back focus for OAG

[Dilsh]

Hi guys,

Just beginning astro again. Life got in the way but got some nice unguided images of M42 on the weekend.

Was looking at guiding next and felt I wanted to try OAG. I wanted to try OAG rather than a guide scope as ultimately I would like to upgrade to a SCT which I apprently works better with OAG.

I currently have a skwatcher ED 80 with a 600mm focal length with a 55mm T2 ring attached to a canon 1100D DSLR.

I was hoping to get a ASI290mm as a guide camera, as it will double up as a planetary camera as well.

If I was to get a ASI OAG, would this increase my back focus too much for a DSLR.

Thanks in advance,

Dilshan

[AdamJL]

Hi Dilshan

Backfocus is more an attribute of the scope/lens, not the camera (hence the name.. it's the "back" of the optical path).. it's usually called the flange focal distance for camera lenses. For scopes, just backfocus.
For example, the ED80 with an Orion field flattener has a backfocus of 55mm, meaning anything between the flattener and your sensor should take up 55mm*. I used to use this same scope/flattener and a DSLR as well.
The same DSLR with a lens requires about 44mm. Only thing that has changed is the lens/scope.
That said, in practical terms for a DSLR, there's not much difference between saying from the back of the lens (not telescope) to the sensor vs from the flange of the camera itself to the sensor, seeing as they are the same thing when the lens is connected.

*give or take. You'll likely need some spacers to get to the correct distance, usually a few mm away.

If you're using a flattener, add up all of the things that you plan to add between the back of your scope or flattener and the sensor. Your camera will consume 44mm of that total distance by itself, so you'll have 11mm to play with to add an OAG and a T2 adapter. Might be difficult unless there are super thin versions about.
Or you could look at something that does both OAG duty and connects to the camera:
https://www.teleskop-express.de/shop/product_info.php/language/en/info/p2722_TS-Optics-Off-Axis-Guider-for-Canon-EOS-cameras---replaces-the-T-ring.html

If you're not using a flattener (and you should :)), then you'll have lots more room to play with.. I can't remember what the naked backfocus is for that scope (without a flattener) but I think it's >100mm.

[Dilsh]

Thanks Adam,

So to get that right, 55mm back focus means that you have to be within 55mm. Right now all I have is a T2 ring between the back of my ED80 and my DSLR and I can achieve focus and the T2 ring is no 55mm?

Sorry about this very simple question.

Thanks,

Dilshan

[AdamJL]

Hi Dilshan

Your focus tube is what matters if you’re not using a flattener. The 55mm only comes into play if you use a flattener
(Btw 55mm is the most common metric but it’s not unheard of to be different. You’ll have to check the specs of any flattener you buy)

[evltoy]

Let me see if I can explain it in another way as I too had issues understanding such a simple thing.


Let say you DONT have no flattener or flattener/Corrector.

The ED80 straight out backfocus is maybe around 100mm. Your camera & T-Ring will aready be 55mm, so to gain focus you will need to rack the ED80 focuser out to about 45mm to gain perfect focus.


Now if you introduce a flattener or flattener/Corrector. Most have a back focus of around 55mm.

So, you connect the flattener or flattener/Corrector to the ED80, now you only have 55mm to play with from the back of the flattener or flattener/Corrector. And the ED80 focuser will be almost racked in with a fine adjustment to gain perfect focus.


The above are not hard fix numbers, but more round-about to give you an understanding of the back-focus concept.

Cheers
Wayne

[Dilsh]

Great. Thankyou. That makes sense