Samyang 500mm/800mm in Astrophotography

[Ati]

Hi there, does anyone have experience using Samyang 500mm/800mm mirror lenses on DSLR (APS-C) for astrophotography ? They're dirt cheap (comparatively), but what's the bang for the buck compared to other solutions ?

[Shadowman]

I used the 500mm Rokinon and it'll do the job. You may require a filter for more contrast.

[James G]

Ati,
I'm no great expert on this, but astronomical telescopes Schmidt/Cassegrain types are similar to catadioptric mirror lenses like the Samyang. The mirror systems though are as I understand it engineered to a much higher spec than the photographic mirror system. Additionally the mirror sizes are much larger than the photographic equivalent in order to get the light gathering capacity needed for astronomical observation.

So, it comes down to what you want to photograph .

If you want Moon shots with good detail a mirror lens will probably give a reasonable result at 500-800 mm. ( have only ever used a standard 500 mm telephoto for this rather than a catadioptric lens.)

I would guess from my experience using a 500 mm standard telephoto (Sigma) that you would not be able to get particularly good shots of, say, Jupiter and its moons, and you would certainly not get Saturn or its rings. To do that you really need a tripod mount with a drive to 'track' objects. Similarly photographing Star Clusters or the Orion Nebula say, would be rather difficult because of the required exposure times. Motorised tracking for any object like this is essential.

That just leaves wide field sky shots, and in simple terms the problem with mirror lenses is the fixed, relatively high aperture compared to standard lenses (which generally increases exposure times to capture star fields).

Realistically, to capture star fields without significant star trails you are looking at exposures of between 15-30 seconds at F2.8 and ISO 1600-3200 with a 35-50 mm lens. Using a 500mm lens/mirror would narrow the field of view dramatically, and extend the exposure time significantly since I think mirror lenses rarely have an aperture smaller than f8 or f5.6. So the problem will still come back to mechanical tracking.

One final point, you should check out the literature regarding 'bokeh' issues with image elements that are not in focus when mirror lenses are used. Arguably not a problem for astronomy, since everything 'astronomical' would be focused at infinity, but for general photography, its the main reason why they are still unpopular.

Pricewise though compared to equivalent Telephoto lenses, I agree that the mirror lenses are very cheap.

[loosecanon]

Mirror lenses were popular - especially when long telephoto lenses were very very expensive.
The do have a problem with out of focus highlights, which appear as bright donuts.
They are fixed aperture.
Camera manufacturers like Canon and Nikon used to sell them, but not now.
A lot, including I believe the Samyang use a T3 lens mount adaptor.
You can find a lot of mirror lenses of high quality on the second hand market quite cheaply - just check as usual the lens elements but also the mirrors.
I still get my 35 year old Tamron 500mm + matched converter giving a 1000mm f16 lens out for the odd job. This lens uses Tamrons Adaptall mount, so when I changed from FD to EF mount I just had to get the Adaptall EOS mount.
At the time the Tamron was rated as very superior to many cheaper mirror lenses on the then market.

[Ati]

Thanks James. This is just tiny tot's first steps in astro' for me. My camera is a Pentax K30. The bad news is that it doesn't have mirror lock (or I haven't found it yet). The good news is that the Pentax GPS add-on has an Astrotracer function which allows star tracking up to, I think, 5 minutes on a fixed tripod by tweaking the shake-reduction unit. I live in a severely light-polluted environment so I'll first see how long I can expose using that before the background saturates. The idea for the time being is to experiment at low cost !

Chris.

[Ati]

Mirror lenses were popular - especially when long telephoto lenses were very very expensive.
The do have a problem with out of focus highlights, which appear as bright donuts.
They are fixed aperture.
Camera manufacturers like Canon and Nikon used to sell them, but not now.
A lot, including I believe the Samyang use a T3 lens mount adaptor.
You can find a lot of mirror lenses of high quality on the second hand market quite cheaply - just check as usual the lens elements but also the mirrors.
I still get my 35 year old Tamron 500mm + matched converter giving a 1000mm f16 lens out for the odd job. This lens uses Tamrons Adaptall mount, so when I changed from FD to EF mount I just had to get the Adaptall EOS mount.
At the time the Tamron was rated as very superior to many cheaper mirror lenses on the then market.

Thanks for the general thoughts on mirror lenses, 'Loosecanon'.
How's the night sky in Norfolk these days ? I can remember when 7 miles south of Norwich the night sky glittered, and the city was just an orange glow on the horizon.... days long gone. Chris.

[ajohnw]

Your looking at the wrong end of the focal range really other than maybe the moon. A number of people take astro shots with a camera plus camera lens but star fields and often have the camera etc mounted on top of an astro scope to make use of it's guiding facilities.

Not something I have done but I did buy a telescope some years ago off some one who did and obtained good results. He used a rather fast zoom lens 70-220mm F3.5 on full frame. I suspect he would have used shorter focal lengths as well but that was the main one. I understand this particular lens by Soligor of all people was fairly popular for this sort of thing on film.

One thing I can pass on. I took a number of 10sec exposures at ISO 250, 50mm F2.8 on m 4/3. The idea was to additively stack them but they didn't capture sufficient light. JPG's were fairly clean but raw had rather a lot of noise and did capture more light than the jpg's but still fell short. That was in dark skies though. 10 secs would probably fog too much where I live. One guide might be how clearly you can see the milky way but it's best to try anyway as the camera will see more than the eye but locating something interesting might be a problem.

A good target to practice on would be the orion nebulae - the "sword" and part of the "belt" as the stars are easy to see. Various types of light pollution filter can help but the size needed on a camera lens is something of a problem. Also low pressure sodium lights are slowly disappearing. Those are easy to filter out.

John
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[ajohnw]

I've been meaning to play with the raw files for some time. This one had a dark frame subtracted and then the exposure increased by 2 1/2 stops allowing the noise to come back up. Then reduced. As the noise is showing the stars that have been captured can be seen -just. In real terms it needs to be packed with them.



Trying to replicate work that people have done with plate cameras in the past.

John
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[loosecanon]

Ah Atl - well now as an economy measure most street lights are turned off after 1pm so not so much of a glow

[ajohnw]

Not round here they aren't. The other problem of course is when they are turned off some one gets run over and the press do the rest forgetting that this happens all the time. Bit like street lights after a fashion. Initially where I lived in younger days the lot went of at 11pm. Surprisingly maybe to some it's easy to see by moonlight most of the time but a bit dark when there is no moon. Some one then had the idea of leaving them on all night on road junctions. At that point it was pretty obvious that one every 100 yds was adequate providing it wasn't too bright. That did reduce accidents So deals were made with electricity companies based on special off peak prices and the lot were left on and even more added. Also sections of main roads which resulted in people complaining that driving past a section of those was like driving into a black hole. Now lighting engineers get involved so the light levels get more even and as a result more power is needed. And so it goes on.

Curious thing is if things went back to ancient gas street lighting levels there would be some point in having car headlamps and every body would be able to see each other along with saving a small fortune.

I remember my grand father driving around town on side lights as he could see perfectly well via the street lights. Not recommended but after a fashion a mute point. Then along comes high powered low pressure sodium lighting where people wearing certain colours of clothing are hardly noticeable to a driver. Hence the change to other forms. Some councils are even fitting high CRI led lighting which is way way less efficient than other types which don't render colours so well and both aren't as efficient as low pressure sodium. They don't pay the bills of course so why should they care.

John
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[Tringa]

Can't comment on the quality of this particular lens, Chris but its focal length is enough to get you some decent sized shots of the moon, especially if you are able to crop the shots a bit. As an example, these moon shots are cropped (one very heavily) using a 300mm lens.





To show detail on the planets you'll need some different and from what I have read very expensive gear.



Getting shots of stars and the milky way is possible with just a camera. There are a few ways of stopping star trailing.

As you've mentioned the K30 has a certain amount of inbuilt capability in that direction so I'd try that first.


If you are going for a single shot without using your camera's ability the 600 rule is a good place to start.

Max exposure in seconds = 600/(focal lens of lens x crop factor of the camera).

It is more a guideline than an absolute rule and depends on what part of the sky you are photographing, but a reasonable starting point.

As you can see from this you can get a longer exposure with a shorter focal length lens but this is not necessarily a disadvantage.

This was taken with my kit lens at 21mm. Exposure 20s, f6.3 at ISO 12800. It is not that good and others produce some really amazing images.



There is also free software, such as Deep Sky Stacker, which allows multiple shots of the sky to be combined. No guidance is needed as the software aligns the images and helps to handle noise.

For moon shots, light pollution is not an issue. The shots above were taken about 7 miles from central London, but for star photos a really dark site is essential. Camera sensors are very good at picking up light pollution. You can offset the light pollution a little by changing the white balance but you will still lose a lot of stars compared to somewhere dark.

One advantage of a Pentax is there are loads of old manual lenses around for very little outlay.

Dave

[Ati]

One thing I can pass on. I took a number of 10sec exposures at ISO 250, 50mm F2.8 on m 4/3. The idea was to additively stack them but they didn't capture sufficient light. JPG's were fairly clean but raw had rather a lot of noise and did capture more light than the jpg's but still fell short. That was in dark skies though. 10 secs would probably fog too much where I live. One guide might be how clearly you can see the milky way but it's best to try anyway as the camera will see more than the eye but locating something interesting might be a problem.

A good target to practice on would be the orion nebulae - the "sword" and part of the "belt" as the stars are easy to see. Various types of light pollution filter can help but the size needed on a camera lens is something of a problem. Also low pressure sodium lights are slowly disappearing. Those are easy to filter out.

John
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Thanks for your experience with stacking, John. I've got Deep sky stacker sitting on my computer waiting to be used, I'm now warned of the traps...

As for Orion's Sword, this is the 'best can do' my 18-135mm kit lens pointing out the window with a 2" exposure gives - you recognise that pink blur round the trapezium ? :-) . I'll be happy to move up the quality curve a little....

Chris

[Ati]

One advantage of a Pentax is there are loads of old manual lenses around for very little outlay.

Dave

Dave, thanks for all the tips. I really like the Milky Way + meteor shot.

Any recommendations on old manual Pentax / Pentax adaptable lenses in the 300mm+ range I could hunt for ?

Chris.

[ajohnw]

Thanks for your experience with stacking, John. I've got Deep sky stacker sitting on my computer waiting to be used, I'm now warned of the traps...

As for Orion's Sword, this is the 'best can do' my 18-135mm kit lens pointing out the window with a 2" exposure gives - you recognise that pink blur round the trapezium ? :-) . I'll be happy to move up the quality curve a little....

Chris

What ISO did you use on that shot? just looked at the exif 800. I can see lots of nebulosity in orion in the back garden with a telescope and an interference type light pollution filter. Or could till they changed the street lighting - not sure now. It looks to me that you have the same problem as me - not enough light captured but have a bit more to stack. I did look at that via one package but found that it would only average stacks to reduce noise. I thought that was deep sky stacker.

If you want to try a mirror lens the Tamron BBAR one is well regarded but you would be stuck with F8 as with most. When they are faster the small mirror on the end that returns the image to the camera tends to get bigger which spoils things in a number of ways. The other problem with longer focal lengths is that the exposures have to get shorter unless some form of tracking is available. A bit of time with google can provide info on that aspect. On my shot the 100% view shows the stars trailing but when reduced to smaller size it isn't noticed.

If you google barn door tracker things will come up that can be made at home some more complicated than others. Scotch mount as well. 5 min should plenty though.

Good luck with a decent long lens from a major manufacturer. There was a long Pentax one on ebay recently but I don't think it sold and is no longer listed. That was faster than this one

http://www.lcegroup.co.uk/Used/Penta...-M-_79913.html

They have a few other lenses as well. I use a late film pentax 100mm macro lens on m 4/3

John
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[Ati]

Thanks John, very helpful. Chris.

[tahoetomahawk]

I'm also interested in a higher focal length lens for Astrophotagraphy. I can tell you that I've had pretty amazing results with my Sigma 17-70 f2.8

Attached is an image of the Orion Nebula at 70mm, I can imagine how much better it would be at 200mm

This was several 45 second subs using a Vixen Polarie star tracker.



And a link to my flickr
https://www.flickr.com/photos/124631...posted-public/

[Inkanyezi]

I'm also interested in a higher focal length lens for Astrophotagraphy.

You might appreciate, that as focal length and image size on the sensor increase, the focal length determines only the size of the distant obects, while resolution, or sharpness is a function of lens diameter. An astronomic telescope, refractor or reflector, with a physically larger aperture, will render more detailed images. Most reflector lenses, whether in telescopes or for cameras, are in the realm of f/8 or f/5,6. For telescopes the f/-number is often not mentioned, but its physical diameter. Larger is generally better.

[Ken MT]

You might appreciate, that ....... while resolution, or sharpness is a function of lens diameter..

Can you explain this, I am having trouble understanding why.

[James G]

Ken,

I'm going back to my school physics/optics so apologies if I'm teaching you to suck eggs...

A 100mm focal length f/4 lens has an aperture diameter of 25mm and a 200mm focal length f/4 lens has an aperture diameter of 50mm.

The 200mm lens's aperture is effectively four times the area of the 100mm lens's aperture, so collects four times as much light of whatever (stars Planets etc), is in the lens's field of view.

Compared to the 100mm, the 200mm lens projects an image of each object twice as high and twice as wide, covering four times the area, and so both lenses have the same luminance at the focal plane.

In astronomy, f-number is commonly referred to as the focal ratio (or f-ratio). It is still defined as the focal length f of an objective divided by its diameter D or by the diameter of an aperture stop in the telescope

The principles of focal ratio are always the same, but applied differently for astronomy as opposed to photography.

In photography the focal ratio varies the focal-plane luminance (or optical power per unit area in the image) and is used to control variables such as depth of field.

When using an optical telescope in astronomy, there is no depth of field issue, (effectively everything is at an 'infinite' distance), and the brightness of stellar point sources in terms of total optical power (not divided by area) is a function of aperture area only. As such it is independent of focal length. Focal length establishes the field of view and the scale of the image that is presented at the focal plane i.e. to an eyepiece, film plate, or CCD.

So, the larger the lens diameter, the more light it can collect, which for astronomy, is probably the primary design requirement before other factors come into play.

Does this help?