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wide-field astrophotography: Focus assistance

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#1 Nico_or

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Posted 22 November 2018 - 08:31 PM

I'm have done some wide-field astrophotography on a tripod and planning to try piggybacking over my telescope (manual tracking, low focal length).

 

I use a Pentax MX with the stock focussing screen (matte+microprism+split) and the lenses I got are a 28, 50, 135 and 75-300mm in focal length.

Locking forward the next local star party I was thinking of getting something to assist focusing. Options available here are a small bahtinov mask or a 90 degree viewfinder with 1x and 2~3x  magnification. Both are somewhat more expensive than I think they should be.

So the questions are:

  • At those focal lengths, can I get significative improvement over just using the viewfinder?
  • It's even possible to use a Bahtinov mask on a SLR or the viewfinder is too dark/small to even see the diffraction spikes?
  • Are there any other focus assistance that you guys could recommend?

Thanks in advance for any suggestions,

Nico

 

 



#2 Cfreerksen

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Posted 22 November 2018 - 09:24 PM

I'm have done some wide-field astrophotography on a tripod and planning to try piggybacking over my telescope (manual tracking, low focal length).

 

I use a Pentax MX with the stock focussing screen (matte+microprism+split) and the lenses I got are a 28, 50, 135 and 75-300mm in focal length.

Locking forward the next local star party I was thinking of getting something to assist focusing. Options available here are a small bahtinov mask or a 90 degree viewfinder with 1x and 2~3x  magnification. Both are somewhat more expensive than I think they should be.

So the questions are:

  • At those focal lengths, can I get significative improvement over just using the viewfinder?
  • It's even possible to use a Bahtinov mask on a SLR or the viewfinder is too dark/small to even see the diffraction spikes?
  • Are there any other focus assistance that you guys could recommend?

Thanks in advance for any suggestions,

Nico

Bahtinov  masks are pretty cheap. If near perfect focus for under $20 is too pricey for you, then do it yourself.

 

http://www.deepskywa...tinov-mask.html

 

Just find a bright star and adjust your exposure long to see the diffraction spikes. Then go to your target. No refocus needed.

 

Chris


Edited by Cfreerksen, 22 November 2018 - 09:27 PM.


#3 fmeschia

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Posted 22 November 2018 - 10:00 PM

 

Just find a bright star and adjust your exposure long to see the diffraction spikes. Then go to your target. No refocus needed.

The problem is that, with a film camera, test exposures are not really an option.

Francesco


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#4 Mark326

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Posted 22 November 2018 - 10:15 PM

What is film?  Haha... 

seriously, possible idea, a flip mirror... could visually verify focus via bahtinov mask and flip mirror, assuming distance to film focal plane would be same focus... just a thought.

 

good luck and Clear Skies.


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#5 gezak22

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Posted 22 November 2018 - 10:25 PM

I know nothing about film photography, so take this with a grain of salt:

 

Add a Vernier scale to your focusing mechanism. Do a couple of test shots during the day to determine the perfect focus for infinity. This will probably only be half accurate as temperature changes will move the perfect focus position. But maybe it's accurate enough for the shorter focal lengths?



#6 Michael Covington

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Posted 22 November 2018 - 10:27 PM

You'll need some magnification.  The Canon Angle Finder 3 is very good (gives a brighter image than most) and fits almost all SLRs with rectangular eyepieces, not just Canons.  It comes with adapters for (if memory serves me right) 3 kinds of eyepiece frames.

Once you have magnification, you can put a Bahtinov mask to good use.



#7 Cfreerksen

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Posted 23 November 2018 - 12:09 AM

The problem is that, with a film camera, test exposures are not really an option.

Francesco

Film, yeah, I had a camera like that a few decades ago. I take it you have a screen and not a viewfinder, right? Even with a screen, I think you could see a very bright star diffraction pattern. If your camera screen has a zoom function use it. Sorry, I wish I could do better for ya. I know i had a terrible time 30 years ago getting a good focus at prime focus through a viewfinder.  But then I didn't have a bahtinov mask. Had a ton of fuzzy moon photos from the fotomat.

 

Chris

 

Edit: I do recall something. I had a issues with vibration too. When the SLR mirror and shutter moves it would vibrate my scope. I learned the "hat trick". Hold a hat or other light blocker in front of the scope. Opened the shutter, remove the hat for your desired exposure. Move the hat back to block the light, then close the shutter. It's pretty old school but it works. Good luck!


Edited by Cfreerksen, 23 November 2018 - 12:16 AM.


#8 telesonic

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Posted 23 November 2018 - 12:59 AM

I'm with Michael C. on this one...

 

Since you don't already have a right-angle magnifying finder for your setup, get one - it's a useful tool to have in your camera bag, and you won't get a stiff neck trying to look through the viewfinder. 

 

Not sure on the Bahtinov, it would work for most lenses I would imagine... but for cheap, definitely go with a little do it yourself and make a Scheiner or Hartmann mask - which is probably less accurate than the Baht, but you could make a test one out of thick construction paper to see how much it dims the image and if it suits you.  

 

There is another more elaborate method that uses a process of a timed exposure and then covering the lens, and repeating as you move closer to infinity - but that requires making a focus scale for the lens, then examining the exposed film to determine which is the best. Probably not for the faint of heart, and I haven't tried it personally - just recently read about it.

 

Hope you get some keepers.

T



#9 Michal1

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Posted 23 November 2018 - 02:47 PM

In my experience, Bahtinov spikes on the split screen are too dim. Relying on the viewfinder is somewhat risky because it has to be calibrated properly, i.e. if the image is focused in the viewfinder, it doesn't have to be necessarily exactly focused on the film. I do this:point the camera to a sparsely populated field of bright stars (e.g. the Big Dipper). Set the focusing ring in the first limit position. Expose for ca. 30 sec. Cover the lens by a cap or so. Write down a note how the focusing ring was placed. Move the mount in R.A. Set the focusing ring in the next position. Uncover the lens carefully and repeat. When all focusing ring positions are tested, one has to mark the end of the series, e.g. by moving the mount while the lens is uncovered. The result is a film frame with a series of differently big points ending by a line. A Vernier scale is usually not necessary. There is usually some symbol on the focusing ring that can be used for defining the position. If it is, for example, the infinity symbol, then your lens positions can be 'the beginning of the first circle', 'the middle of the first circle', etc. This allows you to find the best focus position using a single film frame.

 

In a more advanced version, you use a special film frame for every tested position and use the exposure time that you use for taking regular astrophotos.

 

For those without a mount, there is a similar star-trail method:

http://www.astropix....us/methods.html

At this link, you can find more inspiration for focusing.



#10 Michael Covington

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Posted 23 November 2018 - 03:15 PM

While you're at it, take note of which one looked best in the viewfinder.  That will tell you whether you can trust the viewfinder.

Back in film days, I found that Beattie Intenscreens were the best focusing screens for astrophotography.  Second best were overall fine-matte screens.  (Crosshair screens, as used in photomicrography, were useful only for planet imaging at high f-ratios.)  Microprisms and split-rings were useless -- focus on the matte area outside the central microprism or split ring.


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#11 Michael Covington

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Posted 24 November 2018 - 02:16 PM

The best setup is probably a Nikon F3HP with "stovepipe" finder (which you look straight down in) and an assortment of fine-matte screens.

 

(Does anybody want to buy this setup from me?  I am no longer using it... might be persuaded to part with it...)



#12 SteveInNZ

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Posted 27 November 2018 - 03:46 PM

You could try advertising for a Stiletto knife edge focuser. There must be a few of those kicking around, waiting for the time that they'll become useful again.

It's probably blasphemy but, could you put a DSLR in to focus and then swap out for the film camera ?

 

Steve.


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#13 TxStars

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Posted 27 November 2018 - 10:55 PM

You 

 

You could try advertising for a Stiletto knife edge focuser. There must be a few of those kicking around, waiting for the time that they'll become useful again.

It's probably blasphemy but, could you put a DSLR in to focus and then swap out for the film camera ?

 

Steve.

You can swap most if not all of the Nikon SLR and Nikon DSLR cameras.. the body's are the same thickness from the front mount to sensor/film plane..*at least the ones I currently have..


Edited by TxStars, 27 November 2018 - 10:59 PM.


#14 Alen K

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Posted 01 December 2018 - 07:07 PM

The DSLR idea will work. I confirmed that infinity focus with my Pentax lenses on my Pentax DSLR is at the same position it is (was, since I no longer use it) on my Pentax K1000 film camera.

Another thing you could do that does not require a Pentax DSLR is a knife-edge focus test for all your lenses with film not in the camera. You can easily make a test device out of a piece of plexiglass and a film chip. See the article attached to the following post here in the film forum: https://www.cloudyni...2/#entry8879773

What will that tell you? It will tell you if your lenses are actually focused at infinity when set to the infinity stop. This turned out to be true for all of my old Pentax-M and Pentax-A manual-focus lenses. If that's not true for any of your manual lenses or if instead you have autofocus lenses, it will tell you where they actually focus at infinity and you can permanently mark the barrels appropriately. Now focusing at night is as easy as turning the lens barrel. That's all I do.

BTW, my OM-1 has a Beattie Intenscreen and I have an Olympus Varimagnifier right-angle magnifier that also fits on the K1000. These were not sufficient to judge accurate focus with whatever I attached, be it a lens or my telescope. (For the telescope I always used the home-made knife-edge focuser described in that article.)

Edited by Alen K, 01 December 2018 - 07:17 PM.

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#15 Michael Covington

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Posted 01 December 2018 - 08:18 PM

BTW, the Canon Angle Finder 3 fits Olympus and Pentax and gives a considerably brighter magnified image than the Varimagni.

 

Best of all is the Nikon DW-4 "stovepipe."

 

But these just show you the focusing screen.  Testing at the actual focal plane is better.




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