Using a t-adapter and no eyepiece (prime focus) you can not achieve focus with a Celestron Regal F-ED spotting scope. This applies for t-adapters with and without a barlow lens attached.
To take images with your SLR camera through the Regal spotter is to use the eyepiece and adapter ring provided. Another way is to use single magnification eyepieces. We recommend the Celestron X-Cel 25mm, in combination with the Hawke HX3705 adapter together with a compatible t-ring
You can purchase Celestron Spotting scopes from Northern Optics HERE