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Old 15-11-2021, 08:23 AM
bluesilver (Peter)
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Working out back focus?

Hi, I am in a little bit of a muddle with figuring out back focus,

I understand what it all means in general, but i kind of have 3 different numbers here and just hoping someone might be able to help me sort this simple issue out please.

My scope is a Skywatcher Esprit 150, and it has the Field flattener installed.
So on page 8 of my paper book manual that comes with the scope, it gives me two figures of back focus for this scope.

This is exactly how it is worded:
3) A length to ensure that the distance from the last lens of the field flattener to the CCD sensor is exactly 100mm

As it can be difficult to evaluate the position of the field flattener lenses inside the frame, the distance can be more conveniently measured from the lenses mount edge, as shown in the picture. ( basically the measurement is taken from the base of the field flattener to the CCD )
https://inter-static.skywatcher.com/...1416868668.pdf
From there to the sensor, the correct distance should be exactly 96mm

So i have two figures there, and i am guessing 96mm is the one to use as it is measured from the base of the field flattener ( not from the end or the thread ) to the CCD

Now after all that, the camera that i have is the ASI 2600mc, according to ZWO, that correct back focus is 55mm and the sensor is 17.5mm inside the body of the camera.

So i have all the included a photo of how it is all layed out:

On the far left is the adaptor that screw onto the scope,
Followed by the Field Flattener
Followed by the adaptor to go from the Field Flattener to the spacers ( this is 43mm)
Followed by a 21mm spacer
Followed by a 16.5mm spacer
Followed by the asi2600mc camera

So all up i have a total of 98mm from the base of the field flattener to the CCD

So how do i work out if i have the correct back focus with all those numbers?
98mm seams close to the figure in the book of 96mm, but what about this figure of 55mm back focus for the asi 2600mc that the ZWO site gives me?

As you can see, i am in a bit of a confused mind here at the moment, or will the setup i have work?

Any help would be appreciated.
Thanks.
Peter.
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  #2  
Old 15-11-2021, 09:20 AM
AdamJL
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For what it’s worth, I use 55mm back focus from the field flatterers of my Esprit 120 to the same camera and it works fine. The Field flatterers actually come with required “extenders” built in.
Others can chime in but in a week or so I can take pictures and measure up for you; we’re moving house right now so everything is packed away
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Old 15-11-2021, 09:28 AM
JA
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Hi Peter,

All that matters is that you use the correct (presumably) the specified distance from the flattener, which is specified as 100mm from the last optical surface or 96mm from the flattener rear mounting surface to the ZWO 2600's sensor which is buried 17.5mm within its body. Using the suggested 96mm requirement that means you need to make up 96-17.5 = 78.5mm with spacers/adapters from the flattener rear mounting surface to the ZWO 2600's front mounting surface.

If it were me I would measure all the spacers and adapters, etc to be 100% sure of things. I would even try to verify that the flattener rear element is 4mm below its rear mounting surface.

Whatever you do is predicated on what the manufacturer has told you about the Scope/flattener back focus being 100mm (or 96mm from its mounting surface) being correct, but there may be more optimal positions that people have found in use, and maybe even the 98mm you suggested you had is acceptable.

1st Job - measure all the bits to be sure what you have.

Best
JA

Last edited by JA; 15-11-2021 at 09:43 AM.
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Old 15-11-2021, 09:52 AM
bluesilver (Peter)
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Thanks for the replies and advice, appreciated.
Yes, i have measured up everything and the total amount of spacers come to
80.5mm
So i might be actually 2mm too long in my case as you mentioned, 78.5mm is the number i would be looking for then.

Will it make any noticeable difference being 2mm too long?
I have tried doing a bit of a search to find out what happens if you are too long or too short from the optimal back focus, and all i get is that you won't reach optimal back focus.

So not sure what actually happens there, will the stars be out of focus or something? as you can adjust focus via the focuser itself, a tad confusing there.

If i am the fore 2mm too long, i think the best option would be to shorten the spacers made by ZWO as they appear to be a common size, so either go for a M42-M48 14.5mm extender or a M42-M42 19mm extender, just finding then at this size could be a challenge though.
The Flattener has a 68mm thread, so finding something to fit that would be much more of a challenge.
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Old 15-11-2021, 10:08 AM
JA
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Quote:
Originally Posted by bluesilver View Post
Thanks for the replies and advice, appreciated.
Yes, i have measured up everything and the total amount of spacers come to
80.5mm
So i might be actually 2mm too long in my case as you mentioned, 78.5mm is the number i would be looking for then.
Based solely on both manufacturers' data, YES, but you never know it may work acceptably at differing spacing. People are always tuning their spacings: An Esprit 150 with that Flattener owner could perhaps comment.

At the risk of telling you how to suck eggs, given the way the manufacturers have specified their data you need to be certain your 80.5mm stacked adapter & spacer measurement is indeed from mount surface to mount surface and NOT perhaps from mount surface to the end of the male thread.

I will try to edit a copy of your photo to mark this - just in case. Added below. Required Spacer Dimension excluding Male end thread.

Best
JA
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Last edited by JA; 15-11-2021 at 10:22 AM.
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Old 15-11-2021, 10:49 AM
bluesilver (Peter)
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Thanks again for the rely.
I can see what you are referring to and just went and did another measurement just to be 100% sure.
I am 80.5mm, so basically 2mm too long.
I think i can find some adaptors to make up a 18mm M42 -M42 spacer.
Or some adaptors to make up a M48 13mm spacer
This would be in place of the 21mm M42 - M42 spacer.

It would give a distance of 77.5mm, I would then just add a 0.55 tin shim to bring it to 78mm

So that 55mm back focus on the ZWO site in regards to the asi2600mc doesn't really effect or mean too much in this setup then?

I think that sounds about right?

Last edited by bluesilver; 15-11-2021 at 11:53 AM.
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Old 15-11-2021, 01:08 PM
JA
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Before you buy or possibly cut anything, you might also try to measure the Flattener Rear Optic dimension, just to make sure all that is being said in the manual "adds up" logically. Maybe you will find your lost/added 2mm there. According to what's said in the manual, that dimension should be 100-96 = 4mm.

You could measure it (x) in the diagram by CAREFULLY measuring from the surface of the rear optic in the flattener to the end of the threads (Dimension B in sketch below) and subtracting Dimension A in the sketch below (the dimension from the end of the threads to the rear mounting surface of the flattener. If you get 4mm then that's one less issue to be concerned with OR if you get 2mm, then you have your 100mm overall.
Click image for larger version

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If none of that works out, or even if it does, try to get in contact with other Esprit 150 users to get an opinion.

Best
JA

Last edited by JA; 15-11-2021 at 01:19 PM.
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Old 16-11-2021, 06:47 AM
bluesilver (Peter)
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Thanks for the explanation and also the diagram, I can see what you are referring to.
I have measured it up and surprisingly it is all spot on to what the manual says which is a first.
So what i plan to do is take the first adaptor closest to the camera and put in a lathe and spin off 2mm, this will get me to the spot on measurement.
I think it is the easiest way and also the cost effective way.

I appreciate all the help and explanations, It has helped out a lot,
Very much appreciated.
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Old 16-11-2021, 03:23 PM
JA
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Quote:
Originally Posted by bluesilver View Post
Thanks for the explanation and also the diagram, I can see what you are referring to.
I have measured it up and surprisingly it is all spot on to what the manual says which is a first.
So what i plan to do is take the first adaptor closest to the camera and put in a lathe and spin off 2mm, this will get me to the spot on measurement.
I think it is the easiest way and also the cost effective way.


I appreciate all the help and explanations, It has helped out a lot,
Very much appreciated.
Hi Pete,

When/if you do that just make sure that there is still enough female thread left inside the spacer that you trim such that the male thread from the mating part doesn't bottom out and negate the whole exercise, other wise you may need to trim it as well. Hopefully not, as that wouldn't leave much male thread remaining.

And another last reminder (before cutting anything) that given the stories/posts one always hears about users testing for correct flattener and reducer spacing in telescopes maybe you should simply give your existing 102mm set of spacers a try before shortening them the possible 2mm to 100mm.

Over and above all that, I'd still try to reach out to some Esprit 150 users out there to get their stories of what they've used and/or works for them.

Are there any comments from any ESPRIT 150 Users for Pete on the issue?????? .....

Best
JA

Last edited by JA; 16-11-2021 at 03:39 PM.
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Old 17-11-2021, 12:16 PM
bluesilver (Peter)
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Thanks for the advice,
I did miss out a critical component when i was lining everything up for the image of what i had.
I replace of the 21mm spacer i actually have a ZWO filter housing with filter.
While this filter housing is still the same 21mm, i didn't take in account of the thickness of the actual filter itself.
So that there adds roughly 0.5mm
Worked out that i really only need to replace the 16.5mm spacer with a 15mm spacer which was fairly easily to come by.
So problem all solved, now just need to wait until some clear skies to put into practice and see how it performs.
Very much appreciate all the help and advise, was enough to get the mind thinking a bit more.
Appreciated.
Peter.
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