Antares f/6.3 focal reducer for 8" SCTs

[StuartT]

Does anyone know what the rear thread (male) size is on the Antares focal reducer? I have a Canon EOS T adapter with an M48 female thread, but the reducer seems to have a slightly larger diameter and won't fit into it.

Thanks

[powerlord]

I get one tomorrow coincidentally.

I assume its SCT thread - 2"/24

so youll need a reducer to m48.

stu

[StuartT]

36 minutes ago, powerlord said:

I get one tomorrow coincidentally.

I assume its SCT thread - 2"/24

so youll need a reducer to m48.

stu

Interesting. Thanks. So all SCTs have the same size thread?

So this.. ?

https://agenaastro.com/blue-fireball-sct-female-m48-male-thread-adapter-c-04.html

Edited June 21, 2021 by StuartT

[Padraic M]

Yes SCTs tend to use a 'SCT thread' back interface.
https://agenaastro.com/articles/guides/miscellaneous/astronomy-threads-explained.html#small

Don't forget that you will need the correct back focus between the end of the focal reducer and your SLR sensor. According to this article, that distance is 105mm:
https://www.cloudynights.com/topic/756752-help-a-brother-out-what-is-the-real-backfocus-spec-of-the-antares-f63-sct-reducer/

so if you're ordering a thread adapter, you may also need to order an extender. That Celestron SCT T-adapter could be interesting as it seems to be very close to exactly the correct length, and it has the correct thread on both ends.

The sensor plane is marked on a DSLR like this:
https://www.photokonnexion.com/definition-focal-plane-mark/

 

[powerlord]

well yer in the same boat as me - I was searching through net for same answers.

Yes, all SCTs other than the massive ones have this archaic 2"/24 pitch thread. The most common converters are those to T mount/42mm. And those would then just screw into a standard dslr camera t-ring.

Like you though, I thought that was a waste - why would I want to reduce it to 42mm.. Im using a FF DSLR so would prefer to keep it as big as possible.

Hence, you need an M48 (standard 2" filter thread and usually what's on the inside of 2" eye pieces and the like) to SCT. And then you can use it with you 48mm t-ring.

The one you've linked to is correct, however (again - just be reading up at weekend) I think the antares is around 105mm back focus. Hence, with a DSLR you need an extra 50mm or so. You get that if you go down the 42mm/tring way easy enough. e.g.:

https://www.firstlightoptics.com/adapters/astro-essentials-sct-t-adapter-50mm.html

But you are looking for a similar 50mm length sct to 48mm.. frankly if you find one of those let me know.. otherwise you need the sort of thing you linked to, plus an extension tube (48mm thread female to male, and.. whatever..40mm or so length) i reckon.

[powerlord]

3 minutes ago, Padraic M said:

so if you're ordering a thread adapter, you may also need to order an extender. That Celestron SCT T-adapter could be interesting as it seems to be very close to exactly the correct length, and it has the correct thread on both ends.

 

 

That's a standard t size (42mm) unfortunately - I got one of those as a fall back and that has arrived. it works with my standard canon 42mm rings.

[Padraic M]

I'd go the M42 route with a APS-C camera (OP has 750D).

[StuartT]

Ok, thanks everyone. I think I understand all this. 

I've now ordered the one powerlord mentioned (as I am in the UK). It looks to be the same as the Celestron one mentioned in the CN post.

4 hours ago, Padraic M said:

I'd go the M42 route with a APS-C camera (OP has 750D).

When you say you'd go the M42 route are you saying that because I have an APS-C sensor that I won't see vignetting with the smaller adapter?

(sorry for the basic questions, but I am still on the steep bit of the learning curve 🤣)

[Padraic M]

That looks identical to the Celestron one, so should be fine.

The APS-C sensor at 22.5x15mm has a diagonal of 27mm, so an open aperture of 42mm won't cause any vignetting. 

For a full-frame EOS sensor, at 36x24mm has a diagonal of 43mm so M42 would cause some problems.

I measured the distances with my own old 400D: ~45mm from sensor plane to camera lens mounting plate; 10mm for T-ring; 50mm for the SCT T-adapter gives 105mm overall.

[StuartT]

53 minutes ago, Padraic M said:

That looks identical to the Celestron one, so should be fine.

The APS-C sensor at 22.5x15mm has a diagonal of 27mm, so an open aperture of 42mm won't cause any vignetting. 

For a full-frame EOS sensor, at 36x24mm has a diagonal of 43mm so M42 would cause some problems.

I measured the distances with my own old 400D: ~45mm from sensor plane to camera lens mounting plate; 10mm for T-ring; 50mm for the SCT T-adapter gives 105mm overall.

Brilliant! Thank you Padraic 😀

[powerlord]

Tested today with my APS-C and no vignetting - whether direct or via reducer.

FF 6d though, very mild vignetting direct (50mm tube still), and major vigneting with reducer.

I managed to source an SCT to 48mm adapter so will see if thats any better when it arrives.

[StuartT]

43 minutes ago, powerlord said:

Tested today with my APS-C and no vignetting - whether direct or via reducer.

FF 6d though, very mild vignetting direct (50mm tube still), and major vigneting with reducer.

I managed to source an SCT to 48mm adapter so will see if thats any better when it arrives.

Good to know. Thanks.

Out of interest, can you post a link to the 48mm adapter you located?

[powerlord]

https://www.astroshop.eu/miscellaneous/astoptics-sct-to-m48-adapter-8mm/p,57905

still choking on the price right enough.

[StuartT]

38 minutes ago, powerlord said:

https://www.astroshop.eu/miscellaneous/astoptics-sct-to-m48-adapter-8mm/p,57905

still choking on the price right enough.

ouch! We astronomers are so exploited! 😒