Jump to content

Sketches

Afocal vs Projection vs Prime Focus


Recommended Posts

I've seen a few threads about each of the modes of imagining, I was wondering if anyone had a link to a description of the three and a comparison or recommendations for which to use for different purposes?

I would like to get an image equivalent to what I see through the eye piece so I imagine projection is the path I would like to take. But I thought I should get some advice first.

I would appreciate any advice or tips.

Link to comment
Share on other sites

Afocal imaging = holding camera (point and shoot) up to the ep on a scope. A universal digital camera adapter is a good way of making it easier then just holding the camera.

Prime Focus imaging = attaching DSLR camera to scope (no ep in the scope and no lens on camera) by way of a t-ring and mount adapter.

Eyepiece Projection imaging = attaching DSLR to scope (with ep in scope). I think extention tubes,barlows etc are needed for this method as well as the t-ring and mount adapter.

http://www.astronomyforbeginners.com/astrophotography/prime-focus.php

Edited by LukeSkywatcher
  • Like 1
Link to comment
Share on other sites

cantab said:
I think afocal method and eyepiece projection are two names for the same thing.

Not quite. Afocal imaging - an image taken through an eyepiece using a camera with a lens. Eyepiece projection - an image taken through an eyepiece with the camera body attached directly to the eyepiece - no camera lens attached. Prime focus - camera body attached directly to the telescope - the telescope lens effectively becomes the camera's lens.

Peter

Link to comment
Share on other sites

  • 5 years later...

Thank you so much for making that a little much more clearer LukeSkywatcher.  I know it has been more than 5 years but those definitions helped me a lot.  I am just starting astrophotography and it seems that it is getting better and better with time.  I used the Eyepice Projection Imaging with a T adapter and it seems that the image is not as clear as I wanted them to be.  In fact the best Moon image and video were done (so far) from my bedroom (inside the house) with a tripod (using afocal imaging) attached with my DSLR Nikon camera.  That means there were 4 "filters" before I could see the pictures.  Filter meaning the layers (main window, telescope lens, eyepiece lens and camera lens).  Cannot wait to try this technique (afocal) outside with my camera suppport that I just bought (not delivered yet).  Feeling confident that it will do the trick.  Again your post was a few years ago but shows that someone is reading this and I find that educational.  Keep up the good work!  Happy stargazing!

Link to comment
Share on other sites

17 minutes ago, Alien 13 said:

I have never had decent results with eyepiece projection, the similar method using a barlow lens instead gives far superior results in my view for Lunar/Planetary imaging.

Alan

The more extension that we put between the camera and EP the more the light is reduced.

It is a reasonable method to split doubles and image moon but I found planets not so good.

  • Like 1
Link to comment
Share on other sites

The biggest problem with eyepiece projection is that most eyepieces don't project a flat field, so the edges will be out of focus on a flat image plane.  Pentax made the XP line specifically for eyepiece projection as a rare example of a dedicated projection eyepiece line.  I've also read that microfiche projection lenses can be adapted to this purpose as well.  All in all, I stick with afocal projection using an eyepiece with a large eye lens and long eye relief projecting into a camera lens that has an objective lens at least slightly smaller than the eyepiece's eye lens.  Any other combination results in vignetting.  I've also had good luck using my GSO coma corrector in my dob attached to my DSLR via a T-ring.  It flattens the Newt's field, corrects coma, and allows me to reach focus with only a 10% increase in focal length.

 

Link to comment
Share on other sites

13 hours ago, Chris33benoit said:

 I used the Eyepice Projection Imaging with a T adapter and it seems that the image is not as clear as I wanted them to be.  In fact the best Moon image and video were done (so far) from my bedroom (inside the house) with a tripod (using afocal imaging) attached with my DSLR Nikon camera.  That means there were 4 "filters" before I could see the pictures.  Filter meaning the layers (main window, telescope lens, eyepiece lens and camera lens).  Cannot wait to try this technique (afocal) outside ......

At least part of the reason for the "not as clear", is shooting through the window of your house. A windowpane is not optical quality, the thickness varies over the dimensions of the pane, and depending on the angle of the pane relative to the line of sight, it can induce varying amounts of diffraction, which can blur or distort the image you'll get. Worse if it's a double pane like e-glass. You'll certainly get a better result when you take the scope outdoors.

I'm also getting ready to try some eyepiece projection, will start with the Moon. I have a couple of ways to try it, one with the extension tube that carries an EP forward of the T ring, and I have a Celestron Ultima EP that has threads to screw a T-ring directly to the EP. Hoping to try this out this coming weekend, the Moon will be early 1st qtr so should be some good contrast along the terminator.

Good luck in your experimentation. This stuff is FUN.

Link to comment
Share on other sites

  • 2 weeks later...

I finally had the opportunity a couple of evenings ago to try EP projection photography of the Moon. I was a bit hurried so my composition isn't all that great, plus I was having some focal issues, as I wear bifocals and have difficult focusing when I'm looking at 'live view' on my camera's screen. I also just did a rough polar alignment and manually skewed, rather than align the Go-To and track, so I was using rather fast shutter speeds to compensate for movement.

I found that the biggest difference between shooting at prime focus vs. EPP is the ISO required. My prime photos were good at ISO 400 or 800, with shutter speeds in the 1/125 -1/150 range, but when shooting through my 13mm Ultima EP, I had to increase ISO to 1600 if shooting at the disc, or to 3200 if aiming along the terminator.

1st image is prime on my Edge 8", the light path is through a 2" visual back and 2" diagonal, with my camera mounted on the diagonal.

2nd image is with the camera attached to my 13mm Ultima EP, which is threaded for a T-ring. EP is in the diagonal. That's about 154X. These give you a general idea of image size. I was disappointed I didn't get better focus on the 2nd image. Maybe I should have shot faster shutter speed, maybe I just wasn't looking through the right part of my glasses when I focused.

DSC_0670.JPG

DSC_0678.JPG

Edited by Luna-tic
  • Like 1
Link to comment
Share on other sites

I'm guessing you were using a 13mm Ultima Duo eyepiece?  I don't know if the Ultima LX version had threads on top.  I don't know if the the original Ultima line of Plossls had a 13mm, let alone threads on top.

If the 13mm Ultima Duo is anything like my 13mm AT AF70 which is the same as the Ultima LX except it does have threads on top, it has loads of astigmatism and chromatic aberration in the outer 25% of the field.  The image can't be any sharper than the weakest link in the imaging train, so I'd say that's where the blurriness is at least partially coming from.

Your increased exposure time is a direct result of increasing your magnification with the eyepiece.  The equivalent exposure time to prime focus might be achieved by shooting through a 40mm eyepiece, roughly speaking.  Assuming that, going with an eyepiece of roughly 1/3 of that focal length means you'll need 3 times as long of an exposure, or bump up your ISO by 3 notches.  That's about what you experienced.

  • Like 1
Link to comment
Share on other sites

Thanks for the reply; after I had to bump the ISO, it dawned on me (newbie-ness at AP, but experienced at basic photography) that I was getting less light with the EP. The 13mm is the only Ultima Duo I have, and it was the quickest EP to use to shoot a few quick shots. Given that the middle of the image is also a bit out of sharp focus, I think it was my old eyes and not the EP that were the issue. I have extension tubes and photo adapters for the rear cell, but it takes longer to screw them together and get things right. I did the whole photo excursion, from setting up the mount and tube to packing it all away, in about 45 minutes; like I said, I was a bit hurried, but the Moon looked so nice, it was clear, and I couldn't resist. I'll try it again when I have all night, I'll play with all my EP's (all 4 of them, lol) and may even drag out the Barlows, mix and match and compare. I make notes of all my exposures and lens/EP combinations for any photos I make, AP or otherwise, so I can refer back to what worked or didn't work. My old brain doesn't maintain stuff like it used to.

Link to comment
Share on other sites

I think for me so far (complete newbee!)

The major difficulty that I have to overcome, with either prime focus or  eyepiece projection, is the focus shift between camera (DSLR) and eyepiece.

I can find an object with an eyepiece, achieve reasonable focus, attach (for projection) or switch to (for prime) and have to majorly adjust the focus, which through the viewfinder or on screen with a DSLR is almost impossible!

I have made but not tried a bahtinov mask, and today , received an eyepiece that has M42 threads for my T-Mount!  So as soon as the skies clear I will have another go!

As for the OP.  One Vs the other - in my very limited experience - Prime = Big Things like Nebulae etc. Eyepiece = Planets and the Moon!

 

Cheers MZ 

Link to comment
Share on other sites

4 minutes ago, ManixZero said:

I can find an object with an eyepiece, achieve reasonable focus, attach (for projection) or switch to (for prime) and have to majorly adjust the focus, which through the viewfinder or on screen with a DSLR is almost impossible!

That's one advantage to afocal projection.  If the image is in focus in the eyepiece when using distance corrected vision (wear your eyeglasses if you need them for distance vision) and your camera lens is set for infinity, you'll be able to see an image right away that is nearly in focus.  A slight touch-up with the telescope focuser will usually take care of the rest.  Another advantage is that the camera's lens is much more forgiving of a curved focal plane being projected by the eyepiece thanks to the lens's depth of focus than the plane flat imager itself when using eyepiece projection.

  • Thanks 1
Link to comment
Share on other sites

8 minutes ago, Louis D said:

That's one advantage to afocal projection.  If the image is in focus in the eyepiece when using distance corrected vision (wear your eyeglasses if you need them for distance vision) and your camera lens is set for infinity, you'll be able to see an image right away that is nearly in focus.  A slight touch-up with the telescope focuser will usually take care of the rest.  Another advantage is that the camera's lens is much more forgiving of a curved focal plane being projected by the eyepiece thanks to the lens's depth of focus than the plane flat imager itself when using eyepiece projection.

So, what I need to be doing is attaching a simple (but good) lens to my camera - something like my M50mm f1.4 set to, but not beyond, infinity, - then attaching that via a 49mm to 42mm adapter ring to the eyepiece projection adapter or the eyepiece with the built in 42mm threads?  I have just ordered - This

I will report back once the ring has arrived and I have tried it out!  (I will try it with a couple of lenses (50mm, 35mm, 24mm) and see how I get on (clear skies allowing of course)

Always good to have multiple options!

 

Cheers

MZ

Link to comment
Share on other sites

2 hours ago, ManixZero said:

So, what I need to be doing is attaching a simple (but good) lens to my camera - something like my M50mm f1.4 set to, but not beyond, infinity, - then attaching that via a 49mm to 42mm adapter ring to the eyepiece projection adapter or the eyepiece with the built in 42mm threads?  I have just ordered - This

I will report back once the ring has arrived and I have tried it out!  (I will try it with a couple of lenses (50mm, 35mm, 24mm) and see how I get on (clear skies allowing of course)

Always good to have multiple options!

 

Cheers

MZ

You may see some vignetting because the edge of the camera's front lens is looking beyond the eye lens of the eyepiece.  It's really hit or miss what works well together.  I've found fixed lens cameras with smaller lenses due to smaller imaging chips interface best.  I still use my 2002-era Olympus C-4000 for this reason.  The buttons are dying on it, though.  Don't be afraid to try slow, short telephoto lenses either.  You also want to match the apparent field of view (AFOV) of the eyepiece to the field of view angle of the camera lens.  Your Ultima Duo has a 68 degree AFOV; therefore, any lens with a 35mm equivalent focal length greater than or equal to 32mm (68.1 degrees field of view on the diagonal) will not see past the field stop and the image will fill the frame.  That C-4000 just so happens to max out at 32mm equivalent in a 3:4 format and interfaces marvelously with 65 to 70 degree eyepieces.  You also don't want to zoom a camera lens at all.  It needs to be left at the widest or the eyepiece exit pupil and lens entrance pupil don't mate very well in my experience.  In addition, you can set the camera lens lower than wide open, down to the aperture that matches the focal ratio of the telescope.  Beyond that, you may get vignetting.  Magnification is determined by your eyepiece alone as long as you're taking in the entire 68 degree field and nothing more.  If you take in a wider angle with a lens of less than 32mm and get black in the corners, magnification is will be less.  If you take in a narrower angle with a lens of greater than 32mm cropping the image, magnification will be higher.

  • Thanks 1
Link to comment
Share on other sites

57 minutes ago, Louis D said:

 You also want to match the apparent field of view (AFOV) of the eyepiece to the field of view angle of the camera lens.  

So my DA21mm (31.5mm in 35mm equivalent) should be almost spot on!  I will set it at the lens sweet spot of between f/4.0 and f/5.6 because at the focal ratio of my 4SE (f/13) the lens starts becoming diffraction limited (after f/11!)

All I need is a few cloudless nights to try out the EP Projection and the 49-42mm ring (and clear skies) for the Afocal test!

Thanks Louis for the pointer in the Afocal direction!

Cheers

MZ

Link to comment
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now
 Share

×
×
  • Create New...

Important Information

We have placed cookies on your device to help make this website better. You can adjust your cookie settings, otherwise we'll assume you're okay to continue. By using this site, you agree to our Terms of Use.