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Telescope vs telephoto lens?

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Pete
Pete Site Moderator 1218416 forum postsPete vcard ePz Advertiser England96 Constructive Critique Points
10 Jan 2013 - 10:06 AM

What follows is not an area of my expertise so rather than Google and find the answer I thought I'd ask here and then the answers will be useful for others who hit the same brick walls.

I spotted what appeared to be a bargain telescope in the local Aldi the other week - a 76/350 model. It looked quite impressive so I bought it.

Yesterday I received a telescope to T2 adaptor and mounted it to my Olympus OM-D thinking I was going to be in for a treat taking super star photos. I couldn't get it to work. The T adaptor slips into the chamber where the lens goes so I assume the scope works at its minimal power unless the barlow lens is added? Either option wouldn't allow correct focus. I practiced on brighter distant street lamps.

I managed to use the OM-D by attaching a lens to the telescope, which prevented the adaptor being used and the holding the body over the lens and adjusting position. And then I realised a second issue. The OMD with electronic viewfinder was fine to focus on bright street lamps but I couldn't use it to locate and focus on stars. I then tried the Nikon DSLR and had similar issues so gave up with the telescope.

So first question - how do other members use cameras on their telescopes?

I was in star mood so attached a Tamron 500mm with 2x flat field converter to the OM-D and had the same no star visible problem on the Electronic viewfinder. Next Question
Has anyone use the Olympus OM-D for star photography and if so how?
I reverted to the Pentax K-5 with Tamron 500mm with 2x flat field converter and could look through the normal viewfinder, locate a star and focus.

It then got me thinking (next Q) what is the advantage of a telescope. my combination is effectively 1500mm with the K-5 and 2000mm (if I can get it working) with the OM-D.

I found focusing with the Tamron incredibly difficult as the slightest micro adjustment rendered the star as a halo. This was down to the fact this lens has a before end position infinity marker unlike most lenses that class the last point as infinity.

I was out last night until 3am fiddling about but did end up with four decent single star photos in an uncloudy patch of sky

And last question - is this the correct shape and colour of a star or did I just optically create it due to possible chromatic aberrations, inaccurate focusing, doughnut characteristic of the lens, incompatibility of sensor ISO settings etc.

1-star-2.jpg
The star is that tiny spec to the right enlarged beyond 100% so you can see it on the left

Would those who do it successfully post a good star photo below and what they use to record the shots?

Last Modified By Pete at 10 Jan 2013 - 10:09 AM
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10 Jan 2013 - 10:06 AM

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oldblokeh
oldblokeh  2725 forum posts United Kingdom
10 Jan 2013 - 10:19 AM

Correct shape? Are you joking? Any star other than our own Sun is so far away that it's effectively a point source. All deviations from this are due to atmospheric blurring, optical aberrations, mount vibrations etc.

What you should be aiming for is to minimise the above. As for the colour, it's impossible to say whether it's correct without knowing what star it was. I should add that stars come in such a wide range of brightnesses that, if you use a long enough exposure to capture some of the fainter ones, the brighter ones will be too overexposed to determine colour.

My advice? If the 'scope has a polar mount and a motor drive, use it to piggyback the camera and long lens. Pre-focus the lens on infinity using a distant streetlight and use tape to avoid moving focus. Stop the lens down a stop or two to reduce coma.

oldblokeh
oldblokeh  2725 forum posts United Kingdom
10 Jan 2013 - 10:24 AM

Here's a shot I took of the Pleiades, taken as a bunch of stacked piggyback images.

pleiades.jpg

MichaelMelb_AU
10 Jan 2013 - 10:45 AM

Small telescope like 76/350 has no advantage whatsoever over quality tele lens. It is simply too small. The real thing starts somewhere 150/1200. As for the image - it is overly magnified image taken with poorly collimated system - the result of teleconverter use. If you are seriously into starry sky shots I would recommend you to start with the widefield photos - means 50mm lens or so, and then progress further. Tele-lens can be used for Moon shots. Generally speaking, I don't believe that photography forum is the best place to learn astrophoto. There are more than a few dedicated astronomy forums around.
Have fun!

Last Modified By MichaelMelb_AU at 10 Jan 2013 - 10:46 AM
User_Removed
10 Jan 2013 - 11:02 AM

Pete - the big advantage of a properly set up telescope is that the tracking computer and motor allow longer (or multiple) exposures to be taken than you could achieve with a simple camera/telephoto/tripod set-up.

Whether it is then best to connect the camera to the telescope optics via an adapter or simply "piggy-back" the camera and lens on the telescope is one of the most debated subjects in the field. My own (inexpert) view is that piggy-backing is better unless your telescope has significantly better optics than your camera lens. But many other pros and cons of each will doubtlessly be advanced.

There will be a huge upsurge of interest in astrophotography this week with the BBC2 TV series in full swing.

oldblokeh
oldblokeh  2725 forum posts United Kingdom
10 Jan 2013 - 11:18 AM

Let's do some arithmetic with respect to angular sizes of stars and resulting image sizes.

By my calculation, your OM-D with a 1000mm lens has an field of view of about 0.48 degrees along the long axis. Dividing this by the horizontal pixel count of 4608 gives us a value of 0.000105 degrees per pixel. In order to form a 2x2 pixel image an object would have to subtend an angle greater than this.

The star with the greatest apparent angular diameter, and it's unusually large, is I believe Betelguese. Its angular diameter has been measured, using the Hubble telescope, to be 0.000012 degrees, which is much smaller than a pixel.

The blurring caused by the atmosphere actually limits how much effective magnification you can use anyway. Even at the top of a high mountain in good conditions the best you could achieve is about 0.0001 degrees resolution.

Hope this helps.

Caveat: calculations done quickly on the back of an envelope. Someone with more time might want to check them Smile

Last Modified By oldblokeh at 10 Jan 2013 - 11:20 AM
Pete
Pete Site Moderator 1218416 forum postsPete vcard ePz Advertiser England96 Constructive Critique Points
10 Jan 2013 - 11:19 AM


Quote: Generally speaking, I don't believe that photography forum is the best place to learn astrophoto. There are more than a few dedicated astronomy forums around.

Obviously, but it's photography and ePHOTOzine is all about photography...so why not? As you've already seen we've had a few experienced astro photographers step in with advice. including you good self Smile

This is really interesting because one of the things I try to do in articles I write on subjects I know about is step back to the basics and assume no knowledge. I do this because I remember when I started photography being baffled by jargon that my mate threw at me - simple things like teleconverter, slow shutter speed and tripod meant absolutely nothing.

Now here I am right back at the beginning with no knowledge and I'm reading a post that suggest I piggy back the camera. What the hell does that mean? lol

So for those who're also reading this with a "beginners" interest like me, Piggy Backing (I've discovered) is where the camera is mounted on a bracket on the telescope, so it points in the same direction as the telescope and you use your camera lens to record the photo, not the telescope.

I've also discovered that stars are just pin pricks which is disappointing (thanks oldbokeh), because I thought I'd started to hit on new things to photograph. I have others taken with the same technique that look nothing like this one though so maybe I'll build a collection of abstract blurry dots Wink

And I also discovered that the telescope I bought was a total waste of time!

New question @oldbokeh (or anyone else who wants to jump in) Why did you stack the photos in the Pleiades shot? I assume to avoid movement, but if the scope is on a motorised tracking mount do you need to?

edit

Quote: There will be a huge upsurge of interest in astrophotography this week with the BBC2 TV series in full swing.

Oh right I wasn't aware of that. I saw a local TV news piece on a group meeting up in Doncaster I think it was on an annual star gazing public event and all these kids had turned up and were disappointed because it was raining. The guys there had huge scopes. My interest was triggered pre Christmas when I was playing around with the Pentax K-5 and their GPS unit that has a tracking feature using the movement of the cameras sensor.

Last Modified By Pete at 10 Jan 2013 - 11:25 AM
oldblokeh
oldblokeh  2725 forum posts United Kingdom
10 Jan 2013 - 11:26 AM

The reason I stacked them is to eliminate noise as far as possible. The longer the exposure, the more noise you'll get. But the short exposures limit the faintness of object that you can image. What you can do is to shoot a dark frame under the same conditions and for the same exposure length as the real exposures and run them through some clever sofware which will subtract the dark frame and stack the results to give the effect of a much longer exposure. It also corrects for inaccuracies in tracking by correcting misalignments between individual images.

Last Modified By oldblokeh at 10 Jan 2013 - 11:27 AM
Pete
Pete Site Moderator 1218416 forum postsPete vcard ePz Advertiser England96 Constructive Critique Points
10 Jan 2013 - 11:41 AM


Quote: The reason I stacked them is to eliminate noise as far as possible.

Right thanks. I was finding the exposures to record the stars was around 1/6 to 1/60sec so wouldn't have thought noise was an issue, again I may have been doing something wrong. I was using the K-5 at ISO 800-1600 and it's quite capable at these levels. And the stars I were pointing at were those that were easy to see and focus on. I assume then, that any others will need much longer exposure and that's when noise and movement becomes an issue?
What I really want to end up being able to record are those colourful star clusters / nebulas..but I'm a fair way off that at the moment!

oldblokeh
oldblokeh  2725 forum posts United Kingdom
10 Jan 2013 - 11:58 AM

Yes, you'll need to use much longer exposures to capture fainter stars. I believe I used 8 or 9 30 second exposures for the Pleiades shots. Camera was Nikon D90, kit 18-105 lens at 105mm, F8, ISO 400. I didn't have it accurately focused on ininity, sadly, but the stacking software did a decent job of deconvolution.

The faintest star visible in the shot is possibly about magnitude 11 or 12. A magnitude 12 star gives us about 1/40000th as much light as Betelgeuse.

Newdevonian
10 Jan 2013 - 12:22 PM

I am often amazed at the brilliance of some images which are taken on some very commonplace equipment. I am sure that several of us might be interested in the name and source of your "clever software".

oldblokeh
oldblokeh  2725 forum posts United Kingdom
10 Jan 2013 - 12:26 PM


Quote: I am often amazed at the brilliance of some images which are taken on some very commonplace equipment. I am sure that several of us might be interested in the name and source of your "clever software".

I'll look it up when I get home. Bear in mind that the camera was piggybacked on a Meade 8-inch Schmidt-Cassegrain telescope with motor driven polar mount, although similar results could be achieved with a barndoor device. That's barndoor as in astrophotography, not studio lighting!

Nick_w
Nick_w e2 Member 63723 forum postsNick_w vcard England98 Constructive Critique Points
10 Jan 2013 - 12:52 PM

Funny you raise this Pete as I've been looking at having a go at this for a while. I was going to buy a telescope - but that investment will have to wait awhile (wouldnt get another big ticket past the "boss").

I posted a similar enquiry on an astrophotography website a while back - when I get home I will dig it out and post the saliant points (if not answered in the meantime). But when the D800 arrives I was thinking of using a 300mm +1.4 TC and pointing towards Orion, stacking images at various ISO's with respective blanks / offsets in Deep Sky Stacker

oldblokeh
oldblokeh  2725 forum posts United Kingdom
10 Jan 2013 - 1:04 PM

Ah! Deep Sky Stacker. I think that's the one.

Sooty_1
Sooty_1 Critique Team 31125 forum posts United Kingdom192 Constructive Critique Points
10 Jan 2013 - 1:13 PM

The reason to stack many images (and dark frames) is to increase the signal to noise ratio and to eliminate as far as possible hot pixels etc.

Shooting through the eyepiece is called 'parfocal' photography, and replacing the eyepiece with a camera via a T2 adapter is called prime focus photography. You can gain magnification by using a Barlow lens (2x or even 3x) and adding a teleconverter to the camera, usually at the expense of a little quality. Piggybacking the camera on the driven telescope has the advantage of allowing longer exposures, as mentioned above. The length you can get away with depends on focal length, drive accuracy and background light level, but generally with a wide lens like a 24-35mm you can get up to around 30 secs without trails appearing.

I use DSS as well.

You will never get a good detailed picture of a star (apart from the sun....caveats apply!) as they are so far away as to be point sources, but star clusters, nebulae and other deep space objects are possible, as are most sad system planets and the moon.

One thing you won't ever see with your telescope are the Hubble-esque images on the telescope box!

Last Modified By Sooty_1 at 10 Jan 2013 - 1:13 PM

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