Confused with back focus...help needed please

[dazzystar]

Hi All,

I was watching a video about cameras for telescopes and the subject of back focus was brought up and how it needed to be 55mm from the CMOS chip on the camera. Not 54mm and not 56mm, exactly 55mm to achieve focus. I was then left pondering why on Earth you can't just use the focusing knob on the scope? What have I missed and can someone try and explain it please?

Cheers

Daz

[vlaiv]

Well, that one has not been explained well.

Focal point sits somewhere behind the scope. When focuser is fully racked in - you have some distance from that point to actual focus point of the telescope - that is called back focus of telescope.

In order to be able to reach focus with your camera and accessories - total optical length of all of the gizmos in the optical train need to be less than this distance - if it is larger - camera will sit beyond focus point of the telescope when focuser is fully racked in - and no racking out of focuser will bring it to focus point.

Number that you mention - 55mm is related to flange focal distance of T mount (related to T-thread used with telescopes).

Camera lens have something called flange focal distance - and is very similar thing to back focus of telescope. It is distance from lens thread/connection to where sensor should ideally sit so that lens has full range of focus from close to infinity. It is "operating distance" of lens.

Telescopes adopted T/T2 connection (which is really M42x0.75 in metric thread specification) and with it - majority of simple correctors are designed with T/T2 flange focal distance in mind (for what ever reason as it plays absolutely no part in telescopes). This means that most simple coma correctors for Newtonian telescopes and field flatteners / reducers for refractors require to be positioned at 55mm away from sensor.

This is their operating distance and yes, in most cases - you need to get them to exactly 55mm and change of +/-1mm will be noticeable. On the other hand - some of them are actually designed for slightly different distance and you often need to "dial in" correct distance with spacers - again, they will be sensitive to variation in distance. Third ones state that they are not overly sensitive to distance - and that means they will work ok in range of distances around their prescribed working distance.

All of this is listed in specification of said accessory.

For example at FLO page for Skywatcher CC, you'll see this:

This means two things (which is the same thing when you think about it):

a) coma corrector needs to be placed at 55mm distance from sensor

b) telescope needs at least 55mm of back focus as coma corrector adds 55mm to optical path because of distance from its thread to sensor which is 55mm

This brings us to last point - two are sometimes and somewhat related - telescope back focus and corrector working distance. They are related in two things:

1) Like shown above - you need to have enough back focus of the telescope to be able to properly focus with all your accessories (do include any filter wheels, off axis guiders, adapters - etc)

2) Sometimes, due to their optical properties - reducers / extenders change focus position of the telescope. Almost as a rule - reducers bring focus point inward (shorten back focus of the telescope) and extenders (like barlows) move focus point outward - make back focus of the telescope longer.

Point of all of that is to be able to figure out if you'll be able to reach focus with your planned accessories, and if you'll need perhaps additional extenders if back focus is too long.

Makes sense?

[rsarwar]

7 hours ago, dazzystar said:

Hi All,

I was watching a video about cameras for telescopes and the subject of back focus was brought up and how it needed to be 55mm from the CMOS chip on the camera. Not 54mm and not 56mm, exactly 55mm to achieve focus. I was then left pondering why on Earth you can't just use the focusing knob on the scope? What have I missed and can someone try and explain it please?

Cheers

Daz

assuming you are asking why 55 mm was chosen as a standard and not some other random number, it comes from the fact that the common DSLRs bodies have their sensor places somewhere in the range of 44-45 mm. add another 10 mm for the adaptor. so with 55mm it gives a very straightforward, almost standardized distance between the reducer and the dslr sensor, allowing someone new to enter into astrophotography without having to fiddle with spacers. hence it is 55 mm. just an ease of use, because if we were to use 45 mm as the standarized backfocus, most reducers wounld not work with DSLRs

Edited December 28, 2021 by rsarwar

[newbie alert]

It's spacing distance, to fully illuminate the imaging circle where the sensor should sit

[dazzystar]

Should this back focus measurement be provided by the scope manufacturer as I can't see it on the Skywatcher website? My tube is a Skytravel 102.

[vlaiv]

21 minutes ago, dazzystar said:

Should this back focus measurement be provided by the scope manufacturer as I can't see it on the Skywatcher website? My tube is a Skytravel 102.

Sometimes it is - sometimes it's not.

You can measure it though, rather easily.

Take a piece of thin paper (like tracing paper) next time there is bright moon outside and point the scope to the moon while placing paper over the back of the scope (put it over focuser opening - don't use eyepiece of diagonal or anything just back port of the scope). Focus until you have clear image of the moon on the paper (it will be small). Measure how much you needed to rack out focuser to get that.

Mind you - don't do it on close object - like during the day (and never with Sun! - never point scope at the Sun unless you have proper equipment) because focus point depends on how close object is. We determine true focus for object that are effectively at infinity (meaning just really far away like astronomical objects are).

 

[teoria_del_big_bang]

1 hour ago, dazzystar said:

Should this back focus measurement be provided by the scope manufacturer as I can't see it on the Skywatcher website? My tube is a Skytravel 102.

I think you are still a little confused.
@vlaiv has given a good explanation in his first reply.

Basically if you are using the scope as bought without any additional optics such as a coma corrector, flattener or reducer then really you do not necessarily need to know the focal point.
So long as the scope is suitable for imaging then you can just use the focusser knob to wind the tube outwards, from being fully in,  to achieve focus. No real need to know exactly what that distance is as by design the scope should be capable of achieving focus within the distance the tube can be moved in or out.

To the best of my knowledge the Skytravel 102 is primarily a visual scope for use with eye pieces and not really recommended for imaging which is why you probably do not find any mention of back-focus with it as it just assumes you are using eyepieces and the focal point will be somewhere in the travel of the focussing tube.

The back-focus that people talk about that has to be correct (normally 55mm) is provided by the manufacturer of the flattener, reducer, coma corrector and that is the 55mm you mentioned in the OP. These additional optics are used in imaging to correct for issues of how the light path is focussed that may not be a big issue for visual but becomes noticeable in an image, mostly at the corners or outer edges of the image where stars become elongated, or tear drop shapes, or maybe issues with colour as not all frequencies of light get focussed at the correct point around the peripheries of the image . So the additional flattener, coma corrector or reducer will correct these aberrations, or at least reduce the effect.
But to work correctly and focus all frequencies of light at the same point the distance between the end of the flattener, coma corrector or flattener has to be a specific distance to the image plane (the CCD chip in the camera) and this is often the 55mm you talk about. You still have to achieve focus by using the focus knob to move the tube in and out but 55 mm of the focal point will now be always taken up by this 55mm back-focus required from the additional corrector optics.

Just be aware that some scopes are not ideal for imaging and more usually designed for visual only. That is not to say you cannot use them for imaging but you may need extra extension pieces to add where the eyepiece fits when using a camera to make the camera sit further back than be achieved with the focusser tube itself and that is just due to the fact that the path of light is altered by the eyepieces and the scope is designed for visual.
A good example are the Skywatcher refractors  such as the Sky-Watcher Explorer 130P, 150P and 200P these are really unsuitable for imaging (well I have used them and needed all sorts of extenders to achieve focus with a ccd)  but they do versions for imaging and are called 130-DS, 150-DS and 200-DS.

I don't want to labour that point too much and confuse you more but it is just worth a note as your tube is really designed for visual, although I have no experience of this scope so cannot say whether you will have any issues when using it for imaging but I am sure somebody on SGL will know for sure 🙂 

I think all I have said above is correct but will say I am pretty new to all this and was also asking the same questions only 3 to 4 years ago but I am sure @vlaiv will correct me (I hope anyway) if I have said something misleading.

Steve

Edited January 4, 2022 by teoria_del_big_bang

[dazzystar]

Thanks for the reply. It now makes much more sense and I only need to concern myself with back focus when using a FF or similar additional optical component in the imaging train.

[MKR]

On 04/01/2022 at 11:33, teoria_del_big_bang said:

A good example are the Skywatcher refractors  such as the Sky-Watcher Explorer 130P, 150P and 200P these are really unsuitable for imaging (well I have used them and needed all sorts of extenders to achieve focus with a ccd)

I have a 200P and use a full frame Canon with it. It works fine although I do not have a coma corrector. I am planning on getting one, does it mean that I won't be able to focus after adding a coma corrector? It has maybe 2cm left on the focus inward travel. Do I need at least 55mm left with my current setup in order to add a coma corrector?

 

What about ASI 294MC, which I'm planning on buying too - does it "behave" like my DSLR?

Thanks.

[vlaiv]

5 minutes ago, MKR said:

I am planning on getting one, does it mean that I won't be able to focus after adding a coma corrector?

It's not easy to find coma corrector that covers the whole full frame sensor. Most of these are not 2" but larger like 2.5" or 3" and require suitable focuser.

In any case - look for coma corrector that can be "sunken" into focuser tube.

If you can reach focus with Canon DSLR - you won't have any issues with ASI294mc - all that will maybe be needed is simple extension (depends on your setup but ASI294 has sensor much closer than DSLR).

[MKR]

Thanks. I think that's ok, I don't need to use the whole frame, happy with cropping it to APS-C, as long as I can still focus with my 200P and have coma corrected within APS-C region. Unless I misunderstood?

[vlaiv]

2 minutes ago, MKR said:

Thanks. I think that's ok, I don't need to use the whole frame, happy with cropping it to APS-C, as long as I can still focus with my 200P and have coma corrected within APS-C region. Unless I misunderstood?

That will work.

CC that looks / works like this is what you need:

https://www.teleskop-express.de/shop/product_info.php/info/p6706_TS-Optics-NEWTON-Coma-Corrector-1-0x-TSGPU-Superflat---4-element---2--connection.html

It is 2" body that goes all the way into focuser tube. That CC even pushes focal point 20mm outwards.