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Best RACI finder

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#1 MikeTahtib

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Posted 17 August 2019 - 09:52 AM

I currently only have a Telrad on my Dob.  I would like to add a finder scope.  I am thinking something with some power would be good to be able to see bigger objects directly, but something with a wide field of view would be good to star hop with a chart.  Both in one would be ideal.  What I've seen so far that comes close is a Stellarvue 14 x 80, that would allow me to see a lot of things directly in the finder, but only has a 3.6* TFoV, which seems kind of small for star hopping, and  a Stellarvue 9 x 50, with a 5.5* TFoV, which seems big enough for star hopping, but maybe not enough to see targets directly.  They also make a straight-thru version with a 2" focuser and a couple illuminated-reticle eyepieces.  It seems like it should be possible to cobble together enough pieces to get 12.3 x 80 with a 5.7* TFoV, which should satisfy what I'm looking for.  They don't make a 90* 2" prism, although they do make a 45* one.  This would end up costing quite a bit, though.  I'm wondering if there are other options that I haven't considered that would provide similar magnification and FoV.  I don't have room for 2 finder scopes on my telescope.



#2 vkhastro1

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Posted 17 August 2019 - 10:18 AM

Both the Stellarvue SV50 and SV80 RACI finders accept 1.25” eyepieces.

Just remove the SV 23mm X-hair eyepiece from either finder and insert your eyepiece of choice to get the magnification and FOV you desire.



#3 Migwan

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Posted 17 August 2019 - 10:44 AM

For the price, you might consider a used ST80 at f5.    The F080M2 80 is f3.75 and would give a wider field a view.   Weight is similar between those two, so if that is a concern, then the SV50 might be better.  

 

jd



#4 Don Taylor

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Posted 17 August 2019 - 11:43 AM

I have SV50 and SV60 finders and a friend has an SV80 on his 22" Dob - where I have an ST80 on my 16". Also have the GSO 50mm RACI, AT50mm RACI and a Baader Vario-Finder (I like finders).

 

Directly comparing my ST80 with my friend's SV80 - the ST wins hands down. Much better optics (suspect the F3.75 focal ratio of the SV80 is a significant factor) and while they both can accommodate different eyepieces the limited focuser travel of the SV can be a problem with some eyepieces.  FWIW, I use a conventional dielectric diagonal in the ST80 as I grew up with RA but not RACI finders and don't have a problem with the mental gymnastics needed to reverse the view in my head. But a good quality Amici diagonal would work fine. I like the 24ES68 or 24mm Meade SWA 5000 eyepiece in the ST for finder duty.  Originally I used the ST80 as a finder on the Dob and also for wide field sweeping but I have other scopes for that second purpose so the ST80 lives full time on the Dob now.

 

I hope this helps.  Not badmouthing the SV80 - just my hands-on experience with the one SV80 sample and my one ST80 sample. And my personal preferences. YMMV

 

DSC 2384 2870
DSC 2382 2869

 

I usually use a 1.25" AT or GSO dielectric diagonal now instead of the 2" shown in these photos.


Edited by Don Taylor, 17 August 2019 - 11:58 AM.

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#5 Don Taylor

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Posted 17 August 2019 - 12:15 PM

One more comment about the SV RACI finders.  I added a focuser lockscrew to each of the 3 I have - as the weight of the reticle illuminator would rotate the focuser in some (most) orientations of the scope & finder. I was allways having to rotate it back to position (and focus) every time I looked through the finder. Adding the focuser lock screw fixes that and once focused (and the lock screw tightened), the eyepiece can be rotated to align the reticle then secured with the original eyepiece retention screw (the larger headed screw in the photo). (I'm one of those A-R types that likes the crosshairs aligned with the finder optical axis)  

Added focus lockscrew 4083

Edited by Don Taylor, 17 August 2019 - 12:16 PM.

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#6 Jon Isaacs

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Posted 17 August 2019 - 01:24 PM

Lots of ways to go, I have 50mm, 70mm, 80 mm finders that take1.25 inch eyepiece's plus an ST-80 with a 2 inch focused.

 

My thinking:

 

The vast majority of the objects I'm looking for are not visible in an 80 mm. The finder is for star hopping, wide fields are good, 50 mm is enough aperture. I'm using a modified 24 mm TeleVue Widefield, I get 6.8 degrees.

 

One consideration is the weight of the finder and mounting.  My 22 inch has a 5.5:1 ratio, that means if I add 1 pound to the upper cage, I need about 5.5 pounds of counterweights at the bottom of the mirror box. For a 15 inch, it might be 4:1. An ST-80 with rings etc might be 5 pounds..  that would require 20 pounds or so of counterweights.

 

 

Jon


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#7 MikeTahtib

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Posted 19 August 2019 - 06:30 AM

Thanks for the replies, all.  I don't think I can get the field of view and magnification I want with a 1.25" eyepiece.  I looked for 2" Amici prisms, and found ones by Baader, which sound great, but at $300 for the more affordable version, are more than I feel I should be spending on a finder scope.  Are there lower cost alternates?



#8 MikeTahtib

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Posted 19 August 2019 - 06:31 AM

Or perhaps a way to put an illuminated reticle into a finder scope in front of a standard eyepiece?



#9 MikeTahtib

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Posted Yesterday, 07:50 AM

Lots of ways to go, I have 50mm, 70mm, 80 mm finders that take1.25 inch eyepiece's plus an ST-80 with a 2 inch focused.

 

My thinking:

 

The vast majority of the objects I'm looking for are not visible in an 80 mm. The finder is for star hopping, wide fields are good, 50 mm is enough aperture. I'm using a modified 24 mm TeleVue Widefield, I get 6.8 degrees.

 

One consideration is the weight of the finder and mounting.  My 22 inch has a 5.5:1 ratio, that means if I add 1 pound to the upper cage, I need about 5.5 pounds of counterweights at the bottom of the mirror box. For a 15 inch, it might be 4:1. An ST-80 with rings etc might be 5 pounds..  that would require 20 pounds or so of counterweights.

 

 

Jon

That turned out to be a major factor in my decision to go with the 60mm rather than the 80mm.  Based on geometry, my telescope should be a 5:1 ratio, although when I started testing out weights , it came out to less than that.  I think this is due to the fact that as weight and counterweight is added, the force of the OTA on the altitude bearigns increases, increasing friction, and therefore, the weight it can carry without unbalancing increases.  

I determined that with the 60mm finderscope and 4 lbs of counterweight, I can balance the lightest eyepiece I have through the full range of motion and the heaviest through most of the telescope's range without having to add any temporary weights.  I've got a big chunk of brass I'm about t start cutting.  



#10 MikeTahtib

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Posted Yesterday, 07:57 AM

Both the Stellarvue SV50 and SV80 RACI finders accept 1.25” eyepieces.

Just remove the SV 23mm X-hair eyepiece from either finder and insert your eyepiece of choice to get the magnification and FOV you desire.

 

I was thinking I needed a reticle, because my other finders have something (red dot & Telrad), but perhaps you're right,  those other devices need a reticle because you wouldn't be able to line up the field with the telescope otherwise.  But with the finder, there is only 1 view, regardless of how you position your head, so maybe I don't need a reticle after all.  I calculated TFoV, Magnification, and Exit Pupil for a number of eyepieces, and there are a number of options that look promising.  I tried my 13 mm Ethos last night, and it didn't quite come to focus.  Since I haven't mounted the telescope yet, it's hard to figure out exctly what I will be able to see (pointing in the general direction and trying to rotate around to point the telescope held by hand).



#11 MikeTahtib

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Posted Yesterday, 07:57 AM

Thanks for the advice, everyone!



#12 Jon Isaacs

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Posted Yesterday, 08:35 AM

I was thinking I needed a reticle, because my other finders have something (red dot & Telrad), but perhaps you're right,  those other devices need a reticle because you wouldn't be able to line up the field with the telescope otherwise.  But with the finder, there is only 1 view, regardless of how you position your head, so maybe I don't need a reticle after all.  I calculated TFoV, Magnification, and Exit Pupil for a number of eyepieces, and there are a number of options that look promising.  I tried my 13 mm Ethos last night, and it didn't quite come to focus.  Since I haven't mounted the telescope yet, it's hard to figure out exctly what I will be able to see (pointing in the general direction and trying to rotate around to point the telescope held by hand).

 

I find cross hairs to be critical because being slightly off center in the finder means being way off center in the main scope.

 

Cross hairs are also useful because I can align them with identified stars to more accurately point the scope.

 

Jon


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