Esprit 120 Imaging Rig - focuser in-focus position quite far out

[gilesco]

So, I have the following imaging train attached to my Esprit 120:

Stock Focuser  --> 3" to M74 adapter --> 9mm Filter Cell Adapter --> Esprit 120 Field Corrector --> 20mm Spacer --> ZWO OAG --> ZWO Spacer --> ZWO Filter Wheel --> ZWO ASI1600MM.

With a tape measure I can see that I have the 75mm backfocus needed on the Field Corrector, and my imaging is good.

I have only been out once so far and got good results, but I couldn't hope but notice that my "in-focus" position on the drawtube was at least 75% focus tube extended. It didn't seem to introduce a specific issue, but would rather the in-focus position be with the draw tube less extended.

I suppose that I could try and find either some M74x1mm or M65x1mm spacers to go somewhere between the Field Corrector and the Focuser, but these seem few and far between - anyone have their focus position that far out on the Esprit 120?

Is it an issue? Could it cause draw-tube sag?

Anyone recommend a spacer available in the UK?

[discardedastro]

Draw-tube sag is possible but with a good focuser shoudn't be an issue. I'd try and measure things in terms of the output - either using CCDinspector or the FHWM plotter script in PixInsight - to see if you've got any significant tilt in the images you're getting out. Try a few different frames from different pointing positions to see if there's any significant variation with regards to attitude.

[Adam J]

13 hours ago, gilesco said:

So, I have the following imaging train attached to my Esprit 120:

Stock Focuser  --> 3" to M74 adapter --> 9mm Filter Cell Adapter --> Esprit 120 Field Corrector --> 20mm Spacer --> ZWO OAG --> ZWO Spacer --> ZWO Filter Wheel --> ZWO ASI1600MM.

 

With a tape measure I can see that I have the 75mm backfocus needed on the Field Corrector, and my imaging is good.

I have only been out once so far and got good results, but I couldn't hope but notice that my "in-focus" position on the drawtube was at least 75% focus tube extended. It didn't seem to introduce a specific issue, but would rather the in-focus position be with the draw tube less extended.

I suppose that I could try and find either some M74x1mm or M65x1mm spacers to go somewhere between the Field Corrector and the Focuser, but these seem few and far between - anyone have their focus position that far out on the Esprit 120?

Is it an issue? Could it cause draw-tube sag?

Anyone recommend a spacer available in the UK?

The focus position is the focus position you cant change it, the camera sensor will always land in the same place relative to the lens, I would not thing that any of this would trouble the focuser on an esprit 120 though, they are pritty solid things.

Adam

[gilesco]

6 hours ago, Adam J said:

The focus position is the focus position you cant change it, the camera sensor will always land in the same place relative to the lens, I would not thing that any of this would trouble the focuser on an esprit 120 though, they are pritty solid things.

Adam

Yes, I'm talking about putting a spacer at the focuser end (at the M74 or M65 interfaces), simply so that the drawtube doesn't need to move as far out, I'm not talking about changing the distance between the lenses.

[ollypenrice]

Before fixing it I'd certainly want to be sure it was broke! As suggested, first check for tilt.

Olly

[gilesco]

8 minutes ago, ollypenrice said:

Before fixing it I'd certainly want to be sure it was broke! As suggested, first check for tilt.

Olly

There's nothing in the subs to indicate it is an issue. I just get worried when the tube is extended that far out, I do automated focusing (Sesto Senso 2) when I am absent from the scope, the ticks range from 0 - 97500 or so, focus position is at around 82000 - I would just be more happy if I was achieving focus at nearer the 50000 (around half way the tube extended).

I was more interested in whether other Esprit 120 owners find their focus position that far out.

[ollypenrice]

6 minutes ago, gilesco said:

There's nothing in the subs to indicate it is an issue. I just get worried when the tube is extended that far out, I do automated focusing (Sesto Senso 2) when I am absent from the scope, the ticks range from 0 - 97500 or so, focus position is at around 82000 - I would just be more happy if I was achieving focus at nearer the 50000 (around half way the tube extended).

I was more interested in whether other Esprit 120 owners find their focus position that far out.

They must do. The focal length of the lens is a constant. I'm sure you'd be able to source an extension tube if you were really worried, even if you had to get it made by Precise Parts or another machine shop. But surely something will already exist. FLO and Telescope Service may have something. I used to use one on my TEC140 before fitting the flattener which ate up some of the distance

Olly

[gilesco]

15 minutes ago, ollypenrice said:

 FLO and Telescope Service may have something.

Not seen any M74 or M65 spacers in the UK.

Starizona have one, designed for their reducer: https://starizona.com/store/apexed-esprit-120-150-threaded-extension

Available at Highpoint Scientific: https://www.highpointscientific.com/starizona-apex-ed-esprit-120-150-threaded-extension-for-0-65x-reducer-apex-e150ex

In Europe: https://www.teleskop-express.de/shop/product_info.php/info/p12813_Starizona-Apex-ED-Esprit-120-150-Threaded-Extension.html

No delivery until new year (and I'm in the UK, so will just wait and see for the time being).

[Adam J]

1 hour ago, gilesco said:

Yes, I'm talking about putting a spacer at the focuser end (at the M74 or M65 interfaces), simply so that the drawtube doesn't need to move as far out, I'm not talking about changing the distance between the lenses.

But that won't change anything the mass of the camera and other equipment will still be the same distance from the pivot point irrespective of what spacers you add. The only thing you could do is place a spacer behind the focuser. But as I say above you really should not need to do that, these are solid focusers. 

[gilesco]

18 minutes ago, Adam J said:

But that won't change anything the mass of the camera and other equipment will still be the same distance from the pivot point irrespective of what spacers you add. The only thing you could do is place a spacer behind the focuser. But as I say above you really should not need to do that, these are solid focusers. 

What you are saying is implying that there is a lens in the focuser drawtube (which I don't believe is the case).

We have the lens array at the end of the OTA, and that achieves focus at the camera sensor, we do have the field-flattener in between. All the focuser does is vary the distance between the lens array at the end of the OTA and the field-flattener / camera sensor at the other end.

When I achieve focus with this set up currently the focuser drawtube is almost at full extension, so by inserting spacers between the focuser and the field-flattener focus would be achieved without drawing the focus drawtube as far out of the OTA as it is currently.

[kirkster501]

Is everything screw fit?  If so I do not think you have an issue here on that focuser, like others have said.

[gilesco]

2 minutes ago, kirkster501 said:

Is everything screw fit?  If so I do not think you have an issue here on that focuser, like others have said.

Yes, on page 11 of the Esprit manual https://inter-static.skywatcher.com/upfiles/en_download_caty01416868621.pdf

I'd look for a spacer between either:

Focuser Tube and the DrawTube Adapter M74 male to female
DrawTube Adapter and the Field Flattener M65 male to female

I actually already have a 9mm AstroEssentials Filter Cell adapter connected between the DrawTube Adapter and the Field Flattener, fitted with a 2" IDAS Light Pollution filter.

[teoria_del_big_bang]

I had never thought of this before, but if I understand correctly @gilesco is concerned that the weight of the camera, FW etc on a focusser with draw-tube almost fully extended may cause it to sag and introduce tilt, but by moving the draw-tube in say 50 mm and fitting a light-weight 50mm spacer between the focusser and Flattener then with more of the draw-tube inside the focusser it will be stiffer and less prone to tilt.

It does sound a reasonable possibility to me, I think on my Esprit 100 the draw-tube is only about half way out so never considered it an issue (But so long since we had a some clear nights I have forgotten just how far out it is).

Steve

Edited December 15, 2020 by teoria_del_big_bang

[Laurin Dave]

I'm sure it'll be fine, I have an Esprit150 and ASI1600/EFWOAG etc, the drawtube needs to be a long way out to focus but I have never had any tilt issues with this setup and the stock focuser.   The only advantage an extension tube will give is that there'll be 2" or so (length of the extension tube) extra of drawtube on the scope side of the focuser bearings to possibly add a small bit of counterbalance to the weight of the camera etc.   The camera will still be exactly the same distance from the focuser bearings.  

[gilesco]

Thanks @Laurin Dave if the Esprit 150 is similarly in focus with the drawtube quite far out then I'm guessing it is normal.

It feels like a good sturdy well built focuser without any "give", but as I am new to the Esprit I wanted to satisfy that what I was seeing was normal.

I'll continue as is, but if I see a 20mm - 40mm M74 or M65 spacer at a good price I might just bite for it.

[arrayschism]

4 hours ago, gilesco said:

I was more interested in whether other Esprit 120 owners find their focus position that far out.

Yep, mine's the same - haven't got a precise measurement of what it is as it's not to hand right now... but the drawtube is way out before focus. 75% sounds about right where mine is.

[gilesco]

49 minutes ago, arrayschism said:

Yep, mine's the same - haven't got a precise measurement of what it is as it's not to hand right now... but the drawtube is way out before focus. 75% sounds about right where mine is.

Of course, the upside of this is you have space to add extra gizmos (e.g. a flip mirror) in the train and you will still be able to focus, whereas if the focus was just 25% tube extracted then it limits the number of gizmos that you can attach and not be able to focus at all...

I'm going to stop worrying about this now 🙂 

[rogmet]

Mine is the same also; there's a Pegasus Focus Cube 2 on there and of the 36,000 steps of full travel of the drawtube (or something like that), it gets focus between 30,000 and 31,000. I did think it looked over-extended too, but it's been rock solid.

[cfinn]

Same for me as well and I've had no issues. I have seen some similar discussion about this on Cloudy Nights and it wasn't an issue for anyone commenting there.

[Adam J]

14 hours ago, gilesco said:

What you are saying is implying that there is a lens in the focuser drawtube (which I don't believe is the case).

We have the lens array at the end of the OTA, and that achieves focus at the camera sensor, we do have the field-flattener in between. All the focuser does is vary the distance between the lens array at the end of the OTA and the field-flattener / camera sensor at the other end.

When I achieve focus with this set up currently the focuser drawtube is almost at full extension, so by inserting spacers between the focuser and the field-flattener focus would be achieved without drawing the focus drawtube as far out of the OTA as it is currently.

No you are not understanding at all.

I am talking about the distance from the primary. If you put and extender into any location after the focuser it will move the focus tube inwards yes but it will not have changed how far back your sensor is located at all. It will still be located in exactly the same position as it was without the extention and hence it will still be placing the same forces onto the focuser.

If your still having issues seeing that I am happy to produce a diagram tomorrow for you.

Adam

[gilesco]

11 hours ago, Adam J said:

No you are not understanding at all.

I am not trying to change how far back my sensor is.

Perhaps this (it is an exaggerated scale) will make it more clear. When the tube is extended further there is more sag (upper image of focuser tube and draw tube):

If the tube achieves focus without being as extended as much (lower image) there is less sag.

Of course the lower image means the camera is closer, and so spacers are required to the right of the drawtube to compensate for that distance.

In any case, as others have noted- and this is an exaggerated scale, the Esprit focuser is a very snug fit and others don't expect this issue. But if you had a cheaper not so snug focuser, it would be an issue.

Edited December 16, 2020 by gilesco
diagram

[Hogarth]

I have a Esprit 100 and my draw tube is also quite far out to achieve focus.

also whilst researching back focus on the esprit range , it seems widely accepted the manual is incorrect, well certainly for the 100. The manual says for mine 63mm of back focus when using a FF, but it’s actually 55mm.  I tried mine at 63 and at 55 to test and it’s certainly way better focus at 55mm

not saying the 120mm manual is wrong, but I would investigate this just to confirm.

 

 

[gilesco]

Beautiful set up by the way.

I think it is by design, to just allow for extra non-focus stuff after the tube. Any fears I had are alleviated, just initial quandries.. "why did they do it that way", "why is that like that", etc...

[gilesco]

31 minutes ago, Hogarth said:

 

 

 

 

Can't help but notice you have a Pegasus attached there, how did you go about that?

(I'm choosing to mount it on something attached to the tube rings).

[Laurin Dave]

1 hour ago, Hogarth said:

I have a Esprit 100 and my draw tube is also quite far out to achieve focus.

also whilst researching back focus on the esprit range , it seems widely accepted the manual is incorrect, well certainly for the 100. The manual says for mine 63mm of back focus when using a FF, but it’s actually 55mm.  I tried mine at 63 and at 55 to test and it’s certainly way better focus at 55mm

not saying the 120mm manual is wrong, but I would investigate this just to confirm.

 

 

its both 63 and 55 mm ..   the flattener has an 8mm spacer/ adapter attached that reduces the thread from M66 to M48..   on the 150 there’s a somewhat longer M62 to M48 spacer .(I think the 120 is the same as the 150)    So for the 100 it’s 63mm if your using the M66 thread and 55mm if using the  M48 thread ..