Samyang 135 & 2600MM Focus

[SAW]

Hi,

Just setting up a widefield rig with a Samyang 135mm, 2600MM and ASI Air. I've got the astro Essentials Samyang Lens to M48 then M48 to T2, adapter, 5mm and 6mm spacers, 7x36mm filter wheel, Optolong filters, 2600MM screwed directly to the filter wheel (tilt adjuster removed) so I am around 44mm back focus. With the lens at infinity my stars are still large. When I had my 600d connected focus was around the infinity mark so I know it focuses with this set up. I feel like I need more spacer's ?

Edited January 27, 2023 by SAW

[Adreneline]

My experience would suggest adding 1/3 the thickness of the filters; the 135 is really sensitive to spacing.

Adrian

[michael8554]

If you're not able to pass through infinity focus, you won't be sure that you have best focus.

Since there's no CC or FR involved, which would require precise backfocus:

Try removing the spacers and see if you can pass through best focus.

Or add some more, I can never remember if you have to add or remove spacers to do that.

Michael

[Mandy D]

43 minutes ago, michael8554 said:

If you're not able to pass through infinity focus, you won't be sure that you have best focus.

Since there's no CC or FR involved, which would require precise backfocus:

Try removing the spacers and see if you can pass through best focus.

Or add some more, I can never remember if you have to add or remove spacers to do that.

Michael

Close focusing requires the lens to be moved further from the sensor. We add spacers for macro work. So, if you are not reaching infinity focus, you would need to remove spacers.

[Stefan73]

I've just got a Samyang to fiddle with and am using the astro essentials samyang to M48 adapter too.  I couldn't reach infinity focus with 44mm backspacing from the adapter using the 2600MC either.  43mm (ie 25.5mm of spacers) allowed focus for me (and to pass through infinity). 

I'm not sure it's doing the best it can though as I've apparently got 10-20% tilt (depending on which program you ask).  Although presumably nothing to with back spacing.

[SAW]

I've done this mod to my lens now, hopefully clear tonight to test.

 

[drjolo]

I also found out that SY135 is sensitive to backfocus - if you have sharp stars and the focus ring is not at the infinity mark, they may be distorted in the corners. The larger sensor the more precise backfocus distance should be.