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Image Circle / Sensor Size questions

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#1 Oyaji

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Posted 21 November 2019 - 01:57 AM

All: 

 

I'm confused about image circles and their relationship to sensor size and have a couple of questions.

 

My ASI294 OSC camera has a sensor diagonal of 23.2 mm.  My questions are:

 

1.  In shopping for a triplet refractor upgrade to my doublet refractor, what size image circle should I be looking for to be compatible with this camera?  

 

2.  How does the presence of a flattener, reducer or flattener/reducer change the advertised image circle of a scope?

 

Thanks for any guidance you can give on this.  



#2 artem2

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Posted 21 November 2019 - 03:56 AM

Hello there, hope I will be able to answer your question  " I will try "

 

The best way to see how your FOV would look like with the setup you have, is to go to this link here http://astronomy.too.../field_of_view/

insert all needed data and select an DSO from the list, and see your setup would perform.

 

See how my FOV looks like with the setup I have.

 

Regards

 

Martin

Attached Thumbnails

  • my FOV.png
  • astronomy_tools_fov.png


#3 Chuckwagon

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Posted 21 November 2019 - 04:37 AM

All: 

 

I'm confused about image circles and their relationship to sensor size and have a couple of questions.

 

My ASI294 OSC camera has a sensor diagonal of 23.2 mm.  My questions are:

 

1.  In shopping for a triplet refractor upgrade to my doublet refractor, what size image circle should I be looking for to be compatible with this camera?  

 

2.  How does the presence of a flattener, reducer or flattener/reducer change the advertised image circle of a scope?

 

Thanks for any guidance you can give on this.  

For question 1, I'm not sure what you mean by "compatible" but so long as the image circle is larger than the sensor diagonal it will be "compatible."  The larger the circle, the more likely it is that the "sweet" spot of the image circle will cover the whole frame, and the impact of aberrations will be lessened.  For example, coma tends to be worse nearer the edges of the circle.  Or you may get vignetting more severely if the circle is too small for your sensor. 

 

A reducer will shrink the image circle, and is only capable of producing an image circle that is its clear aperture times its reduction factor.  For example, many of the common .63 and .33 reducers have a clear aperture of only around 41mm, so they would only produce image circles of 25.83mm and 13.53mm respectively.  While the first would cover your sensor, you would likely still see some vignetting, and the second would be far too small for your sensor.  So if a scope doesn't have an integrated flattener/reducer that the maker included in their image circle size calculations, then you will need find out the clear aperture size of the FF/FR and calculate how adding one will impact the scope's image circle.

 

Hope that helps some.


Edited by Chuckwagon, 21 November 2019 - 04:37 AM.


#4 sg6

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Posted 21 November 2019 - 06:01 AM

All: 

 

I'm confused about image circles and their relationship to sensor size and have a couple of questions.

 

My ASI294 OSC camera has a sensor diagonal of 23.2 mm.  My questions are:

 

1.  In shopping for a triplet refractor upgrade to my doublet refractor, what size image circle should I be looking for to be compatible with this camera?  

 

2.  How does the presence of a flattener, reducer or flattener/reducer change the advertised image circle of a scope?

 

Thanks for any guidance you can give on this.  

In absolute terms you need one that delivers an image circle at least the same size as the diagonal. Although bigger will be a lot better and easier. First reason is just that it is easier to get the image over the sensor and cover it.

 

Next is still that bigger is better. I would have said that a 23.2 diagonal needs likely a minimum around 28-30mm. Without a flattener the edge areas will have curvature and the bigger the image circle then as a generallity the flatter the central section is before it all curves away.

 

I recall reading that for a flat field on an APS camera you buy a lens that produces an image that is for a full frame camera.

 

Unsure how a reducer effects the final result - I can see 2 options. You need to check on what the specification is and again the bigger the circle the better. But always over the 23.2 and preferably 28-30mm area. Have never been sure how they classify "flat" on an image, there has to be a tolerance of some variety, and "flat" tends to drift away at the outer sections.

 

Myself I have always just bought a flattener not a reducer. Not overly serious on imaging and so making a scope faster has not been primary to me. Flat yes, fast not really. Not sure what the next scope you get will be but be a little careful of getting into the faster the better idea


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#5 Oyaji

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Posted 21 November 2019 - 11:38 AM

Thanks, gentlemen.  I think I was having brain fog (or, as we might say in Österreich, "Gehirnnebel" laugh.gif ) when I asked this question.  I guess I have been looking at too many pictures--my own and on Astrobin--of nebulae.  That, plus the prospect of dropping some serious money on my triplet upgrade and wanting to make sure I get it right.   

 

Artem2:  Yes, I knew about the FOV calculator in astronomy.tools, but due to brain fog, I had forgotten about it when I posted my question.  This tool is very helpful in analyzing this issue, and I have run a number of simulations using the tool, of scopes with different focal lengths.   

 

Chuckwagon:  Your information about reducers is very useful.  The FOV calculator will show the effect of a reducer on the FOV, but I think it's also important to know that the image circle gets smaller with a reducer, even as the FOV gets wider.

 

SG6:   Glad to know that where image circles are concerned, and as in so many other aspects of life, "bigger is better."  Some of the scopes I was looking at have image circles of 44 mm--almost twice the diagonal of my camera.  If I were to get one of these scopes, I suppose I wouldn't get a flattener, at least at first, until I could see exactly how it performs without one.

 

Thanks again.   


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#6 Im2bent

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Posted 21 November 2019 - 09:28 PM

 I just started a thread about astronomy tools still waiting for more feedback but it seems astronomy tools ignores image circle so I do not know how trustworthy that info is as far as making a purchasing decision.



#7 Alex McConahay

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Posted 22 November 2019 - 12:49 PM

Image circle is an ill defined term.

Is it determined by:
Illumination
Flatness of field
Optical aberrations (coma, chromatic aberration, spherical aberration, flatness of focus)

And, what is acceptable in the degree of aberration?

In a sense, it is akin to the word “sharpness” or “contrast” Or “mount capacity.”

So, take this term and the others as general descriptive and rules of thumb rather than definitions.

Alex
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#8 Im2bent

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Posted 22 November 2019 - 03:51 PM

Image circle is an ill defined term.

Is it determined by:
Illumination
Flatness of field
Optical aberrations (coma, chromatic aberration, spherical aberration, flatness of focus)

And, what is acceptable in the degree of aberration?

In a sense, it is akin to the word “sharpness” or “contrast” Or “mount capacity.”

So, take this term and the others as general descriptive and rules of thumb rather than definitions.

Alex

How about it is the diameter of the image projected by the scope at the focal plane? I suppose "usable" image plane would take into account edge aberrations/flatness?



#9 Alex McConahay

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Posted 22 November 2019 - 08:01 PM

Yes, your definition is good, but one person’s idea of usable is different from others. And one must assume some usability when using the term


Alex

#10 ccs_hello

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Posted 24 November 2019 - 12:00 AM

Old thread on "Image Circle" as an OTA's parameter

https://www.cloudyni...6-image-circle/

 

And...

Indeed image circle size is not absolute. Acceptable image dimming (from fully illuminated, partially illuminated --> correctable by flat, to not illuminated),

various degrees of optical aberration/image degradation, etc. all play parts on an OTA designer/mfg designates that OTA model's image circle size.

 

As real life example, Canon EOS/EF mount lens has EF type and EF-S type.  Latter is more suitable for (image sensor size smaller) APS-C sized DSLRs while former can be used for full-frame sized as well as APS-C sized DSLRs. 

(As an analogy, Nikon has the FX type and DX type lenses.)

 

One could even argue that EF lens, when used in APS-C sized DSLR (a) will burn bigger hole in your pocket book, 

(b) waste a lot of light on that APS-C DSLR, © probably a nice fit, since its smaller APS-C sized image sensor will pick up more of the

sweet-spot lights (  <-- one will notice near the photo's edge, the optical quality degrades more, so it's like using a more capable lens, then crop out the outer region of the image )

 

Back to OP's question 1 on which OTA can offer larger image circle, doublet or triplet OTA?   

Ans: Hard to say, a high quality, pricey OTA may offer better optical quality in small image circle sized format and only willing to do so for reputation/good marketing purpose, while another brand/model may want to push the envelope a bit more to sell (and when customers complain, ask the users "just to crop the resulted image".) 

 

Q2: FR, Barlow, C.C., etc.

Ans: These are OTA optics modifiers.  They first need to match the original OTA's design parameters.

Still they will introduce all kind of effects...   including F.R.'s vignetting (optical as well as mechanical), more aberrations, or improvements (e.g., coma correction if matching/proper C.C. is used) 



#11 mastersniper

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Posted 24 November 2019 - 06:05 PM

lens geometry, glass and coatings is a lot better than the 1920-1940's era lenses I used in large format photo work but if I remember back then I considered 70% of the image circle the boundary of acceptable illumination/minimum abbertations in the corners in other words the outer 30% of the image circle was unacceptable to me for critical review.  Usually this was not a problem as the image circles were huge in those lenses.   shooting 4'x5" film some of my lenses had 10" image circles. so even with large tilt/swing/shift movements used had plenty of image circle to play with.

 

With the better glass/coatings/geometry we have these days I would guess 90% of the image circle is probably a good place to be looking for.   for a 23.2 diagonal sensor that would mean 29mm + image circle should be the minimum more is better to limit gradients/vignetting and aberrations.   

 

Marketing folks definitely get their hands in the spec publishing for example one ota I looked at said "The fully illuminated image circle is 42mm and will cover a 35mm full frame sensor".  They say "fully illuminated", but because the sensor is ~43.3mm diagonal it can't be covered 100% illumination with a 42mm circle. so there must be some image circle outside this 42mm dimension it's just not fully illuminated, probably darkening quite quickly with modern glass. 



#12 Alex McConahay

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Posted 24 November 2019 - 06:26 PM

To bring in a comment from visual observing.....back when 82 degree FOV eyepieces were taking over.

 

Any plossl eyepiece can be an 82 d. FOV eyepiece, even though most of them are rated at about 50 d. Just take out the field stop. That stop is put there to cut out the outer perimeter of the illumination because that illumination level is dimmer, because the optical errors are greater (unacceptable to most). But, if you want 82 d, just cut out the field stop, and that stuff comes through. Not pretty to look at, but it is still illuminated (kinda) out to 82 d. 

 

Note, let's not parse this example too much. I think it is more illustrative than scientific. ("In order to illustrate a point, you must exaggerate a lot, and omit a lot."--Bagehot)

 

Alex




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