Max FOV with 1.25" eyepiece.

[Astro Imp]

I have a 1200mm focal length dob and want to stay with 1.25" eyepieces, what is the maximum TFOV and which eyepiece do I need to achieve this.

I'm sure I have seen @John address this but I can neither remember the answer or find the thread.

Thanks.

 

[Moonshane]

a 32mm plossl with 50 degree apparent field or a 24mm e.g. Panoptic with a field of 68 degree apparent field would give you just over 1.3 degree true field.

[Moonshane]

1200 / 24 = 50

68 / 50 = 1.36

 

 

[Astro Imp]

Thanks Shane.

[ronin]

Some years back there were a line of Antares W70 eyepieces which had a 70 degree field and they did a 25mm.

That would give 48x but a final field of 70/48 so 1.45 degrees.

Not sure how good there were, actually Antares had a decent reputation, and the only place in the UK that sold them was Rother Valley.

A lot wwill depend on the level of "complication" you want to go to. A 30mm 50 degree plossl gives the same as a 25mm 60 degree Starguider. Easy answer is to remain at the plossl eyepiece. But somewhere out there there may be another Antares W70 that just gives a bit more as long as it does it decently.

Edited June 19, 2017 by ronin

[Moonshane]

I'd wager that they were a little generous with 70 degrees. My understanding is that the 24mm 68 degree format cannot be exceeded in 1.25" given the barrel diameter.

[cloudsweeper]

I reckon the theoretical max AFOV for 24mm (1.25") is almost 76 degrees, but that's using the barrel as the field stop!

Alan - why not take the plunge and go 2"?  

A 42/65 Rev would give 2.28 degrees; a 30/82 would give 2.05 degrees.  These would give large exit pupils, and constitute a loss of light, but would still give lovely widefield views of brighter objects.

I like the 2" format so much that I have put adaptors on all my longer FL EPs so I can just plug them in in sequence!

Doug.

[Louis D]

If you are constrained to a 1.25" focuser, you can put the included 1.25" barrel on the 31mm and 36mm Baader Hyperion Aspherics.  There will be some vignetting, though.

[Astro Imp]

12 minutes ago, cloudsweeper said:

Alan - why not take the plunge and go 2"?  

I already have a 1.25" 0III filter and at the moment funds are a bit short so can't see me replacing this plus a new EP any time soon.

[Moonshane]

I bought something like this http://www.365astronomy.com/365Astronomy-2-to-1.25-Filter-Adapter.html for my Hb and UHC filters as in general I don't use them for objects that require a wide field. Also, I hate having to change the filter from eyepiece to eyepiece. The adapter can be screwed into e.g. a diagonal or extension tube and left in situ.

That said, when I need a field more than about 1 degree, I tend to use a different scope anyway. e.g. an 80mm or even a 120mm Startravel f5 achromat would be a lot cheaper for wide fields that a decent 2" eyepiece when bought used.

[Louis D]

8 hours ago, Moonshane said:

I bought something like this http://www.365astronomy.com/365Astronomy-2-to-1.25-Filter-Adapter.html for my Hb and UHC filters as in general I don't use them for objects that require a wide field. Also, I hate having to change the filter from eyepiece to eyepiece. The adapter can be screwed into e.g. a diagonal or extension tube and left in situ.

Great find.  Too bad it costs nearly twice as much here in the states.

Edited June 20, 2017 by Louis D
Corrected link