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Baffled by "desired" size for illuminated field

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#1 EddieRich

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Posted 02 August 2019 - 01:03 PM

I am thinking of building a refractor and came across this page ...

 

http://www.berfield.com/baffles.html

 

The thing I can't understand is "desired" illuminated field of view. That implies a feeling or an opinion or a choice. I'm not looking for someone to tell me what the correct illuminated field size is, I need guidance on how I come to my decision.

 

This same question (and other similar questions) have come up on CN before, but none of the threads answer the question about what the "desired" size is. What factors should influence my "desire"?

 

 



#2 TOMDEY

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Posted 02 August 2019 - 01:40 PM

I looked at the reference there and take their wording as positive, as in, "whatever you want (aka desire)... here's how to baffle for that." And then they go on to show to size and place the baffles so that your "desired field" is both not vignetted and is critically-baffled (aka does not see the tube walls).

 

PS: One's desirement is usually driven by either the largest eyepiece field stop or sensor diagonal that they intend to accommodate.

 

I just made this sketch a day ago, in a similar thread >>>    Tom

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#3 EddieRich

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Posted 02 August 2019 - 03:41 PM

Yes, I saw that thread (and nice cartoon by the way).

 

I will only be using 1.25" eyepieces, and I assume the field stop is significantly less than the eyepiece barrel size.

I don't have any eyepieces yet, so I can't tell what my "largest eyepiece field stop" will ever be.

 

I know I must put baffles and flocking, but I think if I just put a 6mm "margin for mechanical tolerances" I'll be fine.



#4 dogbiscuit

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Posted 02 August 2019 - 03:48 PM

Why not just design for 1.25" desired field?



#5 EddieRich

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Posted 02 August 2019 - 07:15 PM

Most (every?) eyepiece has a field stop, similar to a baffle. If I design for 1.25", and the field stop is 1/2", I will be cropping/blocking/losing over 50% of the "illumination".

 

I'll have to build a telescope, buy some eyepieces, and start experimenting.



#6 howardcano

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Posted 02 August 2019 - 07:23 PM

If I design for 1.25", and the field stop is 1/2", I will be cropping/blocking/losing over 50% of the "illumination".

No.  But of course, you can always use an eyepiece with a larger true field of view, meaning a larger field stop.

 

Here's one way to think about it: If the baffles aren't vignetting the light, then any star that is in the field of view will have all of its light from the objective concentrated at just one point in the focal plane.  The same is true for all other stars that are in the field of view.  Using an eyepiece with a larger field stop just lets you see more stars.


Edited by howardcano, 02 August 2019 - 07:50 PM.


#7 Starman1

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Posted 04 August 2019 - 05:12 PM

Most (every?) eyepiece has a field stop, similar to a baffle. If I design for 1.25", and the field stop is 1/2", I will be cropping/blocking/losing over 50% of the "illumination".

 

I'll have to build a telescope, buy some eyepieces, and start experimenting.

Other way around.

Illuminate a 1.25" field and the 1/2" field stop easily fits within it and is fully illuminated, even the edge of field rays.

 

The point here is that you do not need to illuminate the edge of the field rays to 100% if you want the baffling to be a bit tighter.

Newtonians are designed with a 70% illumnation at the edge, and no one complains about vignetting.

If you design it for the largest field stop ever made in a 1.25" barrel, design it for 1.25" at the focal plane and you'll be fine.

Most 1.25" eyepieces today have 27mm field stops or smaller, but there are and have been those with field stops up to over 30mm.


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#8 dan_h

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Posted 05 August 2019 - 11:51 AM

Most (every?) eyepiece has a field stop, similar to a baffle. If I design for 1.25", and the field stop is 1/2", I will be cropping/blocking/losing over 50% of the "illumination".

 

I'll have to build a telescope, buy some eyepieces, and start experimenting.

Yes and no.  

 

Yes you will be blocking out the illumination for field you are are not interested in.

 

No you won't be blocking any light from the target of interest in the 1/2 inch  field.

 

The telescope primary makes an image of the sky at the focal plane. The size of the total image is unbounded really and the size of individual targets is dependent on the focal length of the objective. A longer objective makes a larger image of any target. For example, the size of the image of the moon  is about one half degree and an image of this is 0.009 X focal length. A 1000mm scope creates an image of the moon (or the sun), that is 9mm diameter. A 500mm scope makes an image of the moon 1/2 this size or 4.5mm.   A 2000mm scope makes an image of the moon that is 18mm diameter. 

 

All targets have an angular size in the sky and the image created by the objective can be calculated as focal length of the objective multiplied by the tangent of the angular size of the target.  The tangent of 1/2 degree is about 0.009. You can easily lookup the tangents of any angle using the Windows calculator. 

 

Deciding on the field size is really determining how much you will see of the primary image made by the objective. So determine the size of your targets and calculate how big the images will be to determine how big an illuminated field is required. Then choose what eyepieces you need to view this image  For a wide field RFT scope, you may want to illuminate 4 degree field. For a refractor to split double stars or for planetary viewing, a much narrower field can be chosen.   Often the moon, 1/2 degree is recommended as the minimum field for a  general purpose scope.

 

It should be added that you can't have a long focal length scope and a very wide field of view.  A 4 degree field in a 1000mm scope has an image about 70mm diameter . An image that size doesn't fit in any eyepiece and would require a monster sized focuser tube.  

 

You need to make choices based on what you desire to observe.  

 

hope this helps,

 

dan.


Edited by dan_h, 05 August 2019 - 12:08 PM.

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#9 EddieRich

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Posted 07 August 2019 - 12:19 PM

Mainly I want to see the moon and planets,. but also some deep sky objects.

I've been thinking about the SurplusShed 150mm x 1200mm telescope objective.

For that lens with a 1-1/4" eyepiece, the max target size would 1.5 degrees (using the above math).

 

Andromeda galaxy is 3x1 degrees, or so they say, I've never actually found it, yet.



#10 dan_h

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Posted 07 August 2019 - 12:39 PM

Mainly I want to see the moon and planets,. but also some deep sky objects.

I've been thinking about the SurplusShed 150mm x 1200mm telescope objective.

For that lens with a 1-1/4" eyepiece, the max target size would 1.5 degrees (using the above math).

 

Andromeda galaxy is 3x1 degrees, or so they say, I've never actually found it, yet.

Your math sounds reasonable.

 

Andromeda is indeed over three degrees in the longest direction but the outer edges are extremely faint and hard to detect in all but the darkest of skies.  I can field M31, M32 and M110 in the same view using my 35mm Pan in my refractor with 1600mm focal length. I know the edges of M31 are clipped but I can't detect them anyways.

 

You say you are planning on only 1.25" eyepieces. There was a time when I felt very much the same way.  Then I discovered low power wide field viewing.  If you are able to proceed with the SS 150mmX1200mm, I would strongly recommend a 2" focuser and 2" diagonal if only to keep your options open. You never know when you will get a chance to try a nice 2" eyepiece even if your kit is only 1.25".

 

Have fun

 

dan


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#11 dan_h

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Posted 07 August 2019 - 12:44 PM

Binoculars provide a low cost  entry into low power wide field observing. They are also great for scanning large areas of the sky to find targets like Andromeda or M33.  I spend far more time with my binoculars than I do with my high power eyepieces.

 

dan


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