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small refractor as a dobsonian's finder

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#1 Guydive

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Posted 27 February 2021 - 03:10 PM

I was thinking of getting a right angled finder for me dob, but then I thought why not enjoying the view while star hopping?

I have the ES 24/68 ep that is never in use. I was thinking of attaching it to a 60-80mm refractor with a ~400mm focal length.

first thought was a Celestron 70mm travel scope. and then, since I'm broke right now, I started to consider the SW 72ED. 

what's your opinion on it? an ED is always nice to have but how difference would it make?

since I'm just star hopping, aberrations are not that critical.. is the view of star fields would be significantly better to justify the extra cost?

I do some EAA but with longer focal length so it's a nice extra but not super important



#2 ButterFly

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Posted 27 February 2021 - 03:34 PM

Fantastic idea, if you can balance it with the extra weight.  For visual only, ED glass won't help very much under these circumstances.



#3 gwlee

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Posted 27 February 2021 - 03:53 PM

That puts a LOT of weight near the focuser, requires a LOT more weight near the mirror to rebalance the scope, which makes a light scope weigh a LOT, LOT more, so it’s not something many people do. Suspect you might be more satisfied with a separate alt/az mount and tripod that are sized to a small refractor, and small light RA finder for your Dob.
 

For example, my AT72ED2 (72mm f6) would weigh ~7# ready to observe, would provide a 3.7* FOV at 18x with your EP, and would require an additional ~7# weight near the mirror to balance the scope. If it only added 1# to the scope, not many people would choose an 18x finder with a 3.7* FOV. 


Edited by gwlee, 27 February 2021 - 04:24 PM.

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#4 Redbetter

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Posted 27 February 2021 - 03:58 PM

ED's are great, but they are heavy compared to short tube achros of the same aperture.  Weight/balance is the big factor since it will be at the end of a Dob.  

 

Realistically, you are looking at a 1.25" system for low power with an RACI prism and a little less than 4 deg TFOV.  ED's are overkill for the purpose.  Now if weight and cost were not an issue, a small aperture ED could be used with a 2" RACI  prism and max TFOV eyepiece (I don't know how well such a prism would hold up for high power, so I wouldn't expect to be able to make full use of the ED's capability that way.)

 

Some feel the need for cross hairs in their finder eyepieces, but I use a 24 Pan or 25mm HD60 without them.  

 

The negative with some of the cheaper/lighter achros is that plastic focusers are often sloppy which can be a problem.  I use an ST80 as a finder, which has a metal focuser, but this adds weight.



#5 sportsmed

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Posted 27 February 2021 - 04:15 PM

Yea you could do it but an ED would be over kill for what you want it for and it would add to much weight to the Dob causing balancing issues like others suggested. If you were to add a refractor I would look at a small 70-80mm Achro. I have an ST80 that is light but I dont use it as a finder though. I just use a Telrad on my Dob but I have other scopes I can set up on other mounts if I want to view through multiple scopes.



#6 jeffreym

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Posted 27 February 2021 - 04:26 PM

I love star hopping with small refractors on all my scopes.  I have mostly 50 and 60mm refractors for this but have used the Celestron 70mm Travel Scope, and have made a 70x300 into a RACI finder that takes standard eyepieces Shortened the tube and changed the fucuser).  I also have a 70mm in a cell from MASILMW like this one:

https://www.cloudyni...-in-metal-cell/

However, that is still a project.

The Celestron is a good scope but I found it kind of larger and extended when mounted as a finder.  The eyepiece tends to not end up close enough to the eyepiece of the main scope.  However, this is made to be light weight.

 

The easy way is to use something small like this 50mm one:

https://www.astronom...-trim-3475.html

This particular version has excellent optics and will bring standard eyepieces to focus.  This one does need a standard eyepiece to be spaced out 1/2" to account for the single crosshair illuminated reticle eyepiece it was designed for.

 

The 60mm or 80mm finders from Stellarvue might be a good choice as well. 

 

I think the available ED scopes will be too heavy and have the same issues I had with the Celestron.  However, there is probably a way to mount anything you want to look through.

 

Have fun,

Jeff


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#7 Jon Isaacs

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Posted 27 February 2021 - 06:07 PM

Some feel the need for cross hairs in their finder eyepieces, but I use a 24 Pan or 25mm HD60 without them.

 

 

Cross hairs add precision and additional tools for precisely pointing the scope. 

 

I use RACI finders with the cross hairs aligned to the vertical/horizontal axes of the sky/scope. Used in conjunction with Sky Safari with a cross hair reticule on screen with the same TFoV, I can use stars anywhere in the field to precisely point the scope, one star for each cross hair and I'm on it.

 

I use standard eyepieces and add my own cross hairs.. I just added cross hairs to an Explore Scientific 20 mm 68 degree, it's quite sharp across the field and in my SV-50 finder provides a measured 6.4 degree TFoV at 10x with a 5mm exit pupil. 

 

I have 70 mm and 80 mm finders but prefer the wider field of the 50mm.

 

And, there's balance.  A 70 mm apo will add 7 lbs or more. That's a lot on the upper cage of a collapsible Dob and a lot to counter balance.

 

Jon



#8 PETER DREW

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Posted 27 February 2021 - 06:13 PM

Depends somewhat on the size of the Dobsonian.  I use 80mm short focus achromats for finders on 20" and 30" apertures where the 80mm scopes with RA diagonals can be mounted on the mirror box and still be accessible.


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#9 stomias

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Posted 27 February 2021 - 06:34 PM

I've used a cheap Meade 60mm F5 (Model 277) for a finder for almost 4 decades.....Works for me... My 20mm Research Grade Meade has been on it for 30 years... :O smile.gif


Edited by stomias, 27 February 2021 - 06:35 PM.

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#10 The Ardent

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Posted 27 February 2021 - 06:43 PM

Look up ads and descriptions of the the Lumicon 50 and 80mm superfinders.


I was thinking of getting a right angled finder for me dob, but then I thought why not enjoying the view while star hopping?
I have the ES 24/68 ep that is never in use. I was thinking of attaching it to a 60-80mm refractor with a ~400mm focal length.
first thought was a Celestron 70mm travel scope. and then, since I'm broke right now, I started to consider the SW 72ED.
what's your opinion on it? an ED is always nice to have but how difference would it make?
since I'm just star hopping, aberrations are not that critical.. is the view of star fields would be significantly better to justify the extra cost?
I do some EAA but with longer focal length so it's a nice extra but not super important



#11 Astro-Master

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Posted 27 February 2021 - 07:25 PM

I was thinking of getting a right angled finder for me dob, but then I thought why not enjoying the view while star hopping?

I have the ES 24/68 ep that is never in use. I was thinking of attaching it to a 60-80mm refractor with a ~400mm focal length.

first thought was a Celestron 70mm travel scope. and then, since I'm broke right now, I started to consider the SW 72ED. 

what's your opinion on it? an ED is always nice to have but how difference would it make?

since I'm just star hopping, aberrations are not that critical.. is the view of star fields would be significantly better to justify the extra cost?

I do some EAA but with longer focal length so it's a nice extra but not super important

I mounted a Stellarvue 60mm RACI on the UTA of my 18" Obsession.  It has a focal length of 210mm F/3.5, it came with a 23mm eyepiece 9x with a cross hair that is almost invisible, and bad field curvature, so I just use my 13 Nagler T6 for a power of 16x and a 5* degree FOV that is nice to the edge. 

 

Most DSO's pop into view at 16x, and if I add a 1.25" OIII filter the entire North America Nebula is quite striking!  I do have to add about 5lbs to the Mirror box, but its not a problem I just slide another 5lb barbell weight on the post.



#12 pweiler

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Posted 28 February 2021 - 11:01 AM

I am currently using SW Evostar 72ED as a finder.
I’ve used a Panoptic 27mm 68’ and similar EPs in it and I thought that the view was impressive! With a TeleVue 2” diagonal focusing worked as expected. In straight through mode additional extension tubes are needed on my SW72. Also, this scope can double for AP work. Some of the photos from this scope on Astro Bin and other sites using a field flattener are incredible.

Edited by pweiler, 28 February 2021 - 11:25 AM.


#13 Guydive

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Posted 28 February 2021 - 01:05 PM

I am currently using SW Evostar 72ED as a finder.
I’ve used a Panoptic 27mm 68’ and similar EPs in it and I thought that the view was impressive! With a TeleVue 2” diagonal focusing worked as expected. In straight through mode additional extension tubes are needed on my SW72. Also, this scope can double for AP work. Some of the photos from this scope on Astro Bin and other sites using a field flattener are incredible.


How did you handle the weight/balancing (which parent scope)?

And how did you align it with the bigger scope, tube rings?
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#14 Mitrovarr

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Posted 28 February 2021 - 10:29 PM

I considered this myself, but ultimately rejected it. There are a lot of downsides. Consider:

 

1. The weight. Any small refractor designed to be used visually is going to outweigh a 50mm finder by a lot. It's going to throw any mainstream dob way off balance just by itself. And, if you're ritzing up your finder, you probably are using huge 2" EPs and a paracorr and all that kind of good stuff so you're already having massive balance problems. This will just make the balance issue you are already having drastically worse. Also, triplet apos are always heavier than regular refractors and ED scopes are usually, too (better build quality) so an apochromatic finder just adds even MORE weight.

 

2. The speed. Finders are generally F/4. Finding an F/4 refractor on the market is next to impossible. You could go F/5 in an achromat, but it's going to have a lot of CA and the ST80 (the obvious choice if you want to go cheap) doesn't take 2" eyepieces. But you'll probably need 2" eyepieces to get a finder-like field of view, since you're not F/4 like a finder and the finder also has less aperture so less focal length there, too. Oh yeah, and if you went apo? You probably won't be able to get under F/6 without spending a fortune on it.

 

3. The diagonal. Assuming you want RACI, you need a different sort than you've probably got. Another expense, and the usual choice for a RACI image diagonal often makes a bright spike appear on stars so you're losing some image quality. And good luck with this if you want 2" capability.

 

4. The crosshair. You probably either don't have an eyepiece with one, or you don't want to use it for this. Also, this usually dashes your dream of ultra-wide fields, because of the limitations on what is available in eyepieces with crosshairs and reticules. You aren't going to be able to get an ultrawide field eyepiece with one, nor a 2" eyepiece, probably (if there an eyepiece with a crosshair in one of these categories, someone mention it). So either you're stuck with a boring kellner/plossl/ortho and a narrow AFOV or you have a finder with a significant deficiency in terms of being a finder.

 

What ultimately ends up happening between the speed problem, the diagonal problem, and the crosshair problem is that the "finder" turns out not to be the greatest finder. Narrow field, no centering, etc. So you need another finder; probably a zero-power finder to help aim the scope (since your finder doesn't have a very wide field). Now you've got even MORE weight...

 

This is why people usually only use small telescopes as finders on giant dobsonians where the overall scope weight makes the finder's weight insignificant.

 

Now, that I'm done crushing your dreams, there is a way you can kind of do it in a practical manner. Stellarvue makes a finder that is basically a small telescope. It's only 50mm, but it has a removable eyepiece (with a crosshair!), a focuser, and a built in diagonal. The optical quality is better than most cheap finders. So you could look into one of those. It allows you to dabble in "What if my finder was a small telescope?" territory while still being a totally competent finder that doesn't cause a bunch of issues to the telescope it is attached to. You can also use it like a tiny telescope on its own if you want. It's expensive for a finder, but not ridiculous. I think it's someone else's product they are reselling so it's probably cheaper under another name somewhere, but I don't know it.


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#15 Jon Isaacs

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Posted 01 March 2021 - 10:01 AM

This is why people usually only use small telescopes as finders on giant dobsonians where the overall scope weight makes the finder's weight insignificant.

 

 

- Even with large Dobsonians, the finder weight is significant, often more significant than with a small scope.  The tracking force will be about the same in a large scope as it is in a small scope so it will need to be counterbalanced. But with a large scope, the center of balance, the altitude trunnion center, is about the same height as a small scope but the scope is much taller so more counterweight is needed. 

 

Some numbers:

 

10 inch GSO: Pivot = 23", Pivot to focuser= 24", pivot to bottom of tube = 16 inches.  For every pound added at the focuser, 24/16= 1.5 pounds will be needed as a counterweight.

 

13.1 inch F/5.5 Starplitter:  Pivot = 21", Pivot to focuser ~ 48 inches, Pivot to bottom of mirror box = 16 inches.  For every pound added at the focuser 48/16 = 3.0 pounds will be needed as a counterweight.  

 

The 22 inch requires about 5.5 pounds of counterweight for each pound added near the focuser.  

 

Peter's solution of mounting the finder on the mirror box would mean I would not be able to work back and forth between the finder and the main eyepiece.  For me, that defeats the purpose of having a finder.  

 

- I have a 70mm finder and a 80mm finder.  Ready to mount, with the eyepiece, the 70mm weighs 40 ounces, the 80 mm weighs about 52 ounces.  With the 13.1 inch, they would require about 7.5 lbs and 10 lbs of counterweights.. 

 

Both these finders work at full aperture, I had to rework both of them, the 70mm was about 63-65mm, the 80mm was only about 60mm.  Many finders do not operate at full aperture.. 

 

- I find a 50mm finder the most effective,  The 50mm SV you mention, that's what I use.  As I stated previously, I use standard eyepieces and add my own cross hairs.  I never expect to see the object in the finder no matter how large the finder, its only for star hopping.  The wider field of view possible with 50mm finder is a big plus for me.  

 

I did try an APM 60mm but returned it.  I measured it at 55mm clear aperture, the 50mm SV measured 50mm so there wasn't much gain there.  I have never measured the 60mm SV but in conversations here, another member did measure it and found it to be about 55mm.

 

(I am using a modified flashlight test using a 4.8mm Nagler which is focused at infinity.  A laser is projected through the eyepiece and the diameter of the projected beam is measured.  It's accurate to less than a 1mm.)

 

Jon


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#16 Bomber Bob

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Posted 01 March 2021 - 12:20 PM

ALL of my reflectors have an R/A Finder, but I don't use the DOB mount -- EQ or standard AZ.  8" F6 Newt + 80mm F5 RFT; 6" F20 Cass + 50mm F4; 6" F4 Newt + 35mm F5; and, the 6" F8 Newt + 50mm F5 that I gave away recently.  I don't need an APO for low-powering sweeping when it's piggyback to a larger reflector.



#17 pweiler

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Posted 18 March 2021 - 05:21 PM

How did you handle the weight/balancing (which parent scope)?

And how did you align it with the bigger scope, tube rings?


The parent scope (Orion CC8) has a top bar across the length of the scope with 2 or 3 screw holes in the center. I used an ADM v/d adapter screwed to the center of this bar. Along with two 100mm adm finder scope rings attached to the green SW short v-bar. The weight of the scope, adm parts and usual 2” 32mm EP helps to balance the focuser/primary mirror end of parent scope on the mount. The arrangement makes it easy to take the SW 72 off and onto a mount (twilight 1) for my kids or quick setup/travel usage without having to redo alignment.

#18 JMW

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Posted 18 March 2021 - 06:13 PM

I used to have a SV70ED on the OTA of a 20 inch Obsession. Made a great finder and wide angle refractor. I added quite a bit off weight off the mirror cell to balance. I had it is adjustable finder rings. It belongs to our club but I kept it in my 5x8 cargo trailer with ramp door at my house and brought it to about 6 club events a year including GSSP.

 

I was at the GSSP in 2008 and a guy was giving 4 of us about a 3 hour tour after midnight on his 28 inch Dob. He had a 4 inch refractor mounted much lower to a bracket on the side of his mirror box. It was well aligned with the 28 inch optics.


Edited by JMW, 18 March 2021 - 06:16 PM.



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