How precise are the Focal Lengths of Esprit refractors?

[tomato]

The reason I ask is I have just set up my Esprit 150 with the OAG but the focus point for the camera is a few mm further in than usual, requiring me to remove the optical window from the ASI 120. The only thing that has changed is I inadvertently changed the scope over to my other 150, so the FLs must be different?

[Earl]

the optical window can alter the focal length.

Correction, where it comes to focus, all glass in the light path will effect it to some extent.

 

 

Edited August 22, 2022 by Earl

[vlaiv]

Only way that I can think of to test if there is difference in focal length between two Esprit 150 scopes is to compare plate solve data taken with same camera (or shoot target at same distance and measure some feature).

There could be other reason for different focus position with respect to OTA - that is placement of cell and collimation.

If there is push/pull mechanism in cell - one lens can be more pushed than pulled in collimation procedure which will change focus position with respect to ota but not focal length.

Temperature can also play a part as tube changes length. If one OTA is carbon fiber and other is aluminum - there could be difference there.

What I don't get is that you mention OAG and ASI120.

If OAG / guide sensor / main camera setup is the same - just changed scope - then it should work regardless of any potential change in FL or change of FL position with respect to OTA as all is compensated with focuser.

 

[CraigT82]

What he said 👆🏻  
 

Did the OAG prism move at all when you were swapping them over? 

[tomato]

No, there was no change in the OAG prism location, but with the main camera at focus, the guide camera had to go further into the OAG helical focuser to achieve focus to the extent that the screw in sensor window had to be removed to get focus, the helical focuser was already at the limit of the travel (at it’s shortest).

I did note that the main focuser was not in the same position as previously noted with the main camera at focus, (different by a few mm) but of course this was the other Esprit, not the one previously used. 

I do see what you are saying about the main focuser compensating for the change in the FL of the refractor, so why does the guide camera have to change it’s position to reach focus?

 

[tomato]

9 hours ago, Earl said:

the optical window can alter the focal length.

Correction, where it comes to focus, all glass in the light path will effect it to some extent.

 

 

The guide camera window only was removed because it had to be to get the camera closer in, but… I may have inadvertently swapped the UV/IR cut filters in the optical train, although I am surprised a different brand could change the effective FL by ~3 mm.

[tomato]

And another variable has just occurred to me, I can’t be certain the same SW Field Flattener was put on the scope, would these be made with such a wide variation, given they are both SW items made for the Esprit 150?

[vlaiv]

Maybe distance thing again.

With reducers - changing distance changes reduction factor and focal length / focus position.

Not sure if similar thing happens with pure flattener.

[inFINNity Deck]

Maybe there is a difference in tube lengths between them? I measured mine, going from the front-end of the lens-cell forward to the front-end of the dew-shield (+0.100m), then back to the filter-wheel (-1.172m), back to the camera (-0.010m) and back to the chip (-0.0065m), so the chip is +0.100-1.172-0.010-0.0065 = 1.0885m behind the front of the lens-cell. The distance from the rear of the "boat wheel" to the camera is 0.297+0.010+0.0065 = 0.3135m, making the tube with "boat wheel" 1.0885-0.3135 = 0.775m (last focused this  morning at around 16°C).

When I plate-solve an image using Astrometry.net I get a resolution of 0.729855"/px. The camera is a ZWO ASI1600MM Pro Cool with 3.8 micron pixels, so the focal length would be 3.8 / 0.729855 x 206.3 = 1074.1mm, which is a bit more than than the specs of 1050mm, perhaps due to the flattener?

Nicolàs

[vlaiv]

3 minutes ago, inFINNity Deck said:

When I plate-solve an image using Astrometry.net I get a resolution of 0.729855"/px. The camera is a ZWO ASI1600MM Pro Cool with 3.8 micron pixels, so the focal length would be 3.8 / 0.729855 x 206.3 = 1074.1mm, which is a bit more than than the specs of 1050mm, perhaps due to the flattener?

I would not trust sensor data either.

ASI1600 is quoted at 3.8um pixel size and following specs (Panasonic data sheet):

But if we divide height with number of pixels we get:

17.6472 / 4656 = ~3.7902

Similarly with width we get:

13.3228 / 3518 = ~3.787

We can't really tell what is exact pixel size. For this reason we can only compare two esprits for focal length - but not measure with precision until we establish actual pixel size somehow.

[inFINNity Deck]

ok, so that would make focal length either 1071.3mm or 1070.4mm, which we can round to 1071mm (provided above dimensions are correct). At least this provides tomato (not his real name I presume?) with a direction.

Lettuce (aka Nicolàs)

[tomato]

Well, I’m taking some of the kit to a star party in September  so when it all goes back on the mount I will ensure I assemble the original Esprit and ancillaries. Then I can report if the guide camera has returned to it’s original focus position.

🍅☺️

[Earl]

12 hours ago, tomato said:

The guide camera window only was removed because it had to be to get the camera closer in, but… I may have inadvertently swapped the UV/IR cut filters in the optical train, although I am surprised a different brand could change the effective FL by ~3 mm.

3mm is a bit much but filters do vary in thickness also.