Can't Achieve Focus using Skywatcher 72ED Reducer

[cwinstone]

Hi, Hopefully someone will help me with this as I'm at the end of my tether here lol

When I use the 72ED reducer I can't achieve focus unless I add extension tubes between it and my DSLR, but doing that gives me horrible coma errors (stars have a blob of light veering off away from center of the image), it does this while tracking long exposures and quick short exposure tests with planets/street lights etc

With the reducer at correct distance from my DSLR the coma error goes away but i cant achieve focus! I can't find anything on the internet to help me.
 
Skywatcher Evostar 72ED
Reducer to telescope tube adapter
0.85 Reducer/Corrector for ED72
m48 adapter
Canon 650D

The image shows the coma error when I use extension tubes to achieve focus

 

 

[symmetal]

The extension tubes need to go before the reducer/flattener and not between the reducer and camera. The distance between the reducer and the camera sensor needs to be at a specific distance to avoid the coma like effects you're seeing. I believe the distance is 55mm (although I couldn't find it mentioned on the adverts) as it's used with a standard M48 to DSLR adapter which provides the correct distance when used with a DSLR. I'm surprised you need to use extension tubes to achieve focus as I would have thought the scopes focuser should have enough travel to achieve this without extension tubes.

Alan

[alexbb]

Hello!

Can you post some pictures of the setup? Unless the OTA's a lot shorter that it used to be in the beginning, there must be something misplaced in your setup. Though, reading your description, it seems correct.

My ED72 is the first iteration and I'm having difficulties to achieve focus, but the other way. I found that the proper distance with the dedicated corrector is more than 55mm. Adding ~5mm doesn't allow me to reach focus due to lack of inward travel.

I knew that newer versions OTAs are 10mm shorter than mine in order to accommodate a 10mm wide rotator, but it's possible that newer ones are even shorter?! The distance between the focuser and the lens cell is ~198mm for my OTA.

[cwinstone]

This is the amount of extension tube i used to achieve focus, I know this is causing the coma issue but wanted to get something out of a clear sky

[knobby]

Pretty sure that the extension should be between scope and reducer - then you'll be able to focus but the distance between camera sensor and reducer will be closer to the 55 mm required. (mine was 61 and still not perfect)

[cwinstone]

8 minutes ago, knobby said:

Pretty sure that the extension should be between scope and reducer - then you'll be able to focus but the distance between camera sensor and reducer will be closer to the 55 mm required. (mine was 61 and still not perfect)

ok so i should invest in a focuser extension tube  and have dslr flat up against the reducer

[KP82]

If the reducer has a 2" push-fit nosepiece, get one of these: https://www.firstlightoptics.com/adapters/astro-essentials-2-35mm-focus-extension-tube.html

If it's a thread-in connection, then you will have to find the extension tubes that match the particular thread size on the ED72 focuser drawtube.

Your M48 DSLR adapter should go right after the reducer and that will give you 54-55mm backfocus distance. You can then insert additional M48 extension tubes if you find the corners are still not flat (stars flying outwards).

Edited November 26, 2020 by KP82

[cwinstone]

2 hours ago, KP82 said:

If the reducer has a 2" push-fit nosepiece, get one of these: https://www.firstlightoptics.com/adapters/astro-essentials-2-35mm-focus-extension-tube.html

If it's a thread-in connection, then you will have to find the extension tubes that match the particular thread size on the ED72 focuser drawtube.

Your M48 DSLR adapter should go right after the reducer and that will give you 54-55mm backfocus distance. You can then insert additional M48 extension tubes if you find the corners are still not flat (stars flying outwards).

it is a thread, been looking for the correct extender

[Peter Drew]

Having the reducer in the wrong place relative to the camera is possibly what is causing the unexpected extension to the focused position.  I would suggest trying it in the correct position before ordering anything, you may find that you have an opposite problem.   Welcome to SGL.             

[KP82]

2 hours ago, cwinstone said:

it is a thread, been looking for the correct extender

I believe this: https://www.firstlightoptics.com/adapters/astro-essentials-adapter-for-sky-watcher-focal-reducers.html

can turn the thread connection on the reducer to a 2" push-fit nosepiece if you're unable to find an extension tube with the right thread size. You can then use the standard 2" extension tubes.

Edited November 26, 2020 by KP82

[cwinstone]

12 hours ago, Peter Drew said:

Having the reducer in the wrong place relative to the camera is possibly what is causing the unexpected extension to the focused position.  I would suggest trying it in the correct position before ordering anything, you may find that you have an opposite problem.   Welcome to SGL.             

Hi! Thanks for the welcome  From what I read the m48 adapter attached directly to the reducer is the exact distance I need.  I just dont have enough travel to focus when I do this.  I hope I'm  just missing something important in my train that I've overlooked as a beginner! 

[fozzybear]

how old is your 72ed? Is it the newer version? Ie shorter tube Mine had a sticker recommending using the camera rotator adaptor. Unfortunately I use the OVL field flattener and not the dedicated one you have, Yours apparently comes with an adapter to attach to the scopes drawtube yes?  then it should be the Flattener then t ring to camera (canon)

Edited November 27, 2020 by fozzybear

[cwinstone]

On 27/11/2020 at 18:18, fozzybear said:

how old is your 72ed? Is it the newer version? Ie shorter tube Mine had a sticker recommending using the camera rotator adaptor. Unfortunately I use the OVL field flattener and not the dedicated one you have, Yours apparently comes with an adapter to attach to the scopes drawtube yes?  then it should be the Flattener then t ring to camera (canon)

I called the retailer and they said it is the most recent version. I have the rotator ordered and on its way. I do have the dedicated one with the attachment yes. Do you mean this sticker? Is it telling me connect the rotation adapter in order  for my camera to work? (apologies for picture quality!) 

[fozzybear]

Hi,

From your photo you do indeed have the latest version same sticker on mine what was provided with the reducer/corrector as I don't have the same one as you

Andy

[cwinstone]

I have now got the rotator and can now focus, but its still having the same issue! Any help would be much apreciated as i have no idea what to do next!

Does my setup look correct?

[fozzybear]

do you have an extra adaptor attached to the drawtube as highlighted

[cwinstone]

1 hour ago, fozzybear said:

do you have an extra adaptor attached to the drawtube as highlighted

yes, that's what connects either the rotator or the reducer to the draw tube. BUT! You have me thinking.. The rotator is the ed80 version, because the 72ed reducer is identical to the 80ed I thought it was worth giving a second hand 80ed rotator a go while the 72ed rotators are out of stock everywhere! It fit perfectly and I assumed the coma issue is only relevant camera side not draw tube side! I'm assuming the 72ed rotator will fit directly on the draw tube then?

(the adapter is needed for the reducer to attach to draw tube so I guessed its normal for rotator to need it also)

I'm being a total beginner here so thanks for being patient with me

Edited December 13, 2020 by cwinstone

[fozzybear]

Ok. I don't know where you are located but I have read that a batch off SW focal reducers had problems with not being assembled correctly check with your supplier. can you shim your connection from the camera t2 to FF using a cut-out from a milk carton/bottle to extend from 55mm a little

[cwinstone]

UK, I got second hand so I guess I cant do anything if it is one of those bad reducers, I'll give the milk carton ago. The Reducer needs 55mm back focus and the camera (i think) gives 44mm, with the m48 ring thats 54mm. Is that what you mean about the milk carton?

 

[fozzybear]

4 minutes ago, cwinstone said:

UK, I got second hand so I guess I cant do anything if it is one of those bad reducers, I'll give the milk carton ago. The Reducer needs 55mm back focus and the camera (i think) gives 44mm, with the m48 ring thats 54mm. Is that what you mean about the milk carton?

 

give it a go with home made shims or you could buy these but wait and see how the home made one's go before spending yet more money

https://www.firstlightoptics.com/adapters/baader-t2-delrin-spacer-ring-set.html

[Stu1smartcookie]

I’m just going to add something which may or may not be relevant. I owned one of these scopes with the dedicated flattener .. but the flattener was faulty .. they actually had a bad batch so I changed it for the OVL flattener, and everything worked fine . I also couldn’t achieve focus with the dedicated flattener. ( I’ve not read all the comments on this post so please forgive me if someone else has mentioned the problem with the dedicated flattener) 

[Alien 13]

It seems that you have an awful lot of adapters in the imaging train, the reducer/flattener should always be connected directly to the camera T ring to get the correct spacing..

Alan

[UCzerw]

Old topic but for future problems ... Nothing is needed if you have a reducer. Only thin pads between the reducer and the camera adapter. And it depends on the camera. For my Nikon D5100 it is about 3.5mm. I personally cut out of cardboard. Below is my focus.

I just have a little problem with the stars on the right.

[M40]

Hello UCzerw and welcome to the site 👍. Thanks for updating this topic.

Looking at your setup, you have between camera sensor and front of camera adapter 55mm then add the 3.5mm card spacer giving a distance of 58.5mm ish between camera sensor and the reducer. I could find no manufacturer dimensions to confirm that distance using the dedicated SW reducer. So thanks to your piece of card, looks like you have got the working distance somewhere between 55-58.5mm.  Anybody know what the actual distance is?

[UCzerw]

Does it not depend on the camera? Each camera has a slightly different sensor distance. It's best to try by trial and error.
Now I remember. The above image of the Bode Galaxy was without shims between the adapter and reducer.
In my opinion, this is quite acceptable. Although when I added 3.5mm the problem is still on the right side. So personally, I probably need a tilt.

Edited May 19, 2021 by UCzerw