SCT focussing and changing focal length: some data

[Adrian]

The fact that SCTs & Maks increase focal length as the focal plane is moved back was raised in a different thread earlier, which reminded me that I took some measurements a few weeks ago – I thought I’d post them in case it helps anyone or if there are any comments.

Chris Lord did some work on this here: http://www.brayebrookobservatory.org/BrayObsWebSite/BOOKS/EFLMAKCASS.pdf

but I thought I would check this myself.

I was collimating my 10" SCT by using an artificial star (ball bearing on a board) recently - while it was set up I put a tape measure across the board and, using the same eyepiece for all, checked the actual FOV for different accessories/setups This allowed me to calculate the focal length for each

1. Original Meade SCT Cell + 1.25" visual back: 2589mm (Taken as normal FL, i.e. 0.0% increase);
   
2. Baader 3.25” to 2” clicklock adaptor + 2” to 1.25” low profile adaptor: 2607mm (0.6% increase);
   
3. Original Meade SCT Cell + 1.25" visual back + 1.25” mirror diagonal: 2849mm (10.0% increase);
   
4. Baader 3.25” to 2” clicklock adaptor + 2” to 1.25” low profile adaptor + 1.25” mirror diagonal: 2871mm (10.9% increase);
   
5. Baader 3.25” to 2” clicklock adaptor + 2” mirror diagonal + 2” to 1.25” low profile adaptor: 2988mm (15.4% increase).
   

Notes:

These are not the actual focal lengths as systemic measuring errors may be significant, however these are the same for all, so the % increase in focal length should be fairly accurate more below). There will also be a difference as it was not focussed to infinity.

Baader adaptor is approximately the same length as the original rear cell + visual back.

4 & 5 show the change due to changing from a 1.25” to a 2” diagonal (117mm or 4.1% increase in FL). I haven't measured the difference in optical length of these diagonals, but it should be around 30-35mm longer, but an increase in FL of much more.

Method

1) With each optical train setup, get the tape measure scale across the centre of the FOV. Note the distance across for each (X).

[1. 142mm, 2. 141mm, 3. 129mm, 4. 128mm, 5. 123mm]

2) Measure the distance from the scope to the board – I paced the distance: 5 paces to my back gate and a further 25 paces into the field. Scope to fence measured as around 4.5m so the distance taken as 4.5m*((5+25)/5)= 27m (D).

3) Eyepiece used was a 15mm plossl (stated afov of 52*) so the focal length of the scope will be given as (15*52)/angular actual fov.

Angular actual FOV calculated as 2*inversetan((0.5*X)/D)

i.e. 142mm gives 2*inversetan((0.5*142)/27000)= 0.30133*

780/0.30133 = 2589mm (rounded).

Errors

The main errors are in measuring the distance to the board (same for all), eyepiece fl & fov (same for all) and determining X (variable error). I think I kept the measuring error below 1mm (so <1%) therefore I think the % increases in focal length can be taken as fairly representative of what is going on.

As noted above, the focal lengths calculated are not accurate – this is because they rely on measurements with serious errors possible, but are the same for all e.g. if the eyepiece FOV is in fact 50* it will give a focal length of 2489mm for the example above! Let alone if it is actually 14mm

I was going to do this again with a BST explorer, but I was put off by the centimeters at the edge of the FOV looking aout 75% of the size of those in the middle - distortion I hope it was exacerbated by the close focussing of the setup...

Phew! If you are still here then you can have a gold star

[Peter Drew]

Good subject to bring up again, it's a function that's not commonly known. I did some similar experiments years ago when I noticed the difference in magnification when using alternative accessories. The magnification increase when using binoviewers is quite considerable.

[damnut]

Hi Adrian,

Nice work I too have the lx200 10", I have removed the original meade back and now use the paterson full 2" back, there is some increase in fl and subsequent mag.

Thanks for an interesting read on a dull day too.

[Adrian]

From what I have read, binoviewers add about the same optical length as a 2" diagonal (around 110mm) so if you put binoviewer straight in the back of the SCT it will increase the focal length about 15% (from 2 to 5 in my setups above - 14.6% increase).

[Merlin66]

I had to do some updates to LORD's original model, based on results with the standard SCT reducers commonly used.....

The spreadsheet shows the variation in effective focal length caused by the primary mirror shift......

HTH

reducers_V2a.zip

[Peter Drew]

Moving the primary mirror to focus a SCT is the same as if you could move the secondary towards the primary, in each case the secondary acts like a Barlow. A reminder as well, the final focal plane position is determined by the inter mirror spacing. This varies by the distance moved by the primary X the amplification of the secondary squared. The SCT secondary amplification is 5X so if the primary moves .001" the focal plane moves .025", not ideal for long exposure photography!

[Adrian]

I have taken some further data this weekend and have the following results:

Scope focal length:

No diagonal 2460mm

1.25" diagonal 2730mm

2" diagonal 2840mm

Full data & workings in the excel sheet attached.

Meade SCT Focal Length.xlsx