30mm wide view 2 inch focus

[nobby2 (Neil)]

Tested out the bintel 30mm wide view lens for the first time last night. I couldn't extend the focus out enough for correct use. Its the standard inclusion on my new Bintel 8 inch Dob. It is using the Crayford Focuser with 10:1 microfocus. Am i missing something?

[mental4astro (Alexander)]

Many solid tube mass produced scopes can see the primary mirror set too close to the focuser. Many reasons for this. One is the OTA set up as a dob is the same one that is used for astrophotography, and this requires the primary mirror to be placed closer to the focuser/secondary for the camera to achieve focus - to make a whole new tube line just for visual use is commercially unviable. Another thing is the true focal length of any mirror expressed in the focal ratio is not exactly true to the millimetre. There will be variations between each, and sometimes significant differences longer or shorter.

If the draw tube does not ruck out sufficiently to achieve focus, you can pull the eyepiece out of the draw tube, even upto half way out. Another option is to get an extension tube. My 8" dob is specifically made for photo, so the primary mirror is closer to the focuser, and I have no way of achieving focus for visual if I don't use an extension tube.

[nobby2 (Neil)]

It works just fine if you pull the eyepiece out of the draw tube a bit. Probably not optimal, so an extension sounds good. Bintel will also replace it, so all's good. The 2" wide view is a new experience.

[bytor666]

You could also try moving your primary mirror back a bit. I used to use an extension tube for all of my eyepieces and someone mentioned this and it worked out for me. I no longer need an extension tube.

[Renato1 (Renato)]

Originally Posted by nobby2

It works just fine if you pull the eyepiece out of the draw tube a bit. Probably not optimal, so an extension sounds good. Bintel will also replace it, so all's good. The 2" wide view is a new experience.

Providing plenty of the barrel is still in the focuser, do you really need an extension tube? I'd just put some insulation tape or thick rubber bands around the top of the barrel to stop the eyepiece going in too far, and leave it at that.
Regards,
Renato

[doug mc]

Renato has got it right. If you want to, get an extender, dont start modifying your tube length. The GSO 30mm superview is a good eyepiece. It has a 70 degree afov. With a GSO 2inch barlow you can use it in the 2x mode, and by screwing the barlow lens into the eyepiece, it works as a 1.5x power booster.