Achieving focus with a Evostar 72ed

[Mr Thingy]

I have various extenders and adaptors but I don't seem to have enough to achieve focus with the 72ED and my ZWO ASI183 GT.

Can any owners of a SW Evostar 72ED advise what extenders and adaptors you use to achieve focus? Please note that I don't yet have a field flattener fitted - I will get one fairly soon but for now I need a solution without one.

I was also hoping that there would be a screw thread for secure fitting between scope and camera but the 72ED only seems to have a 2" fitting for diagonals, etc.

Thanks for any advice you can provide.

[AstroExploring]

I own the 72ED but use it with a DSLR currently. You need 55mm backspace between the back of the scope and the camera. The easiest way to achieve this is to buy the field flattener (noting you don't have one yet). You could try a 2 inch t-adapter? That would give you enough spacing, you'll just need the right adapter to then attached your 183. I don't know if such an adapter exists - sorry, I've only ever used a DSLR. Somebody more knowledgeable might know 🙂

[Mr Thingy]

15 minutes ago, AstroExploring said:

I own the 72ED but use it with a DSLR currently. You need 55mm backspace between the back of the scope and the camera. The easiest way to achieve this is to buy the field flattener (noting you don't have one yet). You could try a 2 inch t-adapter? That would give you enough spacing, you'll just need the right adapter to then attached your 183. I don't know if such an adapter exists - sorry, I've only ever used a DSLR. Somebody more knowledgeable might know 🙂

Thanks. It's helpful to know that I need 55mm of backspace. The extenders from the camera are only 28mm combined and I lost half of that slotting into the 2" hole.

I need to figure out a means to screw the m48 extenders directly to the scope without inserting in the 2" slot.

I suspect more money is needed to find s solution. This sport is a money pit (as if I didn't realise that already!)

Edited November 14, 2020 by Mr Thingy

[alacant]

4 hours ago, Mr Thingy said:

focus with the 72ED and my ZWO ASI183 GT.

Hi

Unless you use a focal reducer, you're going to need about 130mm of extension between the two to reach adjustable focus at infinity. 

HTH

 

[Mr Thingy]

50 minutes ago, alacant said:

Hi

Unless you use a focal reducer, you're going to need about 130mm of extension between the two to reach adjustable focus at infinity. 

HTH

 

Ah! Ok, good to know. Thank you.

In that case I will be needing a focal reducer. No sense in buying umpteen extenders for temporary use.

Any idea how the focal reducer attaches to the scope? There seems to be no thread on the scope.

[fifeskies]

It varies by type but my reducer/flattener screws into my DSLR then the whole lot fits into the 2inch aperture.

(I use a 2in clicklock with my Vixen, my 80 Equinox has a moonlight that holds the camera with reducer/flattener nice and secure)

 

Similar I expect for a ZWO , I believe there are adapters to fit ZWO to a reducer/flattener

 

(I have a ZWO guidescope camera but don't need a reducer with it).

[Skipper Billy]

The end of the focuser is threaded - this is an all screw together system - you will need about 4" of extension.

 

[dodgerroger]

FLO do a camera rotator for the 72 then a field flattener can be screwed to it. I use a generic 2” flattener and some small spacers to achieve back focus and round stars to the edge

[Bruce Leeroy]

9 minutes ago, Mr Thingy said:

No sense in buying umpteen extenders for temporary use.

If you have a couple of barlow lenses you may be able to remove the lens part and use the body as a temporary extender or stack 2 together if 1 isn't enough or draw 1 out to make a few more mm's.

[fifeskies]

Also as a temporary fix remember that adding in a diagonal will take up a lot of extra backfocus until you get some extenders or a reducer.

 

You don't want it there for serious photography , but as a get you up and running for testing its fine

(just be careful not to drop in any extender too far into the diagonal as it could scratch the mirror element)

[Mr Thingy]

14 minutes ago, dodgerroger said:

FLO do a camera rotator for the 72 then a field flattener can be screwed to it. I use a generic 2” flattener and some small spacers to achieve back focus and round stars to the edge

Which generic flattener/reducer do you use? I notice that FLO have a 40-60 working day wait on the dedicated reducer for the 72ED. My HEQ5 had the same estimate when I ordered in mid-August and that's not here yet, so it's not looking good for the reducer 😓.

 

[Mr Thingy]

20 minutes ago, Skipper Billy said:

The end of the focuser is threaded - this is an all screw together system - you will need about 4" of extension.

 

 

Unless I'm missing something the 72ED doesn't seem to have a screw thread at the end of the focuser. I assumed there would be.

[Skipper Billy]

39 minutes ago, Mr Thingy said:

1 hour ago, Skipper Billy said:

 

Unless I'm missing something the 72ED doesn't seem to have a screw thread at the end of the focuser. I assumed there would be.

It should have - mine does !!

You will probably need one of these to make adding extensions cheaper and easier - the M54 thread on the focuser drawtube is an relatively uncommon size

https://www.firstlightoptics.com/adapters/astro-essentials-m42-t-adapter-for-sky-watcher-newtonians-and-72ed-refractor-m54.html

[Mr Thingy]

3 minutes ago, Skipper Billy said:

It should have - mine does !!

You will probably need one of these to make adding extensions cheaper and easier - the M54 thread on the focuser drawtube is an relatively uncommon size

https://www.firstlightoptics.com/adapters/astro-essentials-m42-t-adapter-for-sky-watcher-newtonians-and-72ed-refractor-m54.html

Do you have to remove the bracket for the 2” nosepieces to find the thread? 

I tried earlier but it wouldn't budge and I didn't want to force it.

Cheers

[fozzybear]

it does unscrew

28 minutes ago, Mr Thingy said:

Do you have to remove the bracket for the 2” nosepieces to find the thread? 

I tried earlier but it wouldn't budge and I didn't want to force it.

Cheers

it does unscrew. have a ganders at this youtube vid

 

[dodgerroger]

1 hour ago, Mr Thingy said:

Which generic flattener/reducer do you use? I notice that FLO have a 40-60 working day wait on the dedicated reducer for the 72ED. My HEQ5 had the same estimate when I ordered in mid-August and that's not here yet, so it's not looking good for the reducer 😓.

 

I use the Stella flattener from flo. Altair Astro sell one, they look exactly the same. Just unscrew the thumbscrew holder out of the focus cube on the 72 and the rotator screws in its place. My friend is after a flattener they seem hard to get hold of at the minute 

[fozzybear]

on mine I use https://www.firstlightoptics.com/reducersflatteners/ovl-field-flattener.html but again lead time

[Mr Thingy]

Everywhere is sold of field flatteners at the moment.

Looks like my mount won't be the last thing I'm waiting for before I can start imaging 😓.

Anyway, thanks for all be the great help. Have yourself some rep points for your troubles 😄

[alacant]

4 hours ago, Mr Thingy said:

Everywhere is sold of field flatteners

We use this one with the 72ed. It has the advantage of retaining native focal length and gives you a whopping 130mm back focus. No extra colour or vignetting is introduced and the stars are good to the corners. Well, as good as cheap SW refractors allow!

Yes, it's in stock.

Cheers

Edited November 14, 2020 by alacant

[Mr Thingy]

9 hours ago, alacant said:

We use this one with the 72ed. It has the advantage of retaining native focal length and gives you a whopping 130mm back focus. No extra colour or vignetting is introduced and the stars are good to the corners. Well, as good as cheap SW refractors allow!

Yes, it's in stock.

Cheers

Thanks @alacant 

Do I understand that with the TS flattener I would still need extenders to reach the 130mm back focus?

What would you use for that?

[alacant]

2 hours ago, Mr Thingy said:

extenders to reach the 130mm back focus

Hi

Don't confuse back focus with position of focus relative to the telescope body or camera back focus. The shoulder of the ff needs to be 130mm from the camera sensor. That's it. Set that, then decide on a sensible focus position.

I used one m48 extension tube which fitted into the focuser along with the ff and two m48 extension tubes of a larger diameter. I used a 2" eypiece parfocalising ring hard up against the shoulder of the wider m48 tubes to bring the focuser to the centre of its travel and keep the assembly square to the telescope.

Here is the setup for a dslr with camera backfocus of 44mmm. You're gonna need around 40mm more for your 183. I have an m48 t2 adapter with 11mm optical path. You won't need that but you're gonna need a m48 to m42 ring to adapt to the zwo instead.

For your original request, simply leave out the ff and correct in software. StarTools has a very good lens adjuster module which will correct field curvature, but still IMO better to correct at source and use the module for fine tuning.

HTH

 

 

Edited November 15, 2020 by alacant

[Mr Thingy]

53 minutes ago, alacant said:

Hi

Don't confuse back focus with position of focus relative to the telescope body or camera back focus. The shoulder of the ff needs to be 130mm from the camera sensor. That's it. Set that, then decide on a sensible focus position.

Thanks. I think I'm missing a key bit of theory here as I'm still confused. 

I thought that it would simple be a case of finding the distance behind the scope where the image is in focus. What is the 130mm position then? Is it the point where the image is the correct size for the sensor?

Then how do you set a focus position?

Maybe there is a website that explains this?

Cheers

[alacant]

23 minutes ago, Mr Thingy said:

What is the 130mm position then?

The back focus required for the field flattener to remove field curvature. It is the distance measured from the shoulder of the ff to the camera sensor.

23 minutes ago, Mr Thingy said:

Then how do you set a focus position?

No need for websites. It's a 2 minute job!

• Place the assembly with your extension tubes and adapters -IOW, with the correct ff backfocus- into your 72ed.
• Attach the camera.
• Connect to your app of choice so that you can see a video image.
• Set the focus barrel to about 1/3 of its outward travel.
• Move the ff assembly in or out until the camera is more or less in focus on a distant object.
• Clamp the ff assembly using the focuser thumb screws.

Now all you need to do is fine tune focus on a bright star. Note the ring stop on my setup to maintain the camera square.

If you're not going to use a ff, just measure 420mm from the rear element of the main lens and use extension tubes to hold the camera there. It's going to be the same distance with or without the tsflat2 so the distances in my posts will be the same.

A good quetsion to ask would be, 'how long an extension tube do I need to achieve a sensible focus position using my asi183 with a 72ed?' Answer: about 125mm.

HTH

Edited November 15, 2020 by alacant

[Mr Thingy]

4 minutes ago, alacant said:

HTH

That helps immensely. Thanks for your explanation. 

I knew I'd have some silly questions sooner or later!

If I'm not using an FF, do you know how much extension I need to achieve focus? Is that still 130mm?

[alacant]

7 minutes ago, Mr Thingy said:

Is that still 130mm?

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, and these are just the ones I've kept tidy.

The fewer you have, the more likely you are to fail!

 

 

Edited November 15, 2020 by alacant