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Widest TFOV with 72mm APO?

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#1 raidambrosio

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Posted 27 June 2019 - 01:39 AM

Hello. I am considering to buy a 72mm APO refractor to use as super wide-field scope for star hopping with a C11. I am considering these two:

https://www.astronom...-refractor.html

https://www.astronom...fpl-53-f-6.html

They both support 2" eyepieces and accessories. However, I cannot find exact information on what is the maximum true FOV they can give with appropriate eyepieces. I suppose it also depends on the internal diameter of the focuser they have.

In theory, a 72mm APO f/6 has a f.l. of 432mm. Thus, with an eyepiece William Optics Swan 40mm (72 degrees AFOV) it should return 11x magnification and a 6.5 degrees field of view. Seems nice. But is it really true?

Can anybody with these APO comment on the highest TFOV they were able to obtain?

thanks~~

 



#2 Redbetter

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Posted 27 June 2019 - 02:28 AM

The specs state 420mm focal length for the SW, and 430mm for the AT.    The difference in the true field of view will be negligible.  To calculate it you need the field stop diameter.  I don't know if that is published for the eyepiece you mention, but 46mm is the typical maximum in 2".  The formula is simple:  TFOV = 57.3 * field stop (mm) / telescope focal length (mm).   That yields 6.28 and 6.13 deg respectively for the two scopes.

 

I don't have these scopes mentioned, but I do have the AT60ED f/6 which I have used with 46mm field stops.  It works just fine, but there is a lot of field curvature in these short focal lengths, so one will see a different focus for the edge vs. the center.  When taking in the full field one will typically want to focus somewhere between the two extremes.  To me that doesn't matter much for ultra low power/max pupil where my eyes are the limitation to star shape anyway.  I typically use wide pupils looking for the extent of large diffuse nebulae and such.  

 

Keep in mind that the set up with eyepiece, 2" mirror diagonal, and scope will be in the 7lb range.  I am not trying to discourage you, but these set ups get chunky when you go to 2", so prepare for that.


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#3 james7ca

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Posted 27 June 2019 - 02:54 AM

Well, when using a refractor you can use any eyepiece focal length that you want to give the widest field and/or the lowest magnification. The only real limitation is whether you want to keep under some exit pupil size. For a young person and under dark skies and with full dark adaption you might want to limit yourself to a 7mm exit pupil and thus 72mm / 7mm ≈ 10X. That would mean an eyepiece with a focal length of 432mm / 10 ≈ 43mm. However, you can use a longer focal length but then the eyepiece exit pupil will grow to larger than 7mm which just means that your eye isn't accepting light from the entire aperture of the scope (but, the image will still be as bright as possible given the limits of your vision).

 

In terms of the largest true field, that's controlled by the size of the field stop in the eyepiece and that depends largely upon the limiting diameter of the eyepiece tube itself. Tele Vue likes to advertise that their 41mm Panoptic give the largest true field possible in a 2" eyepiece, but that would be true of any eyepiece that has the same size field stop (46mm in the Panoptic). It might even be possible to find a 2" eyepiece that has a slightly larger field stop, but there is an upper limit just because you're using a 2" eyepiece.

 

So, starting with the 41mm Panoptic we'd have: (field stop diameter / scope focal length) x 57.3 = (46mm / 432mm) x 57.3 ≈ 6.1 degrees.

 

Note, it's unlikely that the WO Swan 40mm would give 6.5 degrees, since that would mean its field stop would be:  (6.5 (degrees)  x 432mm) / 57.3 ≈ 49mm

 

Problem is, a 2" eyepiece has an outside diameter of 50.8mm which means the walls of the tube and any retaining ring to hold the field lens would be just 50.8mm - 49mm = 1.8mm in total thickness (that's the two walls of the eyepiece tube and the retaining ring, so we're talking a faction of a millimeter for each wall and the retaining ring itself, very unlikely).

 

Eyepieces that offer wider apparent fields of view just allow you to use higher magnifications before you reach the limits imposed by the diameter of their field stop. The higher magnification means a smaller exit pupil, which could be an advantage under some situations.

 

All that said, Tele Vue does offer some useful tools to select eyepieces, here are some of their references:

 

Eyepiece calculator:

 

   http://www.televue.c...=212&plain=TRUE

 

Choosing an eyepiece:

 

  http://www.televue.c...ChoosingEPs.pdf

 

Note, you can use the information provided in the links to help guide you in the selection of just about any brand or make of eyepiece, you don't have to buy from Tele Vue.


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#4 Jon Isaacs

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Posted 27 June 2019 - 10:08 AM

My two cents:

 

- If you want the widest true field of view in an 80 mm, an ST-80 with a 2 inch focuser is the way to go..  400 mm focal length, 41 mm Panoptic TFoV = 6.6 degrees, 31 mm Nagler = 6.0 degrees.  These are based on field stops, not AFoV. The Explore Scientific analogs are essentially identical.

 

- Few 2 inch diagonals have the 46 mm clear aperture needed for a 40 mm SWA with a 46 mm field stop.

 

- At F/5, F/6, eyepieces like the 40 mm SWAN are pretty messy.. 

 

- For star hopping with long focal length scopes, I consider an eyepiece with cross hairs essential. The cross hairs have more advantages than just precisely locating the center of the field.  They allow precise alignment with stars that are off Axis..

 

- I find a SV-50 finder with a modified (added cross hairs) 24 mm TV Wide Field to be my favorite finder. It provides a 6.8 degree field at 8.3 x. 

 

Jon


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#5 raidambrosio

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Posted 27 June 2019 - 10:23 AM

My two cents:

 

- If you want the widest true field of view in an 80 mm, an ST-80 with a 2 inch focuser is the way to go..  400 mm focal length, 41 mm Panoptic TFoV = 6.6 degrees, 31 mm Nagler = 6.0 degrees.  These are based on field stops, not AFoV. The Explore Scientific analogs are essentially identical.

 

- Few 2 inch diagonals have the 46 mm clear aperture needed for a 40 mm SWA with a 46 mm field stop.

 

- At F/5, F/6, eyepieces like the 40 mm SWAN are pretty messy.. 

 

- For star hopping with long focal length scopes, I consider an eyepiece with cross hairs essential. The cross hairs have more advantages than just precisely locating the center of the field.  They allow precise alignment with stars that are off Axis..

 

- I find a SV-50 finder with a modified (added cross hairs) 24 mm TV Wide Field to be my favorite finder. It provides a 6.8 degree field at 8.3 x. 

 

Jon

How did you add illuminated cross hairs to the Tele Vue eyepiece?


Edited by raidambrosio, 27 June 2019 - 10:23 AM.


#6 Jon Isaacs

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Posted 27 June 2019 - 11:16 AM

I did not add illuminated cross hairs. I dislike illuminated cross hairs.. I much prefer visible non illuminated cross hairs that are large enough to be seen under dark-dark skies.

 

I used 0.004" diameter hard brass wire. Used calipers to measure the barrel ID, cut them to length. One at a time I positioned them on the field stop by eye and then placed a drop of Crazy Glue to secure each end.

 

The alignment is very good but not perfect. I have also done this to a SV finder eyepiece.

 

The field stop ring needs to be wide enough to glue to, a 24 mm Panoptic wouldn't work.

 

The 24 WF is not the sharpest off Axis in an F/4 finder but it far better than any finder eyepiece you can buy.

 

Jon


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