Back Focus

[AstroComet3]

Hello everyone! I have recently purchased an Esprit 120ED telescope and an EQ6R-PRO mount. Everything seems to be going quite well so far but I have come across a definition that I am not familiar with: 'back focus'. The Esprit 120 has a back focus of 75mm and I was just wondering whether the back focus had to be bang on 75mm or if it could be under? Clear skies  

[Rob Sellent]

You'll probably need an imager to go into the camera side of things, but as a visual observer - for right or for wrong - I understand 'back focus' as 'inward focus' . If I focus on a star, 'back focus' is about how much more 'inward focus' do I have before nothing is left. In this case, almost 3" or 7.5cm. Confused

P.S: Congratulations on the cracking scope and welcome to SGL

Edited September 21, 2019 by Rob Sellent

[Stub Mandrel]

I think it's the distance between the end of the focuser tube and the focus point, so you have up to 75mm for any filters, flatteners etc, plus your camera will probably need some of that distance -  a typical DSLR takes a whopping 55m.

[AstroComet3]

Thanks guys for your help! I actually have a Starlight Xpress Trius Pro 825 MONO camera for my imaging setup. : ) 

[AstroComet3]

12 minutes ago, AstroComet3 said:

Thanks guys for your help! I actually have a Starlight Xpress Trius Pro 825 MONO camera for my imaging setup. : ) 

 

[jadcx]

Hi

I'm not going to claim expertise here, but I think there may be some confusion.  In the case of the Esprit 120 the 75mm back focus would usually be relating to the dedicated field flattener, which requires that any attached imaging plane is spaced exactly 75mm from the end of the flattener in order to achieve focus.

If you do not have the field flattener attached, then the focus point will be at whatever the focal length of the scope is (for the 120 then its 840mm from the front lens element) which is usually achievable with the stock focuser.  Just to confuse the inexperienced (it took me a while to get any sense of what all this meant at first) SW also quite the visual back focus (for the 120 its 225mm) which is the distance from the end of the focuser tube (with 2’’ adapter installed) to the focal plane. This means, all accessories needing 225mm back focus or less can be installed.

Is that in any way clearer? 😕 

[AstroComet3]

Thank you so much! I am learning every day with this hobby and I am so grateful for your help. Yes, this way clearer! 

[AstroComet3]

One more thing, is it the same for imaging (the 225mm back focus)? I thought you were talking about visual observing : )

[jadcx]

Yes it’s the same - it doesn’t theoretically matter whether you are visual or imaging, the back focus number is describing the distance to the focus plane, so whether that’s a camera or an eyepiece is not important (although each will have its own additional requirements). Imaging using a flattener is a special case as this effectively alters the light path of the scope, which is why it has a different (and very specific) figure.

[AstroComet3]

I am imaging with a flattener. The back focus of my scope is 225mm and the field flattener is 75mm. Does this mean I have 150mm left for other accessories? 

[jadcx]

1 minute ago, AstroComet3 said:

I am imaging with a flattener. The back focus of my scope is 225mm and the field flattener is 75mm. Does this mean I have 150mm left for other accessories? 

OK - that’s a clearer case  You need exactly 75mm from the back of the flattener to the imaging plane of your camera. I don’t know for certain where the overall focus point will be but it is likely to be within the range of the stock focuser.  The 225mm figure is irrelevant in this scenario (as this relates to use without the additional flattener).

[AstroComet3]

Fantastic, I have all of the adapters needed to add up to 75mm. If I wasn't using a field flattener, would I have to have exactly 225mm of accessories or could it be under? 

[jadcx]

21 minutes ago, AstroComet3 said:

Fantastic, I have all of the adapters needed to add up to 75mm. If I wasn't using a field flattener, would I have to have exactly 225mm of accessories or could it be under? 

Without the flattener you will be using the ‘standard’ scope specs - which is the overall 840mm.  It’s slightly confusing because (I think) the 225mm refers to what is available, not (just) what is required. So for example, you could construct an optical chain with Barlow,  adapters, filter wheel, diagonal etc. which is longer than 225mm and this would not be able to reach focus (because it’s too long). So as long as you are imaging with an optical chain of less than 225mm, you will be able to reach focus (I.e. imaging plane is at 840mm) using the supplied focuser.
 

I hope that makes sense, I’ve had a very nice beer or two now.

Edited September 22, 2019 by jadcx

[AstroComet3]

Thank you so much for your help! 

Clear skies 

[jadcx]

Any time 😁 This can be complicated to get your head around - at least I found it so.

[AstroComet3]

Yeah, I am finding it complex as well! 

[AstroComet3]

Clear skies and thanks again.