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ES82 30 degree, Field Stop diameter 43mm, through M42/T2

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#1 StarCurious

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Posted 11 October 2019 - 12:04 PM

To accommodate a 2" filter wheel, which has an M48 aperture, I need to find the shortest 2" eyepiece holder, and I found the one in the link below, with external length 40mm, internal depth 35mm, and withT2/M42 constriction:

 

https://agenaastro.c...isual-back.html

 

My Explore Scientific 82 degree 30mm has a field stop diameter of 43mm, according to official specifications:

 

https://explorescien...roducts/82-30mm

 

I measured the, ES82 30mm barrel length is 38mm.  I will use a parfocal ring to insert it 28mm inwards.  This will make my ES82 30mm parfocal with my 1.25" eyepieces.

 

I suspect the light cone should have a diameter of 43mm, but the diameter may be even larger at the point of 42mm constriction.  My question is how much larger, and how bad the effects of vignetting should I expect?

 

 



#2 Astrojensen

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Posted 11 October 2019 - 12:29 PM

I suspect the light cone should have a diameter of 43mm, but the diameter may be even larger at the point of 42mm constriction.  My question is how much larger, and how bad the effects of vignetting should I expect?

That depends on how far ahead of the eyepiece the T2 constriction is. I am in a somewhat similar situation to you, in that I use a 2" diagonal attached to my 63mm Zeiss refractor via an M44/T2 adapter. In my case the T2 constriction sits more than 75mm in front of the eyepiece and the vignetting, while present, is quite subtle visually and not immediately visible. It is quite visible in a 40mm ES68, though, which max out the 2" barrel field stop. 

 

 

Clear skies!
Thomas, Denmark



#3 StarCurious

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Posted 11 October 2019 - 12:43 PM

The constriction should be 7mm in front of the field stop.  This is for my Zhumell 10 inch Dobsonian F/5 1250mm focal length, but with added GSO Coma Corrector which supposedly lengthens the focal length by about 38mm.

 

 

That depends on how far ahead of the eyepiece the T2 constriction is. I am in a somewhat similar situation to you, in that I use a 2" diagonal attached to my 63mm Zeiss refractor via an M44/T2 adapter. In my case the T2 constriction sits more than 75mm in front of the eyepiece and the vignetting, while present, is quite subtle visually and not immediately visible. It is quite visible in a 40mm ES68, though, which max out the 2" barrel field stop. 

 

 

Clear skies!
Thomas, Denmark

Your ES68 40mm has a field stop diameter of 45.5mm!

 

What is the F/ ratio and focal length of your OTA?

 

75mm in front seems like a lot, then I realized may not be uncommon if the T2 is in front of a 2" diagonal.



#4 Antodumont

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Posted 11 October 2019 - 01:20 PM

I'm also looking for a 2" eyepiece holder with M48x0,75 thread , baader has one Clicklock (rather expensive)

 

https://www.teleskop...scope-side.html

 

I'm also interessed if somebody has another option ....  smile.gif



#5 Astrojensen

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Posted 11 October 2019 - 01:35 PM

The constriction should be 7mm in front of the field stop.  

Then the AFOV of the 30mm ES82 will be stopped down to about +70° AFOV. Before getting the 2" diagonal setup for my Zeiss Telemator, I used a T2 diagonal with a T2/M48 adapter screwed into the filter threads of the eyepiece and this stopped down the AFOV quite noticeably. 

 

 

 

What is the F/ ratio and focal length of your OTA?

 

75mm in front seems like a lot, then I realized may not be uncommon if the T2 is in front of a 2" diagonal.

The f/ratio is f/13.3, 840mm focal length. And the T2 adapter is indeed sitting in front of the 2" diagonal. 

 

This is roughly the setup I use now (excuse the Danish text, but I made this illustration years ago for a Danish astronomy forum), except the diagonal has been changed to a prism diagonal and the quick change adapter has been upgraded to the new heavy duty version:

 

gallery_55742_318_1407450214_26780.png

 

 

Clear skies!
Thomas, Denmark


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#6 StarCurious

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Posted 11 October 2019 - 03:54 PM

Thanks, Thomas, for very good illustration.



#7 StarCurious

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Posted 11 October 2019 - 04:27 PM

I'm also looking for a 2" eyepiece holder with M48x0,75 thread , baader has one Clicklock (rather expensive)

 

https://www.teleskop...scope-side.html

 

I'm also interessed if somebody has another option ....  smile.gif

Try this one:

 

https://www.amazon.c...r/dp/B07DFYCK1V

 

Don't know how long it is.  I have something like that included with the 2" filter wheel, external length 49.2mm.  I suspect it is the same Chinese made generic part.

 

The shortest one I found so far is the one that is part of the GSO Coma Corrector - measured external length 46.6mm.  I am looking for something as short as around 30mm internal depth - add a few more mm for external length.  If I can't find it, I may have to stick with the GSO, and use zero profile 2-to-1.25" eyepiece adapter.

 

Come to think about it, I would only gain back focus of 3mm with the adapter in post #1, and losing FOV. So thanks to Thomas, I will not try to use T2 on this large field stop eyepiece.

 

EDIT

I just inserted the ES82 30mm eyepiece into the GSO Coma Corrector 2" eyepiece holder. Visually, there's not even 1mm left in internal depth, even though eyepiece barrel length is 38mm.  That's because the female M48x0.75 thread takes up length.  I now conclude there is no point in searching for another one.   I googled GSO 2" eyepiece holder, I only managed to find something out of Dubai or Indonesia, for SCT,


Edited by StarCurious, 11 October 2019 - 04:45 PM.


#8 Antodumont

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Posted 12 October 2019 - 02:28 AM

Try this one:

 

https://www.amazon.c...r/dp/B07DFYCK1V

 

Don't know how long it is.  I have something like that included with the 2" filter wheel, external length 49.2mm.  I suspect it is the same Chinese made generic part.

 

 

Thank you for the link !

 

One can finf hips of these on eBay or Ali but I'm afraid sellers don't know exactly what they're talking about ... many shows in the title M48 x 0,75 thread but when looking carefully in description it is written for SC telescope

(see here for example https://www.amazon.c...tronics&sr=1-35) .... anyway , these are too long for me (about 2"" ; 50 mm)

 

Another way could be a 2"/SCT adaptator + a SCT threaded eyepieceholder , a short one https://www.pierro-a...pour-sct_detail , sorry , didn't search too long for a link in english)

 

 

I'm actually looking for one to go on my filter wheel , the one provided with the wheel is a bit too long , I can not reach focus with some of my 1,25" eyepiece , I have to unscrew the eyepiece holder and screw a 2"/1,25" adaptator ... I can live with it but it could be a bit easier with a shorter one

 

Anyway , I found this one on Ali , looks different to the other one can find on this site or on eBay or Amazon

 

https://fr.aliexpres....30452e0ea4Wjbb

 

I've asked the seller for the total lengh and confirmetion of the thread ... he never answered , I've ordered one anyway , I can tell you the measurement when I get it (probably not before 2 or 3 weeks)


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#9 Antodumont

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Posted 24 October 2019 - 12:50 AM


Anyway , I found this one on Ali , looks different to the other one can find on this site or on eBay or Amazon

 

https://fr.aliexpres....30452e0ea4Wjbb

 

I've asked the seller for the total lengh and confirmetion of the thread ... he never answered , I've ordered one anyway , I can tell you the measurement when I get it (probably not before 2 or 3 weeks)

 

Ok , I received it , thread is correct m 48x0,75 but i is still too long , about 5 cm (2") .....



#10 StarCurious

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Posted 24 October 2019 - 06:25 AM

Ok , I received it , thread is correct m 48x0,75 but i is still too long , about 5 cm (2") .....

Sorry to hear that. Thanks for letting us know.


Edited by StarCurious, 24 October 2019 - 06:25 AM.


#11 Vla

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Posted 24 October 2019 - 09:02 AM

The constriction should be 7mm in front of the field stop.

What is it that you call "field stop"? Physical field stop in Smyth-type eyepieces is internal, and the quoted stop is virtual, obtained by dividing physical stop by the Smyth lens magnification. Where vignetting takes place is not at the eyepiece end, but on the opposite side.

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  • STOP.png

Edited by Vla, 24 October 2019 - 09:08 AM.


#12 StarCurious

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Posted 24 October 2019 - 11:21 AM

Vla,

 

I don't understand your picture.  Seems like negative offset for the lens illustrated.

 

I must read the wrong page:

 

 

https://explorescien...roducts/68-40mm

 

Field stop diameter 45.5mm for ES68-40

 

https://explorescien...roducts/68-34mm

 

Field stop diameter 38.6mm for ES 68 34mm

 

 

Field stop positions:

 

https://www.cloudyni...corr/?p=7622711

 

+ve means below shoulder of eyepiece.

 

For my ES82 30mm, field stop is at zero.



#13 StarCurious

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Posted 24 October 2019 - 11:37 AM

Ok , I received it , thread is correct m 48x0,75 but i is still too long , about 5 cm (2") .....

I also need a shorter 2" eyepiece holder.  My problem is no longer back focus, as I raised the primary by some 20mm.  The problem is the distance between the GSO CC top lens and the 2" eyepiece holder (46mm long, came with the GSO CC package) is 78mm.  Adding a 10mm high 2" to 1.25" adapter will increase the separation to 88mm, which is beyond the recommended range of 65-85mm.  Incidentally, I tested with ES68 24mm with +3mm offset, and the view is good - at the 85mm separation.  With the LVW65 17mm, with offset of zero, the view is inconsistent, affected by seeing conditions.   Until I find a shorter 2" eyepiece holder (I would like to find 28-29mm barrel insertion depth, 35-36mm length),  I ordered this to use for my 1.25" eyepieces. 

 

https://www.ebay.ca/...872.m2749.l2649

 

With a 7mm length for GSO M48 female to M42 male, this should give me a range of 44-54mm, which should work with my 1.25" eyepieces and make them parfocal with the ES82 30mm in the GSO CC 46mm long holder.  I was going to order a M48 extension for my 2" to 1.25" holder, but the price of this Focuser Adapter is as low as and extension.


Edited by StarCurious, 24 October 2019 - 11:37 AM.


#14 Vla

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Posted 24 October 2019 - 12:43 PM

That is configuration of a Smyth-type eyepiece, such as ES82 30mm. Field stop is, by definition, an opening placed at the ep's front focal plane, physically limiting its FOV. With Smyth-type eyepieces, physical stop is well inside the body, as shown, and the quoted stop is at some, smaller distance beyond the negative front lens. Since it is a virtual stop, there is no way to determine it other to raytrace eyepiece (i.e. to get it from the manufacturer). I rushed it saying that vignetting takes place on the opposite side, looking at the beam as a whole, which is converging. Since individual points' beams are diverging, vignetting does take place at the end opening, although the difference is nearly negligible. As can be seen on the pic, 42mm opening at some short distance in front of the [EDITED: virtual (quoted) field stop] will cause barely perceptible field restriction (less than 0.5mm on the field radius), but will cause 50%+ vignetting at the field edge. If you pull it farther away, it will actually cause less vignetting and no field restriction, because every central ray of the beam converging to a field point originates at mirror's center, so those cones are diverging toward eyepiece. Only when you pull it away far enough that the bottom of the front (toward mirror) opening cuts into the converging beam, the vignetting will start to increase.

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  • stop2.png

Edited by Vla, 24 October 2019 - 03:32 PM.



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