography, I made very nice pictues with the 300mm and I purchased the 200 mostly for astrophotography with really pinpoint stars on the edges at full aperture. When using visually with the Nagler 13 (without diagonal) the image is still crisp with only minor color errors at the edge. It outperforms the Vortex Vulture, but the 70-300L + Panoptic (or this eyepiece) performs better yet.

Well the (ab)use of the telephoto lenses showed very different results. I tested both side-by-side with a Baader Amici diagonal, and a Televue Powermate 2.5x which is required as otherwise the lenses do not have enough backfocus for use with a diagonal. As eyepiece I used the Nagler 13T6.

Canon 70-300L

I tested this lens earlier for visual use and the results were amazing. All tests were done when fully zoomed in. This setup shows crisp and contrasty images sharp to the edges. This setup magnifies 300x2.5/13 = 56x which equals an exit pupil of about 1mm as the aperture of this lens is 54mm (measured with the flashlight method, i.e. shining a flashlight through the eyepiece shows a circle of 54mm projected on a sheet of paper close to the objective lens. Contrasty images (e.g. the USAF 1951 photos but also tree branches against bright sky do not show any blue fringing or other color errors.
Earlier I put a Panoptic 24 directly into the eyepiece adapter (i.e. no diagonal + barlow) under the stars. The result was a crisp image of about 12x power and really pinpoint stars to the edges. Mizar could be separated, not completely, but seen as a double when holding the device steady by leaning on a wall or fence. When using of an OIII filter, views of the Eta Carinae Nebula or other emission nebulas are stunning.

Canon 200L

This lens showed different results. Using with barlow + diagonal was a disappointment. Strange artefacts like looking through a blurry screen, even at 37x. It seems that the diagonal + Powermate deteriorates the image. Both the Baader Amici prism or a William Optics 99% Dielectric mirror diagonal have the same adverse effect on the 200mm and not on the 300mm which is a mystery to me. Very strange as using on the camera the images are really crisp, see the USAF 1951 images which were taken at full resolution 20MP with the Canon 6d and cropped. Without diagonal+barlow (15x) is is usable (and better than most binoculars) as a 15x70 monocular.
Under the stars it is an excellent performer, despite the upside-down image. Stars are really crisp and contrasty. The Great Orion Nebula is really great with this device. And the FOV is mnore than 5 degrees: the whole head of Taurus, including Aldebaran fits in the field. No 15x70 binoculars can achieve this, they reach at most 4.4 degrees.

Preliminary conclusion

I make a final conclusion when testing both lenses visually (and the 200L photographically) under the stars. The 70-300L is really a very good performer as a handheld monocular. I made a handle with a neck strap which can be screwed at the tripod ring of the lens to hold is easier.

For handheld use without barlow+diagonal (i.e. 'straight-through') is can be useful, despite the upside-down image which is not a big deal for astronomical use. Particularly when not much gear can be taken on a trip and a choice between an astronomical binocular and a telephoto lens should be made.
Of course there are some advantages and disadvantages.

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Advantages:

• No hassle with inequal focus in both eyes which is an issue particularly in cheaper binos.
• No collimation issues
• Free use of any 1.25: eyepieces
• Easy use of filters such as UHC or OIII

Disadvantages:

• Searching upside down image slightly less convenient than 'straight through' when using handheld
• Missing the Binocular view which is more relaxed and 'increases' the aperture by 20% with same optical quality due to stacking of two images into one image perceived by the brain.

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Here an overview of my test results:

Quality: ++ = Excellent, + = Good, o = Fair, - = Poor, -- = Bad

Optical device	Tested with	Measured aperture	Magnification / FOV	Sharpness	Edge sharpness	Contrast	Color errors	Notes
Vortex Vulture 10x56 binoculars	Visual	52mm	10x	++	o	+	+
Polarex 80mm modded spotting scope of 1973	Visual with Nagler 13T6	80mm f/6.5	40x	++	++	++	+
Televue Genesis2)	Visual with Nagler 13T6	102mm f/5	38x	++	++	++	++	Builtin flattener I sold this scope recently in favor of the ED 110
	Photographic with Canon 6d		2.6ºx3.9º	++	+	++	+
Robtics ED 110	Visual with Nagler 13T6	110mm f/7	59x	++	++	++	++
	Photographic with Canon 6d		2.2ºx3.3º	++	+	++	+	Use with 0.8x flattener / reducer i.e. f/5.6
Canon 70-300mm f/4-5.6L (fully zoomed in)	Visual (with Nagler 13T6 + Powermate 2.5 + diagonal	54mm f/5.6	55x	++	++	++	++
	Visual (with Panoptic 24 only)		12x	++	++	++	++	Upside down image With Nagler 13T6 (22x) same results.
	Photographic with Canon 6d		4.4ºx6.6º	++	++	++	++	Full aperture (f/5.6) and full resolution 5432x3624
Canon 200mm f/2.8L	Visual (with Nagler 13T6 + Powermate 2.5 + diagonal	69mm f/2.8	37x	+	+	-	--1)	Poor performance, unlike its excellent photographic performance
	Visual (with Nagler 13T6 only)		15x	++	+	++	+	Upside down image Good useable as a handheld 15x70 monocular. Better image quality than the Vortex binos.
	Photographic with Canon 6d		6.7ºx10º	++	++	++	++	Full aperture (f/2.8) and full resolution 5432x3624

1) Colors seen are not from the objective but from the combination.
2) Used from 1995 to 2015, sold