2" vs 1.25" with APS-C DSLR/observation

[jbrazio]

Hello all,

To start I don't know if I'm posting this to the correct sub-forum..

What is the difference between using a full 2" vs 1.25" based setup with a APS-C (cropped sensor) DSLR ?

And how about for observation, does it makes a huge difference ?

Thanks.

[jgs001]

I've shifted over to camera discussions...

With an APS-C DSLR you will get vignetting with a 1.25" setup, flats will help deal with it. However, with a 2" setup, the vignetting is just about gone, if not quite gone entirely. If you can, and you want to image with an SLR, get the 2" fittings. I can't comment for visual, I've only got 1.25" eyepieces, and just use an adapter to fit them into the scope.

[jbrazio]

Thanks for shifting the topic to the correct section.

If you still get vignetting with a 2" setup, how do people with FX cameras deal with it ?

[jgs001]

I put that in just in case ... Personally, I've never spotted it on my APS-C images on my 80ED with a 2" setup, although I know people that have had some vignetting. If I'm getting any, it's so mild as to not be visible, and as I always take flats, if there's any there it's removed in the stack. My RC has a 3" tube on the back, and there are 3" focusers around.

[jbrazio]

[OFFTOPIC]John, what's the weight of you telescope and camera ? Basically what weight are you putting on top of your HEQ5 mount ?[/OFFTOPIC]

Maybe I'll go with that setup, use 2" fittings from the focuser to the DSLR and buy 1.25" eyepieces for observation.. for sure this is the most cost effective solution as 2" eyepieces are expensive.

Nevertheless if someone uses 2" eyepieces that could provide their experience vs 1.25" would be helpful.

[jgs001]

the 80ED, 450d, dual mount bar, ST80 and QHY5v is about 7Kg I think... the RC weighs in at about 5Kg, compared to the 80ED of about 2Kg, but it still works fine for imaging with... I think it's cos the RC has pretty short tube.

I think you only need 2" ep's if you go beyond about a 32mm

[Gina]

I got some vignetting with ED80 and 2" fittings with my DSLR (1100D). When I got the focal reducer it needed a special DSLR adapter ring to increase the light path size from 42 to 48mm to avoid vignetting. I could overcome the vignetting with flats as it wasn't that bad but I think 1.25" fittings or filters would cause significant vignetting though I haven't tried it.

[jgs001]

I think I've got an image I shot with my ST80 clone, 1.25" fittings before I worked out flats that shows how bad the vingetting is... I may have a single sub, unprocessed using 2" to compare, if you want me to try and dig them out.

[jbrazio]

I think I've got an image I shot with my ST80 clone, 1.25" fittings before I worked out flats that shows how bad the vingetting is... I may have a single sub, unprocessed using 2" to compare, if you want me to try and dig them out.

John, if you don't mind posting them.

I got some vignetting with ED80 and 2" fittings with my DSLR (1100D). When I got the focal reducer it needed a special DSLR adapter ring to increase the light path size from 42 to 48mm to avoid vignetting.

How did you calculated the exact extension requirement for your setup ?

[jgs001]

NP, Here's the one with the 1.25" vignetting

and a single sub with 2" fittings, as I recall nothing has been done to this image

Both taken with my 450d

[Gina]

How did you calculated the exact extension requirement for your setup ?

With just the ED80 I found the focus was nearly obtained with the focuser right out. I measured the focuser range (75mm) so guessed 80mm for focussing point. So decided to go for a bit over half of this and got a 60mm extension tube. That gave focus with the focuser about a third to a half of fully out.

With the FR/FF no extension tube is required. The FR goes directly on the focuser tube itself and the DSLR directly onto the FR using a Canon version special adapter ring. This gives a very rigid connection to the camera and has a wide light path to avoid vignetting.

[jbrazio]

NP, Here's the one with the 1.25" vignetting and a single sub with 2" fittings, as I recall nothing has been done to this image Both taken with my 450d

I can see the darker sides on the first picture, for AP 2" is a must; I'm just missing the feedback from someone who could compare 2" VS 1.25" on visual observation.

With just the ED80 I found the focus was nearly obtained with the focuser right out. I measured the focuser range (75mm) so guessed 80mm for focussing point. So decided to go for a bit over half of this and got a 60mm extension tube. That gave focus with the focuser about a third to a half of fully out.

That's dependent of the DSLR on the end of the focuser ?

[jgs001]

I think the Canons will all need the same amount of extension on the same scope, they all use the same lens mount, so the distance from lens to sensor should be the same. I can't comment on Nikon, but I'd expect it to be somewhat similar.

[jbrazio]

But not related with the WxH of the sensor nor the MP count ? It just related with the internals of the camera, meaning distance from the lenses mount to the sensor for instance. I'm asking this because I have the risk when ordering, for instance, a ED80 OTA that it will not focus correctly.

[jgs001]

As far as I know, that's correct, an EF mount lens will work on any APS-C, APS-H or full frame body, irrespective of the sensor size or MP count, be it a 6Mp 300d or a 22Mp 5Dmk3.

I believe Nikon use the F mount (??) on all their bodies, so the same would hold true here, the main difference as I understand it, some lenses won't autofocus on the lower end bodies, something to do with the AF mechanism locations..

I bought a 50mm tube for my Celestron 80ED.

[jbrazio]

Yes within Nikon is the F mount basically since the beginning of times, what changed was the AF motor location that nowadays in built-in on the lenses and before was screw-like built-in on the body itself; only higher models (for instance my D7000 is capable of AF on all Nikon lenses) have the little screw (thank god, Tokina 116 PRO is a must :-).

Could you please post a link to a extension tube so I can see what we're talking and make sure I'll not screw it up ?

[jgs001]

Something like... http://www.firstlightoptics.com/adaptors/two-inch-focus-extension-tubes.html

[jbrazio]

Thank you John.