Question, Focal Reducers & FOV

[lenscap]

I use a 200p/F5 for visual only. My longest FL EP is a 25mm Plossl with a 22mm field stop which gives a FOV of about 1.25 deg.

I would like a much larger field to view the larger open clusters & similar size targets.

Expensive SWA EP's & all 2" EP's are beyond my meagre budget.

I could find say a 32mm (1.25") EP with a 27mm field stop which would deliver about 1.5 deg; better, but not enough.

But (unbranded) focal reducers are not too expensive. If I used say a 0.5 reducer with my 25mm EP this would give me about about 2.5 deg which is just what I'm looking for.

Is my analysis correct?

Would this give pleasing views?

Or would the shadow of the secondary be too noticeable at a Mag of 20X ?

Any advice would be welcome. Thanks for reading.

[drjolo]

Can you link the reducer you refer to? I found for example such reducer https://www.teleskop-express.de/shop/product_info.php/info/p676_TS-Optics-Optics-TSRED051-Focal-reducer-0-5x---1-25-inch-filter-thread.html but it illuminates field of approx. 12mm diameter, so it will not give much benefit. I think this one is for some small sensor cameras only.

[lenscap]

9 minutes ago, drjolo said:

Can you link the reducer you refer to?

This is the link. I dont have a specification for the item.

http://ensoptical.co.uk/1.25-0.5x-focal-reducer-

[Louis D]

First off, you'll need a 2" focal reducer to make any real difference.  Second, I have one and have tried it will my various 2" (or 1.25" with an adapter) eyepieces and haven't been able to get anything to come to focus due to the massive in travel requirements.  Even if I could, the edge distortion would probably be way worse than a cheap 30mm 80 degree or 40mm 70 degree 2" eyepiece.

All this assumes you have a 2" focuser.  If you only have a 1.25" focuser, it limits you to a 27mm field stop as in the 32mm plossl.  No amount of focal reduction will change that.

[michael.h.f.wilkinson]

To get a wider FOV, you will need a 2" focal reducer, simply because only a focal reducer with a 2" outer diameter will grab a sufficient area of the focal plane of the telescope. Threading a 1.25" focal reducer into a 25 or 32mm EP will not give access to a larger section of the focal plane of the telescope.

[michael.h.f.wilkinson]

1 minute ago, Louis D said:

First off, you'll need a 2" focal reducer to make any real difference.  Second, I have one and have tried it will my various 2" eyepieces and haven't been able to get anything to come to focus due to the massive in travel requirements.  Even if I could, the edge distortion would probably be way worse than a cheap 30mm 80 degree or 40mm 70 degree 2" eyepiece.

All this assumes you have a 2" focuser.  If you only have a 1.25" focuser, it limits you to a 27mm field stop as in the 32mm plossl.  No amount of focal reduction will change that.

Snap!! must type faster

 

[michael.h.f.wilkinson]

I would go for something like a cheap 40mm 68 deg EP rather than a cheap 30 mm 80 degree EP. The latter are OK in slow scopes, but in an F/6 (I assume) 200mm Newtonian they show an awful lot of seagull-shaped stars at the edges of the FOV. If you can stretch to a 40mm Aero (clone of the 40mm TMB Paragon I used to have) that would be great, as it worked very nicely indeed in my 80mm F/6 scope. Another option is the 38mm Panaview. Not quite as good as the Aero, but still a good option, I am told.

[Louis D]

The only thing I've found a 1.25" 0.5x focal reducer useful for is with binoviewers to expand the field again when using a 2x nosepiece to reach focus.  I put the 2x on the front of the 1.25" diagonal and the 0.5x on the front of the binoviewer in my refractor.  For my Dob, I put about 50mm to 60mm of spacer rings between the two, with the 2x again closer to the secondary mirror.  In both cases, there is significant vignetting at the edges, but the 1.25x overall magnification factor is so worth it to binoview larger objects.

[lenscap]

5 minutes ago, Louis D said:

First off, you'll need a 2" focal reducer to make any real difference.  Second, I have one and have tried it will my various 2" eyepieces and haven't been able to get anything to come to focus due to the massive in travel requirements.  Even if I could, the edge distortion would probably be way worse than a cheap 30mm 80 degree or 40mm 70 degree 2" eyepiece.

All this assumes you have a 2" focuser.  If you only have a 1.25" focuser, it limits you to a 27mm field stop as in the 32mm plossl.  No amount of focal reduction will change that.

Thanks for that. Yes, I have a 2" focuser.

[Louis D]

1 minute ago, michael.h.f.wilkinson said:

I would go for something like a cheap 40mm 68 deg EP rather than a cheap 30 mm 80 degree EP. The latter are OK in slow scopes, but in an F/6 (I assume) 200mm Newtonian they show an awful lot of seagull-shaped stars at the edges of the FOV. If you can stretch to a 40mm Aero (clone of the 40mm TMB Paragon I used to have) that would be great, as it worked very nicely indeed in my 80mm F/6 scope. Another option is the 38mm Panaview. Not quite as good as the Aero, but still a good option, I am told.

Absolutely true about the 30mm 80 degree.  I have one, and it is miserable from 50% out at f/6.  However, it probably still looks better than just about any narrower eyepiece with a 2" 0.5x reducer screwed into the filter threads IF you could reach focus.  These cheap focal reducers tend to introduce field curvature which the 30mm 80 has loads of natively.  I was able to improve mine by using my 0.0x field flattener and extension tubes leaving only residual astigmatism.  If they flattened the field, those 30mm 80s wouldn't be so awful.

The older 40mm Meade 5000 plossl with a 60 degree field is quite a nice performer as well.  It has a flat field, but astigmatism again starts at 50% out, but it doesn't build so strongly.

I have never looked through the 40mm TMB Paragon or clones, but I'd like to someday to compare it to my 40mm Meade 5000 SWA.

[Louis D]

If you could live with the massive exit pupil at f/6 and the crazy long eye relief, a 50mm to 56mm Plossl might also be a cheap option to maximize true field of view.

[lenscap]

My 200p is f/5.

To summarize what I've learned here;

A 1.25" focal reducer wont work

A 2" reducer is impractical

I will have to save my pennies & try to find a (second hand) 2" 68-70deg EP like an Aero or similar. Goes to show there is no such thing as a free lunch.

Thanks, guys for the brilliant & very rapid response. You have pointed me in the right direction.