Achieving focus with a Evostar 72ed

[Mr Thingy]

3 minutes ago, alacant said:

Yes. that will do fine. Remember that you also need some distance inside the 72ed so that you can clamp it.

After a few year's doing this for any telescope which comes along, you end up with this number of extension tubes. The fewer you have, the more likely you are to fail!

 

 

Awesome. Thanks a lot.

[Mr Thingy]

FLO will love me this year. 

This hobby is expensive!

[alacant]

11 minutes ago, Mr Thingy said:

expensive

These aren't. Nor are these. One set of each will easily get you over the winning line;)

Not sure if the asi comes with this. 

Cheers

Edited November 15, 2020 by alacant

[Mr Thingy]

I bought an adaptor and a bunch of extenders and now everything works perfectly (I also learned a wee bit more theory on telescopes and focus) 😀

Thanks for all the great advice!

Edited November 21, 2020 by Mr Thingy

[jpwilliams.com]

I've just taken delivery of the 72ED and OVL FF which @Rustang listed in the classifieds last week. I want to use it with my Altair 183c Pro Tec, which has 17.5mm front flange to sensor, and an Altair filter holder which is 17mm thick with M48 front & T2 back threads. I was planning on adding a 15.5mm M48 spacer, to give the desired 55mm back focus. I've discovered the FF has a M/T?42 thread at the back , luckily I have a suitable adapter which is only a couple of mm deep, so I'll be able to connect my 15.5mm tube (indirectly) to the back of the FF. 

Plan will be  FF > M/T42 > M48 15.5mm > M48 Filter Holder T42 > Hypercam

Is the 55 mm distance critical? I've seen a few discussions that ~60mm might give better results. With having to convert the rear side of the FF back to M48, I'm gaining the thickness of the conversion ring, I do have the M48 variable ring I could use instead of the 15.5, it's got a minimum distance of 13.85mm, so would give me 1.65mm back.

Thanks

John

[fozzybear]

ED72 backfocus Diag

[jpwilliams.com]

Thanks, I've spacers to make the 55mm, just need a clear night to try them out.

[Ckr]

I just got the same scope and couldn't find focus with my ASI294mc pro last night. It's my understanding that "back focus" applies only when using reducers/flatteners/correctors, and remains fixed for that reducer/camera combination. Please correct me if I'm wrong.  What I'm missing is "out" focus. I'm going to set up (in daylight) and add extensions 'til I reach focus in the mid range of drawtube travel. I have found that it's easier in daylight, after finding a distant target, to assemble, adjust, and note (WRITE DOWN!) distances/extensions required to reach focus for all combinations of camera, scope, reducer, diagonal etc. that I expect to use. 

So, with the SW 72ED focal length of 420mm and the distance from the lens to back of drawtube (at center of drawtube travel)  being around 305mm, 420 - 305 - 6.5mm (ASI294 flange focal distance) = 108.5mm approx. extension required, plus insertion distance, call it 130mm (deja vu). At least on paper, with a broken measuring tape, in dim light with bad eyes, no glasses, and a headache.  We'll see.

Good luck!

 

 

Edited October 1, 2023 by Ckr
Better accuracy

[fozzybear]

49 minutes ago, Ckr said:

I just got the same scope and couldn't find focus with my ASI294mc pro last night. It's my understanding that "back focus" applies only when using reducers/flatteners/correctors, and remains fixed for that reducer/camera combination. Please correct me if I'm wrong.  What I'm missing is "out" focus. I'm going to set up (in daylight) and add extensions 'til I reach focus in the mid range of drawtube travel. I have found that it's easier in daylight, after finding a distant target, to assemble, adjust, and note (WRITE DOWN!) distances/extensions required to reach focus for all combinations of camera, scope, reducer, diagonal etc. that I expect to use. 

So, with the SW 72ED focal length of 420mm and the distance from the lens to back of drawtube (at center of drawtube travel)  being around 305mm, 420 - 305 - 6.5mm (ASI294 flange focal distance) = 108.5mm approx. extension required, plus insertion distance, call it 130mm (deja vu). At least on paper, with a broken measuring tape, in dim light with bad eyes, no glasses, and a headache.  We'll see.

Good luck!

 

 

The best solution of 55mm back focus length

you need approx 134mm from end of scope drawtube to your sensor  your camera has 17.5mm so you need 116mm extra extension if no other items ie FF or filter wheel

[Ckr]

I don't believe the 55mm comes into play here as I'M NOT USING A REDUCER/FLATTENER/CORRECTOR.

If back focus is the distance from the last optical component to the image plane, then my objective lens back focus is 420mm and my telescope length is about 305mm at mid focuser travel, leaving 115mm between my image plane and the end of the draw tube. Subtract the camera ffd (6.5mm) and add for insertion into eyepiece holder (20ish mm)

 

Edited October 1, 2023 by Ckr
Accuracy

[fozzybear]

41 minutes ago, Ckr said:

I don't believe the 55mm comes into play here as I'M NOT USING A REDUCER/FLATTENER/CORRECTOR.

If back focus is the distance from the last optical component to the image plane, then my objective lens back focus is 420mm and my telescope length is about 305mm at mid focuser travel, leaving 115mm between my image plane and the end of the draw tube. Subtract the camera ffd (6.5mm) and add for insertion into eyepiece holder (20ish mm)

 

As per the diagram above you need to add 134.4mm from end of the drawtube to your Camera lens minus 6.5mm