Username:

Password:

Altair AstroDIO DehumidifiersAtik CamerasModern AstronomyDavid HindsNe3 Filters

Author Topic: DSLR Focusing - A Quick Guide!  (Read 6085 times)

0 Members and 1 Guest are viewing this topic.

Offline the fordster

  • Galactic Poster
  • ******
  • Posts: 20128
  • while (!(succeed=try ()));
    • 56 Degrees North
DSLR Focusing - A Quick Guide!
« on: 10:07:02, 13 June, 2007 »
Was tidying through my website and found this tutorial which I thought I'd update & share...

Ask any astrophotographer what one of the main causes of ruined images are, and chances are they’ll say “poor focus”. The main problem is that you are usually focusing on a dim pinpoint of light with a very forgiving and advanced optical system - your eye... However, there are many ways to achieve this tricky feat, and as you'd guess - Some work far better than others.

I've tried them all and, below, I have listed them in order of accuracy / ease of use (according to my tests :cool:)

1)   A Parfocal Eyepiece

A parfocal eyepiece is simply a regular eyepiece that has a focal length the same as your DSLR backfocus distance.  This distance is achieved by trial and error focusing of the DSLR to nail focus, then replacing it with an eyepiece and a parfocal ring (these rings are available from many of our sponsors).  Don't touch your focuser, but focus the eyepiece by sliding it in/out then tightening the parfocal ring.

Unfortunately, setting the proper distance with a parfocal ring is usually a hit-or-miss operation, and parfocal rings are always susceptible to slipping.

Also, even with a good design, proper calibration, and skill, all parfocal eyepieces will fail to perform properly in the end, as your eye automatically compensates for images that are out of focus by up to 3%.  So, even though you think you have attained a pinpoint image through a parfocal eyepiece, this still may not correspond to the proper film-plane or backfocus distance necessary. 

2)   Magnify the Image

Many people get a better focus by magnifying the image through the viewfinder. This attempts to compensate for the smallness of star images through a viewfinder, and also reduce the effects of the eye’s focus compensation.

There are two ways of magnifying the image on a DSLR – One is to use a magnifying assembly that attaches to the viewfinder.  The other is to “playback” the image and check star focus by “zooming in".

The latter is certainly cheaper than the former, and far more accurate, as most of the magnifying assemblies on the market do not provide enough magnification to be useful. Furthermore, they dim the image, making focus even harder.

With a bit of practice, you can achieve very good focus by the iterative process of snapping an image, playback & zoom in, check focus, adjust focuser, snapping an image etc… Once focus is reached, lock the focus mechanism and you’re done…

3)   Make a Hartmann Mask

A homemade Hartmann mask is an incredibly effective piece of kit, providing you have a bright star to focus on.  If you don’t, simply slew to a bright star, focus and slew back to your object.  Providing your ‘scope doesn’t suffer from mirror flop or other pains, you should be still in focus.

The mask simply consists of two or more identical circular or triangular holes cut into a piece of card, wood or metal that fits over your OTA (there are already many posts and links on UKAI for Hartmann mask designs).  As the mask blocks out all light - except the light entering through the holes - you actually see an image of the holes when you look at a bright source.  As you focus your ‘scope, the holes appear to come together.  Simply focus until only one circle appears instead of many. This technique can even be used on camera lenses.

4)   Focusing Software

There are some excellent programs out there for focusing your DSLR using FWHM routines (FWHM means Full Width at Half Maximum, and refers to the width of an object with soft edges, judged by brightness) - Simply tweak focus until you get the lowest possible FWHM number and you have focus nailed.  My choice for this is MaxIm DL, but others include the excellent Images Plus.

Whilst this is undoubtedly the best way for CCD imaging, the trouble with a DSLR is that focusing using software can be a VERY lengthy process due to the snap a pic, download a pic cycle (though this is less of a problem now with the new run of USB2 DSLRs).  I have found myself wasting over half an hour trying to get a decent focus with software.  There are just too many iterations necessary to get a pinpoint image - As you are constantly fighting seeing, you may think you have focus nailed, but still need to tweak a bit - Therefore every focus adjustment needs at least three or four snaps to ensure a good average FWHM

5)   Diffraction Focusing

Diffraction focusing is a relatively easy procedure.  In this approach, the cross pattern of the vanes in the secondary holder of a reflector telescope are used for focusing.   For other telescope designs without vanes, twine or cotton can be placed in a cross in front of the telescope to simulate them.

In this procedure, a very bright star must be used.  A three to five second image is taken, and then you refocus repeatedly, until the four diffraction spikes generated by the interference are distinct and bright. In a broad sense, the procedure is very similar to using a Hartmann mask except for the pattern.   

It is one of the most accurate and cheap methods out there.  However, like software focusing, it can be very lengthy, as again you need to take repeated exposures and refocus in between each one.

6)   A Ronchi or Knife-edge Focuser

This is simply the most accurate way of focusing, and the one “the professionals” use.  It is also my preferred way of doing things with DSLRs. Stellar Technologies International have introduced an incredible piece of kit for CCD and SLR/DSLR focusing, called the Series 4 Stiletto focusser.  They are fairly expensive new, but come up fairly regularly second hand - They can be used for Prime Focus, Eyepiece projection, and even to focus camera lenses.  Also, they work equally well with any optical system – Refractor, Reflector or Hybrid (such as the SCT)

Details can be found on Richard’s site here:

http://www.stellar-international.com/

Please note  - I'm not in any way affiliated with Rich Shell at STI. It's just the only product I know of that does this :cool:...

Hope this tutorial helps the DSLR imagers!  :urock:


Offline Hoohoo

  • Poster God
  • *****
  • Posts: 1412
Re: DSLR Focusing - A Quick Guide!
« Reply #1 on: 10:37:08, 13 June, 2007 »
WOW, now thats how a tutorial should look like.

Have som karma.

Kim
Kim

Skywatcher 190MN
Skywatcher HEQ5 Pro
Canon EOS350 Cold Finger modded and removed IR filter
QHY5 Finder Guider

Offline the fordster

  • Galactic Poster
  • ******
  • Posts: 20128
  • while (!(succeed=try ()));
    • 56 Degrees North
Re: DSLR Focusing - A Quick Guide!
« Reply #2 on: 10:45:41, 13 June, 2007 »
Thanks Kim - Seems a shame to have that info kicking about unused so thought I'd share :cool:

Offline tasco

  • The Regulars
  • Full Member
  • ***
  • Posts: 383
    • astro rejects
Re: DSLR Focusing - A Quick Guide!
« Reply #3 on: 12:41:53, 13 June, 2007 »
wow! thats a great comparison of different focusings- i was looking for something like this as they say 'in a nut shell'- ive been doing this with the best success so far- after i snap the image i use the camera zoom (10X)- then use a 4" magnifying glass over the viewfinder- this seems at least 3X- so i guess i can see the star at 30X or better? its actually works great and is simple enough--- problem with this is-- with such a large zoom and the limited # of pixels on the viewfinder, doing any in/out tweaks on the foucuser really doesnt change the image that much!!- so in the meantime i just found 10d remote and downloaded it- it shows a rather large clear image on my  comp screen- so im going to try this next time and see what happens- i got the 'unregistered' version which is a freebie-its actually better- LOL, because it wont save any images, thay way i wont have to worry about deleting anything or cluttering up my comp with test images
if at 1st you dont succeed- join the club!

Offline dciobota

  • Daniel
  • Galactic Poster
  • ******
  • Posts: 9722
Re: DSLR Focusing - A Quick Guide!
« Reply #4 on: 06:37:25, 14 June, 2007 »
  Excellent writeup Fordy!  How bout making it a stickie too?  :urock:  :cool:
PLEASE REMOVE ME

Offline NickH

  • Galactic Poster
  • ******
  • Posts: 8566
  • It aint just the weather that's CaK
    • The Cherhill Observatory
Re: DSLR Focusing - A Quick Guide!
« Reply #5 on: 09:16:19, 14 June, 2007 »
And 100-% agreed on the ST1V...corking bit of kit...

Celestron C11: DSI-C/Toucam Pro 2 ICX mod and SC1.5 mod/ Meade LPI/EOS300D/ED80/WO80ZS/Shoestring guider. PST/PST CaK/ART285

No time....always bloomin cloudy...need to build the Obsy..

Offline coxellis

  • The Regulars
  • Galactic Poster
  • ***
  • Posts: 16819
  • Rowlands Gill.
    • Coxellis Astroimaging
Re: DSLR Focusing - A Quick Guide!
« Reply #6 on: 09:49:45, 14 June, 2007 »
yup get it in the stikies mate - good work :D
Obs: EQ6 SkyScan; Field: EQ5 SynScan
Scopes: MN76 : Equinox 80
Cams: ART285 : BWSC1 : 1000D : 450D

>coxellis.net<
>astronomiser<

Offline the fordster

  • Galactic Poster
  • ******
  • Posts: 20128
  • while (!(succeed=try ()));
    • 56 Degrees North
Re: DSLR Focusing - A Quick Guide!
« Reply #7 on: 09:55:54, 14 June, 2007 »
Now stickied 8) Thanks for the thumbs-up, guys!

Offline NickH

  • Galactic Poster
  • ******
  • Posts: 8566
  • It aint just the weather that's CaK
    • The Cherhill Observatory
Re: DSLR Focusing - A Quick Guide!
« Reply #8 on: 11:51:35, 14 June, 2007 »
Great stuff Fordy...

(sleep deprivation..baby's got an ear infection....please excuse...)

 :lol: :lol: :lol:

Celestron C11: DSI-C/Toucam Pro 2 ICX mod and SC1.5 mod/ Meade LPI/EOS300D/ED80/WO80ZS/Shoestring guider. PST/PST CaK/ART285

No time....always bloomin cloudy...need to build the Obsy..

Offline the fordster

  • Galactic Poster
  • ******
  • Posts: 20128
  • while (!(succeed=try ()));
    • 56 Degrees North
Re: DSLR Focusing - A Quick Guide!
« Reply #9 on: 12:42:42, 14 June, 2007 »
  kjhugh :lol:

Offline cygnusx1

  • Senior Member
  • ****
  • Posts: 556
Re: DSLR Focusing - A Quick Guide!
« Reply #10 on: 14:15:28, 29 August, 2007 »
Ok maybe this is obvious, but what about an angle finder with a 2x ??

Not cheap, but it does have other uses.
Kevin

"Freedom is not the right to do as you please, but the liberty to do as you ought"


C9.25 SCT,WO 110 ED APO,Meade 8" F/4 SN
EQ6 Syntrek w/EQMOD
450D,modded 400D,DSI I,SPC9k
18-55 IS,135,80-200,70-300 APO

Offline philjay

  • Poster God
  • *****
  • Posts: 1428
    • http://www.geocities.com/philjay_2000
Re: DSLR Focusing - A Quick Guide!
« Reply #11 on: 19:19:41, 29 August, 2007 »
Good article Andy youve covered all the points well.

Hi Cygnus, I have tried an angle finder as well as most of the other options but the angle finder still relies on your eyesight, the Ronchi screen costs about the same and does not rely on getting a pinsharp image, it in fact does the oppossite, you need to have just a white screen and no dark bars to be in focus, much easier to judge.

After trying all the options over the pat few years I am in agreeement with Andy, the STI Focuser, (Ronchi) has got to be the quickest accurate method.

Phil
Grow old disgracefully and make it worthwhile

www.geocities.com/philjay_2000

CGE 1100, Zenithstar 80 APO, HEQ5, 127 Apex Mak, Hutech Canon 300d, Toucam unmoded, Mintron 12V6EX, Orion 102ED

Offline twinsen

  • Full Member
  • ***
  • Posts: 464
    • uk-astronomy - COMING SOON
Re: DSLR Focusing - A Quick Guide!
« Reply #12 on: 22:52:31, 13 September, 2007 »
I was wondering i havnt been able to get my sti to work properly. It says you are meant to see alternating bands (diffraction pattern?) but it does not seem to produce any kind of banding at the eyepiece. When i look though it seperate from the scope it appears to work and splits the image.

Am i doing something wrong or is it just not getting enough light in to create a visible pattern?

I have a 100mm f9 refractor so it should work with the lower resolution grating right?

Thanks
Alex
Vixen GP - Celestron 102 f9.8  - Canon EF300f4

Offline brianday

  • Full Member
  • ***
  • Posts: 182
Re: DSLR Focusing - A Quick Guide!
« Reply #13 on: 21:35:23, 09 October, 2007 »
I have looked at all these and ended up with software -
I am using focusassist which is free
Its not perfect - you need to be nearly focussed first whch I do by looking in the viewfinder

early days yet - I now have to do some drift aligning to enable shots over 60 secs
I do notice that the focus can drift in 30 or 40 mins

http://www.xmission.com/~jstanley/focusassist/

there is a discussion on flickr
http://flickr.com/groups/deep_space_astrophotography/discuss/72157600001044075/

not bad for nowt
Brian

Offline astropilch

  • Galactic Poster
  • ******
  • Posts: 9561
Re: DSLR Focusing - A Quick Guide!
« Reply #14 on: 13:47:50, 21 June, 2008 »
Good stuff mate!!
Alan :D
Nothing.......zilch :(

 

ukbuysellRemote Imaging from AustraliaSharpSkyblank APTUKAI on Facebook
Powered by SMF 2.0.15 | SMF © 2006, Simple Machines LLC
DarkBreak by DzinerStudio. Theme modified by The UKAI Team

Page created in 0.407 seconds with 37 queries.
TinyPortal © 2005-2012