Back focus issue

[Sitt]

I have a Skywatcher Espirit 120 that I'm having back focus issues with. I've read numerous sites and read the Espirit instructions over but having difficulty getting focus for some reason when used with a Celestron OAG with a filter drawer.
Everything I've read says 75mm back focus from start of the threads on the end of the reducer to the sensor. I obtained focus this afternoon playing with various adapters and managed to get focus on a house around 1km away with 100mm back spacing. Yet to test 100mm out on the stars but I think this spacing is going to cause me issues.
Any pointers?

[Joshua Bunn]

Hi Simon.
Maybe try testing on the stars, that might bring the focal plain closer to the scope.
Is the reducer an add on that you have put in the image train?
Are you saying that documentation says the sensor is to be 75mm from the reducer threads but you have done it at 100mm from the threads?
Where abouts do you have the reducer in relation to the OAG and filter wheel?
Josh

[Sitt]

Sorry it's a flatner not reducer. Starting at the camera I have the filter drawer, then the OAG then my spacing. There's an adapter that comes with the skywatcher which fits on the flatner The flatner comes with the Espirit as a package so not after market.
Here a thread on cloudy nights with the 75mm back focus https://www.cloudynights.com/topic/539916-esprit-120-flattener-spacing/

[Joshua Bunn]

So it goes filter to OAG to flattener to camera?
Any reason why you are guiding through the filters?
So currently you have focus with the camera 100mm behind the flattener, that's a long way from 75mm... Did you contact skywatcher?
As always, I'd be doing a star test.

[Sitt]

No, the filter drawer is after the OAG. I worked backwards in my previous description from the camera to the scope which may have confused you. From the scope
Flatner - SkyWatcher Adapter - Spacing - OAG - Filter Drawer - Camera

[Joshua Bunn]

Okay, makes sense.

[Astronut07]

Simon

The correct back focus as stated in Skywatcher manual is 75mm from
flattener to sensor

What camera are you using?
What is the width of the OAG, filter drawer & spacers?

According to manual 75mm is total needed

Cheers
Ben

[Sitt]

Camera is a ZWO ASI 533 MC Pro, the width of the Celestron OAG is around 45mm.
If I used 75mm back focus then I can't get focus on an object around 1km away - I know 1km is not as far as a star when comparing distance but surely I should be able to get focus on something as close as 1km?


* Corrected size of OAG (depending on what adapters are used will affect size)

[Joshua Bunn]

Simon, if you can not get focus, you have to much or too little back focus in your image train for your scope backfocus, or you're trying to focus on something too close.

2 things: 1) what's the back focus of everything in your image train, flattener, adapter, spacing, OAG , filter wheel and camera.
2) try focusing on the stars.



Josh

[Astronut07]

Simon

According to ZWO information camera sensor is 6.5mm inside housing
So if u add 6.5mm plus OAG 29mm ( 35.5mm ) total

Minus 35.5mm from 75mm= 39.5mm

Therefore if Im correct you would need 39.5mm in spacers to get correct focus

[Sitt]

Guys, I really appreciate your help with this. Here's a link of a photo with measurements showing my image train that I'm using for clarity. Just add 6mm to the sensor
https://www.dropbox.com/s/qw5iuk2hha3h36d/Image%20Train.pdf?dl=0

[Joshua Bunn]

Hi Simon.
It appears you have 120mm of space between the flattener and the sensor. This may allow you to reach focus on something close, but I'm unsure if it would work on the stars, i dpnt think your ficuser would even have enough travel to allow focus on the stars with 120mm of space beyond the flattener. Also it's the incorrect distance for your flattener, which requires 75mm of back focus. Have I missed something, as to why you have this much space?

[Sitt]

Josh, using 75mm back focus I had issues with focusing on stars then it got very late so I packed up and tested it out yesterday afternoon.
With my Celestron 9.25 I can focus no problem on a house around 1km away and also focus perfectly well on a star by adjusting the focus. With my SkyWatcher I can't achieve focus on a star or the property 1km away using the recommended 75mm
Logic tells me that I should be able to focus on something 1km away and also a star without having to adjust the back focus as per my 9.25 which has a lot higher magnification than the SkyWatcher.
Of course I intend to get as close to the recommended 75mm as I can but sods law its been cloudy since. I just wondered if anyone on here had the same combination as me and wondered if they would give me advice...

[Joshua Bunn]

The 9.25 has a lot more focus travel with moving mirrors (I'm.guessing this is how you ficused) and so that may be why you were still able to focus.
My recommendation would be to point at the moon tonight and without your camera in the image train, see where the scope form an image of the moon, how far behind the flattener.



Josh

[Nikolas]

Is it inward travel or external travel?

[Sitt]

Using 75mm both inward and outward travel can't achieve focus. It looks like focus is starting to appear but gets to max outward which is why I increased spacing. hopefully tonight will be clear so i can test