EFW 7*36 & L-OAG

Didier_SC

Is it possible to link the EFW with L-OAG with the M2.5 screws provided with EFW ?
T&R,
Didier S-C

Tech@ZWO

Didier_SC If you have the new 736mm EFW, you can connect them with the M2.5 screws.
The screws came from the EFW are M2.56, usually for connecting the EFW to the camera, and there are also some M2.5*8 screws came from the OAG-L, you can use these to connect EFW and OAG-L

Didier_SC

Thanks a lot very clear,
I'm waiting Christmass now ;-)

Didier_SC

One more question :
I'm calculating the back-focus and learned that 1/3 filter's thickness must be substracted.

Is it the same for OAG-L ? and 12/3=4 mm.

w7ay

Didier_SC I'm calculating the back-focus and learned that 1/3 filter's thickness must be substracted.

No. You must increase the mechanical distance of your spacers by 1/3 of the filter's thickness. I.e., add a spacer or washer to the train.

See page 8 of Katie Schwertz's Masters' thesis:

http://www.loft.optics.arizona.edu/documents/journal_articles/2010_Katie_Useful%20Estimations%20and%20Rules%20of%20Thumb%20for%20Optomechanics.pdf

Notice that adding a glass plate will throw the focus farther by a distance of 1/3 of the plate's thickness if your glass has a refection index of 1.5. The camera sensor needs to be farther away when you add the glass.

Refraction index of FPL-51, FPL-52 and FPL-53 optical glasses are about 1.496, 1.46 and 1.44 respectively; so 1.5 is a good enough approximation.

Filter thickness is usually between 1mm and 2mm. The Astrodon and Chroma filters are 2mm, for example.

(Ignore the "55mm backfocus" examples given by ZWO in their documentation -- they are wrong, and do not take into consideration this property of optics even when they show a filter wheel or filter tray that obviously will have extra glass. Caveat emptor.)

Chen

Didier_SC

Very interesting,
could you confirm that I need to add 4 mm (12/3=4) due to OAG-L prism ?

Didier S-C

w7ay

Didier_SC could you confirm that I need to add 4 mm (12/3=4) due to OAG-L prism ?

No. The light cone from the telescope to your main camera does not go through the OAG prism. Therefore, add no additional spacer.

The OAG prism is positioned just outside of the light cone that is intended for the main camera, otherwise it will cast a shadow.

The light for the guide camera does go through an internal reflection through the prism glass, and does indeed need to be compensated because of the glass. However, the OAG designer should have taken care of that for you by lengthening the mechanical distance to the guide camera with respect to the distance to the main camera by the appropriate amount.

(Because the guide camera is picking up light outside the good part (inside "image circle") of the light cone from the telescope, it usually gives distorted images of stars.)

Chen