After using this high quality diagonal, I realize the 1.25″ star diagonal included with my Celestron C11 does not do the telescope’s optics justice.

I did not know this, and for some months after I bought my C11 SCT last year, I kept experimenting with different 1.25″ eyepieces and filters, as I was unwilling to make the jump to a 2″ diagonal and eyepieces.

Unfortunately, I kept coming up dissapointed on images, not seeing the brightness and clarity that I expected from such an expensive (and highly rated) telescope.

I purchased this 2″ Diagonal mirror last weekend. Celestron includes a small locking aluminum case. This is a nice touch, and I feel it hints at the quality of the diagonal. At the least it is impressive for the new buyer, albeit maybe a little pretentious.

I used this from my backyard for my first views. Even with the extreme light pollution from nearby Phoenix (you can’t see all of the Big Dipper with the naked eye many nights — it’s that bad!) I was seeing objects that I had never see even in deep space dark sky viewing sites before. I was surprised when I unexpectedly resolved the double star in Polaris. I admit to mixed feelings about this, as I wondered how much more I had been missing with the stock 1.25″ diagonal.

Earlier attempts to see M31 from my backyard resulted in blurred and indistinct images, but with a F 6/6 focal reducer and this 2″ diagonal it was bright and well defined.

Speaking of the focal reducer: I was concerned after I bought this diagonal that I would be unable to use the reducer, but the reducer screwed on the optical back threads as usual, and the diagonal screwed onto the reducer easily. I did find that I almost reached the end of the C11′s focus adjustment when I added a CCD camera onto this setup. However, I was still able to get a sharp focus with several turns left on the focusing knob.

I used this combination with an Oxygen III filter and viewed M42. Again, previous attempts from my backyard resulted in a partial view of M42, with the outer edges fading rapidly. However, the addition of this 2″ diagonal (and a 10mm televue eyepiece) resulted in a full eyepiece image of M42, with the entire nebula clearly and brightly obvious across the field of view.

I did not expect this dramatic of an improvement in the views, and feel I got more than I expected from this modest addition.

I considered a high end diagonal claiming 99% reflectivity instead of the 96% reflectivity of this diagonal. The more than doubled expense might be worth it, but I did not think so at the time I bought this diagonal. I didn’t think I’d get this much more out of adding this diagonal, and I can’t comment on what an additional 3% of reflectivity might add.

I have not compared this to other 2″ diagonals, but the incredible increase in quality compared to the stock 1.25″ diagonal is so spectacular that I feel as if I have a new (and far superior) telescope.

You can continue to use your old 1.25″ eyepieces, as there is an adapter included. Now that I’ve seen the improvement in upgrading to a 2″ diagonal, I’m planning on upgrading to 2″ eyepieces as well. Anyone want to buy a few hundred bucks worth of 1.25″ eyepieces?

I highly recommend this diagonal. If you’re using a 1.25 viewing system, consider upgrading to the 2″ diagonal and eyepieces before adding more aperture with a larger telescope.
Rating: 5 / 5