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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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