Finally had a night were I could try out the Orion 8" imaging Newt at f/2.8. I had extended the screws on the primary to raise it up a few weeks ago and had a quick test to make sure it would reach focus with the ASA corrector, but at the time I did not really do more than test it. Last night was the first time I had a chance to view any nebula with it.
Conditions were so-so. At 1x with the binoculars, the Milky Way was easily visible with a of structure, but I could tell that the transparency was a bit off. The view was just a bit dull. Summer in Austin is almost always a bit hazy and humidity, while not high in absolute terms (probably about 40%) was high by Austin standards. Ambient temp was about 90 (sad for 10:00 PM and the tube I was using was not the lowest EBI tube I had, but last night, sky glow and transparency were the limiting factors). What all of this means is that it as not a great night for nebula but it was what I had, so I used it.
As with the 6", the ASA corrector really does produce a very nice well illuminated field of view with sharp stars across most of the field. I did a vignetting test and I estimate that the fully illuminated circle is probably about 12mm in diameter and drops off by maybe 25% at the edge of the field. Now this is not the scope... This is the filter that does this. I use a filter wheel, and at f/2.8, the filter opening is far enough in front of the photocatode that at f/2.8, it becomes the limit for off axis illumination. For general work, I always use the filter wheel because of the convenience. I have a 1.6x Barlow position, open, 610nm (which I really need to change to a 650nm or 695nm, a 12nm H-a and a 6nm H-a. (I really need to bump the 610nm up a position and fill that position with an agressive long pass). For dedicated nebula under dark skies, I ditch the filter wheel and move the filter to very short 1.25" nose and this can allow almost full photocatode illumination, but when I am observing from home, I always run the filter wheel.
Given the so-so conditions I was pretty pleased with the performace of the 8". While the power is only a little more than 25% higher, I could see the difference in several objects. I won't bore with repeating all of them, but the Trifid was the best example. In the 6", the nebula is rather small and the dark lanes are not separated out so well that they appear stark and black, but with the larger scope, I do feel like the extra scale gave me a view that was much brighter than in the 12", and while the scope was probably 50% magnficatgion of the 12", it was still good enough to provide a very enjoyable view. Cresent was another case where even though the change in scale is small, it does make a nice difference.
Where the scope did really well was on rich fields. The 6" does great under dark skies, giving fields that are dense with stars but under city skies, the inherent baseline limiting magnitude of a 150mm aperture is always going to dull down star clouds. The best example where I saw a big benefit on non nebula was the Ink Spot and the surrounding area. The Ink Spot itself was not darker in the bigger scope, but it was larger which I think gave the impression that it was a bit darker, but the field around it appeared richer than when I have viewed it with the 6". Same for Sagittarius Star Cloud. The field is a bit smaller, but the 8" does produce better star density simply due to the greater limiting magnitude of the larger aperture.
The good news though is that modifying the scope for use with the ASA corrector was easy and the results were excellent. Buying an 8" on the used market and adding the ASA corrector is a fraction of the price of buying an 8" Boren Simon. The only problem with going with an 8" is that few Alt-az mounts will handle this setup well. The IOptron Az Mount Pro does great with it, but it would not work for a 10", and since there are no 10"f/4 dobs on the market, there is not an easy solution to getting a 10" to work other than a GEM or one of the new convertible GEMs so it can be alt-az mounted.
Easy to make the ASA work with the 8" imaging Newt though and I am guessing it would be the same for a 10".