Determination of Focal Length from Image...

[JamesF]

5 minutes ago, inFINNity Deck said:

I was surprised Astronomy.net managed to solve the original image with all that annotation and hand-drawn circles! 🙂

I didn't think it would, which is why I didn't try myself

James

[JamesF]

1 hour ago, Captain Magenta said:

Those are the numbers that come out from the equations and measurements, and they combine to produce the following table, which accords with reality as far as how many turns of the knob gets me to focus:

This also gets me wondering...  The quoted focal length for the Skymax 180 is 2700mm.  I wonder if that is because 2700mm is (close to) the point at which the optics are considered "best corrected"?  If so you can presumably then determine the optimal backfocus.  I wonder if there is some test that could be done to determine the "best corrected" position optically?

James

[inFINNity Deck]

  

4 minutes ago, JamesF said:

I wonder if there is some test that could be done to determine the "best corrected" position optically?

 

Yes, that can, see

Nicolàs

 

Edited August 20, 2020 by inFINNity Deck

[JamesF]

5 hours ago, inFINNity Deck said:

Yes, that can, see

An interesting thread.  Almost makes me glad I don't own an RC

I'm assuming you're particularly talking about the use of the Ronchi grating to determine the "best correction" position?  If so I think I might need another read through (or several) to get my head around how that would work.  For the moment I'm assuming that there would be some backfocus distance (where the grating is placed) and the telescope brought to focus where the image from the grating gives lines that are closest to parallel?

James

[inFINNity Deck]

Hi James,

indeed, that is correct, as very well explained by David at the second page of that thread:

Quote

... I think you should look at your focal length. Your focal length of 1607mm is close to what mine was (1604mm) when I first started my investigations. At that focal length my scope was very 'over-corrected' - the mirrors were too far apart. Es Reid checked my scope with a Ronchi grating but over the past month I've been helping someone collimate his scope including checking the focal length. You'll need a Ronchi eyepiece; Gerd Neumann makes one. Focus on a bright star and rack the focus in and out; more and more lines appear as you move away from focus. My guess is that as you go inside focus, you will see this pattern:

A barrel-shaped Ronchi pattern inside focus = overcorrected scope, too short focal length, mirrors too far apart.

Outside focus, I think you will see this:

 

a pinched pattern outside focus, again overcorrected.

As a first guess, let us assume your scope will be properly corrected at the specified focal length of 1624mm. So you are 17mm too short. Look at the chart I gave plotting focal length vs. number of turns of the centre screw of the secondary (page 1 of this thread), you'll see that about 3/4 of a turn of the centre screw should bring you to 1624mm. So unscrew each of the three secondary collimation screws one turn, and then unscrew the centre screw 3/4 turn. This pushes the secondary mirror towards the primary by about 3/4mm.  Now tighten the three collimation screws by an equal amount and check on the star again with the Ronchi eyepiece. The lines should be parallel inside and outside focus. If so, your scope is working fully corrected. If you do not get parallel lines, you'll need to keep adjusting.

The Gerd Neumann eyepiece come with a manual which you can download from the Teleskop Service website.

Note that if the patterns are the other way around - pinched pattern inside focus and barrel pattern outside - you've gone too far and the scope is now under-corrected and the mirrors are too close together. 

(I had to move my secondary by four full turns of the centre screw and adjust the primary mirror up the tube until I got a corrected image. My corrected focal length is 1660mm. I'm not far off not being able to focus without another extension tube.)

I use Pixinsight to tell me the focal length of the scope. Take a single image of a star field, say an open cluster, and use Script/Image analysis/Image solver.

Once you are at the correct focal length you will need to recollimated the scope, hopefully by only adjusting the secondary.

David

 

 

This week I received my photographic ronchi-eyepiece and last night I put it to the test using my 11" EdgeHD SCT. On that SCT I have a MoonLite focuser, so I was wondering what the best combination of mirror and focus setting would be. Last year I made a contraption that allows me to do afocal imaging using a ZWO ASI290MC and it appeared to function equally well on the ronchi eyepiece. The ASI290MC was equipped with ZWO's CCTV-type lens and focused at infinity. Then the mirror and focuser were moved until I got a ronchi pattern. Seeing was terrible and as below video shows, it was far from easy getting proper results. During the recording I played with both the mirror and MoonLite focuser, so the grating goes in and out of focus and even the SCT's mirror-shift can be seen. Clearly it would be beneficial to put the SCT in front of my collimator to do this test, but that means I have to take it of my rig.

The video plays at 3 x actual speed.

Nicolàs

ronchi.avi

Edited August 21, 2020 by inFINNity Deck