Focal reducers and field of view

[Ags]

I've seen a couple of threads lately where it's been suggested that a focal reducer can be used to make a Mak a wide® field instrument and I felt that was just wrong. The maximum field is a limit imposed by the scope, not the eyepiece.

I have a 24mm hyperion that gives the maximum field for a 1.25" fitting, so I attached a 0.5 reducer to it to see if I was wrong. The hyperion has a field stop of 29mm which matches the field stop of my Mak which is approximately 28mm (measured by projecting a sunny day onto paper).

As expected, the true field of view was unchanged, with the outer 50% of the apparent field vignetted away - all the reducer was doing was showing me the scope's internal baffling... So I had the same view, but in miniature.

What I didn't expect was the drastic change to eye relief - the reducer reduces eye relief too... My eye lashes were mashed up against the eye lens.

So no, reducers do not increase the maximum field an instrument can show. I guess if I had a 12mm EP, I could use the reducer to change it to a 24mm EP without any vignetting, but the eye relief would be horrible.

Anyway, using the reducer was as unpleasant as I thought it would be, and I would not recommend it!

[ollypenrice]

Yes, this has come up before with regard to SCTs. On these, if you don't have a facility for 2 inch widefield EPs, then the F6.3 reducer does widen the field. However, if you have a good 2 inch widefield EP and 2 inch back then the reducer will not increase the field.

Olly

[Ags]

Yes, in my particular case the scope field stop is the same as the EP field stop so the reducer has no effect. If the scope's field stop had been just a bit more - say 33mm - then the reducer would have clawed a bit more field of view, but I think it would do so unpleasantly.

[Macavity]

Yes, the budget (0.5x Atik etc.) reducer provides the impression of being "drunk and peering through a drinking straw".

However it's anecdotal (and true!) that the MAK127 (and MAK150!) will work with common 2" eyepieces, given a suitable adaptor and/or diagonal. Of course, you're only going from a true field of 1.1 deg to 1.5 deg. But the extra 30% can be useful for "framing" a fair number of objects.

Of course, the MAK + 0.5x reducer idea will work quite well, with the latter placed on the end of a webcam / videocam nosepiece. It's then well possible to illuminate a so-called 1/2" chip at (effectively) F5-6. A tight squeeze, but helpful for many a globular cluster. And the real-time image is *notably* brighter on video cam...

I believe some, more expensive SCTs are rather better "baffled" so this... "fortuitous feature" of Skywatcher MAKs may not always apply.

[Mr Spock]

My C9.25 with the 42mm LVW gives 1.3° free of vignetting. That's as good as it gets with a SCT.

A focal reducer just adds more glass and can reduce the field due to its limited lens size...

For deep sky I tend to use a 22mm Nagler anyway and reserve the 42mm for open clusters. For wide field I use the 80ED (5° and 3° with the above mentioned eyepieces).

[michael.h.f.wilkinson]

My C9.25 with the 42mm LVW gives 1.3° free of vignetting. That's as good as it gets with a SCT.

A focal reducer just adds more glass and can reduce the field due to its limited lens size...

For deep sky I tend to use a 22mm Nagler anyway and reserve the 42mm for open clusters. For wide field I use the 80ED (5° and 3° with the above mentioned eyepieces).

Almost identical to my set-up: The C8 works fine with the 40mm Paragon to give 1.36 deg FOV, for real wide-field, I use the APM 80mm (5.6 deg TFOV in the Paragon ). In the past I used a 36mm 1.25" Plossl, combined with the 0.63x reducer. Whilst this works, the views are much better with a 2" EPs.

[CGolder]

I manage to get 1.12 degree's of perfect field in my 10" SCT and 41mm Pan and it certainly gets a lot of use and frames and double cluster lovely.

i used to have a 24mm hyperion and with a reducer it provided the same FOV as the 36mm hyperion but the image wasnt as nice and meant removing the reducer when going in high power so was more pain than its worth