John

The diagonal would look a bit like the one pictured below. The 2x Barlow lens should have a glass element in the bottom end of it. If you have 2 eyepieces with different focal lengths (eg: 20mm and 12mm or similar) then you can get started. The diagonal would make viewing through the scope at the sky easier though (less back / neck breaking !).

The offset device is an erecting prism for terrestrial viewing. It turns the image that the scope produces into the orientation that our eyes see. I would do everybody a favour and destroy the sun filter. One more of these dangerous devices less is a good thing. Does it come with a diagonal prism / mirror (goes in between the end of the scope and the eyepiece) ? This will make viewing through the scope at the sky somewhat more comfortable.

The Stellarvue ED102 is an older model I think. The 102ED Access has replaced it and I think they changed the glass in that one.

If the glass type is not mentioned assume FPL-51 or a Chinese equivalent. The price would indiate that too. FPL-53 (or equivilent) objectives are quite a bit more expensive due to the cost of the ED glass blanks.

Mike has confirmed my feelings on the differences between the AZ-5 and the AZ-4. The latter will hold more and hold it steadier IMHO. No slow motion controls though. Skytee 2 is a much more heavier duty mount again - mine holds my 130mm F/9.2 triplet refractor pretty well and that scope would completely swamp an AZ-4.

Canon Optron make the fluorite objectives for Takahashi and Vixen as well.

"... for DSO, which makes up 95% of my observing... " Is this balance likely to change ? If not, going for an ED for 5% of your viewing interests might mean accepting aperture compromises to DSO viewing. With your desire to upgrade the focuser and continue to use the AZ-4 mount, it's a tricky balancing act William Optics used to market a 110mm F/5.95 FPL-51 ED doublet which might have fitted the requirements and it sells, when it comes up, for £500-£600.

It would for planetary / lunar/ double star viewing but not really for observing DSO's.

I replied to your other thread with this title yesterday (or maybe the day before) but I'm a bit confused what you are trying to get at with this new thread with the same title that you have started Also much of the above discussion in this thread seems now to relate to high power planetary type performance which does now really relate to dark skies particularly.

I do use my 3.5mm XW and 2-4mm zoom in my 12 inch dob from time to time for specific purposes such as spotting the faint planetary moons of Uranus and Neptune. For more regular planetary / lunar observing I find that 5mm (318x) is usually as much as I want to use.

My 12 inch dob comprises of an Orion Optics optical tube assembly (bought pre-owned) and a mount made for me by a skilled SGL member in a similar style to the Orion Optics ones. It's my least expensive scope but has given me more enjoyment and "wow" moments than any other that I have owned. It's also quite light and easy to move around for a 12 inch aperture scope weighing about the same as a 10 inch Skywatcher dob. That weight difference makes a lot of difference to me because I need to be able to set the scope up and tear it down quickly and move it about during a session. I found the 12" Skywatcher and Meade dobs a bit too heavy to move around with ease, for me at least. The primary in mine is what Orion Optics call Research Grade in terms of quality. It performs very well Having had dealings with Orion Optics over some smaller orders I would not buy one new from them though. Nor would I buy a new one at all to be honest with you. They depreciate a lot so waiting around until a decent used one crops up is the way that I would play it.

They work OK in slow scopes, eg: F/10, F/12 etc. The field of view is limited to around 40 degrees maybe a little less. This assumes that it is a 1.25" fitting eyepiece. You are right that the K means a Kellner design. The Circle-T ones were well made. Not worth a great deal today - just a few £'s really.

I use Stellarium version 0.18.3 and Cartes du Ciel version 4.0-3575. I update the cometary data daily. Hope that helps.

I've never tried the 35mm in this series. The 30mm is not quite as well corrected in faster scopes as as the 40mm I've noticed that. Sounds like I'll continue to give the 35mm a miss. Not that I need that focal length. I was using the 30mm with my ED102 F/6.5 at an urban outreach event last night and the views were lovely. The 4 Trapezium stars very crisp amongst the nebulosity even though the magnification was only 22x. Based on a combination of mine and your experiences Louis, looks like the ranking of the 3 focal lengths in terms of overall quality would be: 1st: 40mm 2nd: 30mm 3rd: 35mm And there we have it

William Herschel described Beta Mon as “one of the most beautiful sights in the heavens” when he discovered it