10 replies to this topic

[mike_k]

I’ve seen back focus bandied about a lot. I’ve just finished my third “beginning in astrophotography” book and two of then failed to mention it, and a third basically said “it’s where the telescope focuses”. Looking into various fora it’s mentioned a lot. It’s obviously important. Why is obvious. But exactly how is it important … well I guess that’s obvious too. But maybe “what” is it.

You always see diagrams of scopes focusing to a point somewhere in the back of the scope (where we burn the ants). Is that where the back focus is? Or is it where that cone fills the chip/film, which is past the intersection of that cone.

And I keep seeing 40-some odd mm this and 60-something mm that. But for some reason the number 55mm shows up a lot. Is that just chance, or is it important. That’s really what got me going on this - “If 55mm is important why are they setting back focus at 63mm?”). At first I thought everyone was doing all that shimming and extension to get to 55mm. Or a lens was purposely placed in the optical train to only extend the 55mm. But 55mm was the magic number.

Now, the more I’ve read the more I think that not it. But I don’t know for sure. And I’ve searched this and another online fora, and with my terms got nothing or got dozens and dozens of hit.

Can anyone point me to a web page/site/article, or a particular book, that might cover this. I really can’t believe 3 beginner books just barely touched on it.

Thanks in advance.

Edited by mike_k, 25 April 2025 - 01:20 PM.

[dx_ron]

If your telescope has no post-objective corrector lens (no field flattener, no reducer/flattener, no coma corrector), then usually all you need is to get your camera sensor to the focal plane of the objective. Many scopes need some sort of corrective optics - refractors need a field flattener, newtonians need a coma corrector - in order correct for off-axis abberations. That corrector lens is designed so that the corrected / flattened field is restricted to some fixed distance from the corrector lens. That required distance is the backfocus.

 

Anything in the light path after the corrector lens "uses" some of the backfocus distance. In fact, the distance from the T-connector to the sensor inside a typical dslr is 55mm - which is why the most common design for corrective optics use 55mm as "standard backfocus". Most dedicated astro cameras have the sensor ~17.5mm from the threads you use to connect to the telescope, so you have 37.5mm that either needs to be added in the form of spacers, or can be used to add other things you want to have in the light path, such as a filter drawer or wheel and an off-axis guider.

[bobzeq25]

Change the word to "spacing". Makes it much more intuitive.

Refractors typically need a flattener or flattener/reducer, because their plane of good focus is curved. The spacing ("backfocus") between the back of the flattener/reducer and the camera sensor needs to be precise within about a millimeter to get the flat field, and good stars across the whole image.

The spacing of a typical DSLR with a t-adapter is 55mm. Many reducers are designed to accommodate this by having a spacing of 55mm. The t-adapter screws into the reducer. the DSLR is attached, and the spacing is automatically correct.

Most astro specific cameras have much shorter spacing, so you need to add an appropriate extension tube to move them back. It can be useful to adjust that spacer a bit with washers, a reducer with a specified spacing of 55mm may work a bit better somewhere between 54 and 56.

Finally. Flattener/reducers tend to work best when they're used with the specific scope they were designed for. If your scope has a flattener/reducer the manufacturer makes/recommends, that's ALWAYS your first choice. Using an unmatched flattener can work, but if you don't have to risk that, don't.

Edited by bobzeq25, 25 April 2025 - 01:45 PM.

[kathyastro]

"Back focus" is a confusing term, because it has multiple meanings, and the most common meaning is not the official meaning.

 

The focal point of a telescope is a specific distance (the focal length) behind the primary lens or mirror.  In refractors and Newtonians, the focal point never changes.  (Maks and SCTs are a bit different because their focal length changes.)  The official technical definition of "back focus" is the maximum distance between that point and the draw tube of the focuser, which occurs when the draw tube is fully retracted.

 

For focusing a camera, the aim is to place the camera's sensor at the focal point.  That only occurs at one point.  Obviously, the distance between the draw tube and the focal point must be greater than the depth of the camera's body, or the camera won't fit and focus at the same time.  If you are using a DSLR, its body (with the necessary T-ring installed) is 55mm deep.  That is where that number comes from.  So the minimum required back focus for a DSLR is 55mm.  A little more allows you to tweak the focus.

 

Things change when you add corrective optics, such as coma correctors or field flatteners.  Those devices are designed to flatten the image field at a specific distance, also known as "back focus".  In this case, it is less a measured characteristic of the telescope and more a manufacturing specification.  To get the best results from the device, you need to place the camera's sensor at that exact distance from the back of the device.

 

Again, 55mm is a common specification because it matches the depth of a DSLR.  But other common specifications are 85mm or 105mm.  You need to know the requirement of your device.  You arrange the correct spacing for your device and camera using extension tubes, which might include OAGs or filter wheels.  Then you mount the entire rigid combination of corrective optic, extensions and camera onto the scope and focus it.

[Marcelofig]

Cuiv has a recent video explaining backfocus:

 

https://youtu.be/6xE...?feature=shared

[matt_astro_tx]

This is the effect that too much/too little back focus distance has on your field.  

 

[Alex McConahay]

You have good information above. Here is one of the  YouTubes from the Astro Imaging Channel that may help.   

 

https://youtu.be/5J654T0UpBc 

 

Alex

[Myk]

Wow.. "Fora" I haven't seen that since high school! 2nd Declension, I think.  

 

Looking into various fora it’s mentioned a lot.

[mike_k]

T hanks all.

I THINK I understand it now. Probably will better when I actually DO something, rather than only theory. Though the graphic I think really helps.

And… forums ??

[Myk]

T hanks all.

I THINK I understand it now. Probably will better when I actually DO something, rather than only theory. Though the graphic I think really helps.

And… forums ??

Fora AND Forums are both right from what I understand. More power to you for using the, less common but cooler, fora!

[AstroFromHome]

Does your telescope use a corrective optical element like a flattner, focal reducer or coma corrector that you need to put on the end of the focuser?

 

If so the magical 55mm of backfocus are measured from the back of that element (without threads) until your camera sensor. It is 55mm as most manufacturers of those corrective optical elements somewhen started to design them in that manner that no other manufacturers has any advantage through the customers available spacers or spacers easy to get for them to combine with what they have.

 

Than there is the focus plane. It measures from where the focuser tune is exiting the telescope. That distance is the 55mm of backfocus plus what is needed to get your camera into focus.

 

If you are using a Petzval design telescope a backfocus range is stated. Usually between 50 an 65mm. So instead of the fixed 55mm you can have diviations. Then you fust bring the camera into focus and you are done. Using a backfocus at the low or high limit of the range may cause that you cannot exactly meet the point of best focus.

 

A flatfield telescope that does not require additional optical elements the manufacturers also state the 55mm of backfocus. So the distance from the end of the focuser to the camera sensor. Why is it here as well the 55mm? These telescopes have the corrective lenses built into the focuser. So the way to approach those is the same like the first set of telescopes for that you need to purchase the additional optical element separately.