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Evostar 72ED PRO Focus question

beginner eyepieces refractor
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#1 vetie

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Posted 20 September 2020 - 10:25 AM

Hello everyone,

 

First, thanks to everyone on these forums for the assistance in helping me learn this new hobby. 

 

I recently purchased a Evostar 72ED APO Pro which I eventually will be using for astrophotography and visual but have a question related to focusing the scope. 

Yesterday I was able to get the scope out for the first time and found that I am unable to properly focus on stars or planets as the focus does not move back far enough. 

 

My setup was the following: 72ED with a 2" to 1.25" adapter to a 1.25" standard star diagonal and a 32mm eyepiece. With the focus all the way out I could ALMOST get perfect focus, if the focus was able to move just a little more it would work (same thing happen when connecting my D5300 to the scope). I did pull the star diagonal out as far as I could. 

 

My research shows that I need to add an adapter (move the star diagonal further from the focuser) or use a filed flattener. 

If I want to use the scope for visual, what is the best method or adapter that will allow me to focus the scope (move the star diagonal further from the focuser)? I have seen adapters for cameras but I don't see adapters for star diagonals. 

 

Again, thanks for the help. 

 

 



#2 sg6

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Posted 20 September 2020 - 10:56 AM

I believe the scope is supposed to be used with a 2" diagonal thus the path length through the diagonal is longer, the 1.25" diagonal and focus travel simply cannot make up the difference - some eyepieces will work, others will not. Found this out myself.

 

With a 2" diagonal you are likely to have to wind the focuser fully inwards - I had to have 2.5mm skimmed off my 2"-1.25" converter to get semi reasonable settings.

 

The scope is a bit of a pain as it "just" manages to operate but around 80% of the time. The other 20% you are up a certain creek.

 

Put a 2" diagonal in for eyepieces. Have fun.



#3 vetie

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Posted 20 September 2020 - 12:37 PM

Thanks for the information Cosmos!

 

I will purchase a 2" diagonal to use with the 72ED.



#4 gnowellsct

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Posted 20 September 2020 - 08:29 PM

Thanks for the information Cosmos!

 

I will purchase a 2" diagonal to use with the 72ED.

Get yourself a dielectric.  Astronomics has a 2" refractor diagonal for $119.

 

If you want to buy upper end I like the Max Bright sold by Astrop-physics $300+.

 

Greg N



#5 Shorty Barlow

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Posted 20 September 2020 - 08:48 PM

I doubt the 72ED DS Pro was intended exclusively for 2" diagonals, as they can have in-focus problems with eyepieces of 3mm or less focal length. I tend to use mine mostly with 1.25" diagonals.

 

med_gallery_249298_10284_82567.jpg

 

There are solutions with some diagonals. TBH most short tube refractors suffer from this.

 

med_gallery_249298_10284_9538.jpg

 

Which is why the extension tube was invented.

 

med_gallery_249298_10284_50749.jpg

 

These can vary with size.

 

med_gallery_249298_10284_57536.jpg

 

I bought these for my ST80's and ST102. They're rarely used now as it's just a matter of trial and error to find which eyepiece/diagonal combinations work.

 

med_gallery_249298_5348_66321.jpg



#6 Redbetter

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Posted 21 September 2020 - 02:34 AM

From what I have gathered listening to owners the root problem with the Synta 72ED's is that the 38mm focuser draw tube travel is too short to handle either 1.25" or 2" diagonals well across the range of eyepieces.   Extension tubes can fix one problem but not necessarily the other.  Folks using 2" diagonals or extra long 2" eyepiece holders or 1.25" adapters might have trouble with inward focus travel--lower profile or even recessed adapters might be needed.

 

Contrary to what was stated ealier, most short tube refractors do not suffer from this problem in my experience, because they attach focusers with enough draw tube travel...and they design the OTA to handle the inward travel requirements of 2" diagonals and 2" eyepieces as well as 1.25" diagonals with 1.25" eyepieces and everything in between. 

 

How do I know?  I have an AT-60ED, AT-72EDII, as well as an Orion ST80, and Meade 80 f/5 (same as the ST80 except for color and badging.)  None have this sort of trouble.  I have converted the ST80 to a 2" GSO 2-speed diagonal which covers the full operating range.  The difference is that the GSO focuser I chose has nearly 80mm of draw tube travel.  I worked out what the distances were before I bought the focuser.


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#7 Shorty Barlow

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Posted 21 September 2020 - 12:32 PM

med_gallery_249298_5348_96212.jpgmed_gallery_249298_5348_106239.jpg

med_gallery_249298_5348_87441.jpg



#8 Redbetter

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Posted 21 September 2020 - 04:24 PM

Those photos with extensions demonstrate the problem with the TS/GSO 2" linear bearing focuser's shorter drawtube: 

  • The max outward travel is ~29mm less than that of a GSO Crayford two speed.  They have the same overall length from where the focuser flange mates to the OTA.   
  • The original Sytna 1.25" focusers for the ST80 have 60mm of draw tube travel.  They are also longer overall with their standard end connection, at about 113mm from where the OTA and focuser flange mate.
  • GSO 2" focusers are about 95mm from the edge of the OTA's tube to the end of the 2" focuser (fully drawn in) without 1.25" adapter height.  Add about 10mm for the adapter.  So the GSO Crayford with adapter provides 184mm past the flange, vs. about 155mm for the GSO linear bearing, and 173mm with the standard 1.25" Synta focuser.

This nicely demonstrates the problems with the SW/Evostar 72ED's short focusers as well, since they have only 38mm of draw tube travel.



#9 Shorty Barlow

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Posted 21 September 2020 - 04:34 PM

med_gallery_249298_10284_152588.jpg

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med_gallery_249298_10284_48695.jpg

med_gallery_249298_10284_116002.jpg

med_gallery_249298_10284_667464.jpg


Edited by Shorty Barlow, 21 September 2020 - 04:36 PM.


#10 Bowlerhat

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Posted 21 September 2020 - 11:42 PM

My local vendor posted that they're rolling th esecond versions of this with 10mm drawtube difference to address the focus problem-but at th emoment with old stock you'll need extension tubes.




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