Is it worth it? 1.25" diag vs 2" diag for an 8" SC

Hello All,

I'm sure this question has been asked and answered but I can't find it anywhere. I'm a couple of years into the hobby, gone from a 4.5" reflector to an 8" SCT with a 1.25" setup. Wondering if I'll see a big difference if I put a couple hundred into a 2" diag, and an ep.

Nagging question is...if the hole in the back of the OTA is same, how does more light get througt the 2" stuff?

Any major difference in mfg'rs once you're above the standard Meade/Celestron group. Don't want to throw a ton of money into a top of the line model. What's a good middle of the road brand.

Any articles on the net or reviews would be much appreciated.

Thanks -- Jeff

The rear "hole" in an 8 inch SCT is actually made to give a clear "total unobstructed illumination" image to a 35mm camera frame. That's because when these scopes were designed back in the '60s and '70s most astro photo types would use a 35mm camera to take photos, and that standard dictated that size as the maximum for the real field size for SCTs.

The figure many folks seem to use these days is that plus a little extra or about 45 to 48mm as the maximum usable size of the "real" illuminated field or about the maximum field stop on most 2 inch EPs. You may get some light cut off at the edge of a 48mm field but most folks don't notice it too much.

So, if you stick with 1 1/4 inch EPs you should not see any darkening at the field edges, but are limited by the maximum size of the field you can "cram into" the 1 1/4 inch EP barrel. On the other hand with a 2 inch EP you can cram more real field into the EP barrel (bigger field stops with very wide-field 2 inch EPs) but you might get dimming at the edges of the field as the SCT was not really designed to fully illuminate such a large real field.

Roughly translated (my understanding here) is that if you use EPs of 25mm to 32mm or less (depending on how WF they are) the field stops will always be able to fit into a 1 1/4 inch EP, so the cheaper 1 1/4 formats works just fine. But and if you want longer focal length (lower power) EPs with very wide apparent fields (for example a 40mm with a 70 degree apparent field of view) then you will need the 2 inch format to accommodate the bigger physical field stop size the EP requires. This means the EPs will be bigger, heavier, and cost a whole lot more money for that larger field stop a 2-inch EP format can give you.

So, (as an example) if you are content with about 60X with a 32mm Plossel (55 degree apparent field of view) then you can stay with the 1 ¼ inch format. If you want wider field views then you can go to 40X with a 50mm Plossel (same 55 degree apparent field of view). If you use EP with wider apparent field of views (like those nice but $$ TeleView Eps) you will have to reduce the EP focal lengths to get the same results.

You can also get a 6.3 reducer/field flattener here on AM for about 100 bucks that makes the SCT seem to have a shorter focal length. This will make a 1 1/4 inch 32mm EP act like a 50mm and give you the same apparent field of view but a wider real field of view at a lower power. Be aware, however, that such devices will not make the REAL field size (the illuminated spot in the back of the SCT) physically any bigger, so EPs that worked fine without the reducer may end up with a lot of dark edges with it.

This is my understanding - FWIW - TM

Consider a F6.3 focal/reducer corrector for your SCT. Flattens the field and reduces the focal length to about 1250mm, allowing you to achieve those same widefield views without getting tangled up with expensive 2 inch eyepieces.

Uncle Rod (Mollise) swears by 'em.

jon

I bougth a 2" visual back, diagonal, and 34 mm 2" eyepiece for my C8. For me, it was worth it. I get a wider field of view and I don't see any vignetting.

If you buy the $99 William Optics 2" diagonal for SCTs, you don't have to buy a special visual back; it just screws right on, saving you about $50. I didn't get that one (it was out of stock when I ordered), but I do have a regular 2" William Optics diagonal, and it provides the same (to me) views as my more expensive Televue diagonal.

So you should be able to get a WO diagonal and a decent 2" eyepiece for around $200.

Thanks to all for the input. I'm going with a 2" diag and 2" 32mm ep from scopestuff (Antares models). I'll post the results of my little experiment once I get a chance to use them.

Man, there's a lot of stuff to learn, f ratio's focal lengths, almost took my eye out measuring my pupil against an exit pupil 8O . (kidding)

Best wishes to all.