Back Focus - EdgeHD 925 with Baader Steeltrack

[wornish]

Starting this as a separate thread as requested.

I am struggling to understand what the published back focus figure for my scope actually means.  According to  Celestron the back focus distance on the Edge HD925 is 146.05mm.

In the base config (pic 1) the distance to the sensor is 80mm and I can achieve focus using the Celestron's main focussing knob.

If I use the Baader fully retracted the sensor distance is 115mm. (Pic 2) Again I can achieve focus by using the main focussing knob on the scope.

So what is the 146.05mm all about? What am I missing?

If I then add a Barlow in to the mix the same thing happens.

[PeterCPC]

If you can achieve focus, why worry - be happy.

Peter

[wornish]

But is it the best possible focus?

What is the back focus figure meant for ? and does it change with the focal length setting?

[PeterCPC]

This is enlightening: Very large in focus range. Came from here

Peter

Edited September 11, 2020 by PeterCPC

[wornish]

Thanks for this it seems to be exactly what I am looking for!

I have been struggling in poor seeing conditions and starting to worry why I can't get the detail I hoped for on planets.

It's the first time I have heard it called the Nominal Back Focus.  I will read with interest.

Edited September 11, 2020 by wornish

[PeterCPC]

Seeing is your biggest enemy. I tried a couple of nights ago and Jupiter looked awful - I gave up.

Peter

[wornish]

Me too.  Looks like Sunday might be the next chance🙏

[JamesF]

The text with Peter's drawing pretty much says what I was going to say.  What it doesn't mention is that as the focal plane moves closer or further away from the back of the OTA, the effective focal length also changes.  I suspect that at the quoted nominal backfocus distance you actually achieve the quoted focal length of the OTA.

What I would try to do is to set things up so that the secondary focuser has a small amount of travel either way when the primary focuser is used to bring an image to focus on the camera.  So set the camera sensor to be at the quoted backfocus distance and then focus with the primary focuser.  At that point you shouldn't need to move the mirror to focus any more and it can hopefully be left well alone (and the mirror locked, with the Edge models?).  After that any fine tuning to the focus can be done with the secondary focuser.

James

[ollypenrice]

9 hours ago, PeterCPC said:

If you can achieve focus, why worry - be happy.

Peter

You're probably right but an SCT can focus outside the range in which it is diffraction limited.

Olly