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newt as finderscope

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#1 rob1986

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Posted 02 August 2021 - 08:09 AM

any thoughts on using newts as finder scopes? (coma induced comments included! flowerred.gif )

 

I see that they were once used for that function. something like orion's starblast 114mm f4 (at maximum exit pupil 28 mm EP gives about 16x and something like 3.6 degrees with a ~60 degree AFOV EP but if you use a longer ep you could get 40mm plossl with 4.5 degree FOV



#2 Jon Isaacs

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Posted 02 August 2021 - 08:22 AM

any thoughts on using newts as finder scopes? (coma induced comments included! flowerred.gif )

 

I see that they were once used for that function. something like orion's starblast 114mm f4 (at maximum exit pupil 28 mm EP gives about 16x and something like 3.6 degrees with a ~60 degree AFOV EP but if you use a longer ep you could get 40mm plossl with 4.5 degree FOV

 

A 1.25 inch 40mm Plossl will not increase the TFoV over a 24mm SWA or a 32mm Plossl, they all have the same diameter field stop, ~27mm.  I calculate that these eyepieces will provide a 3.43 degree TRoV in a 4.5 inch F/4 Starblast.  

 

I think there are a number of issues to consider.. 

 

-  The OTA weighs 4.0 lbs and it will have to be mounted so it can be aligned.  That's a lot of weight to add to a telescope.

 

- It's not a correct image finder so it will be difficult getting it straight with a star chart.  

 

- There will be quite a bit of coma even with high quality eyepieces.  

 

Jon


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#3 Supernova74

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Posted 02 August 2021 - 08:40 AM

You be better off using a small Apo refractor something like the William optics redcat.


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#4 LIVE LONG

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Posted 02 August 2021 - 09:23 AM

   I agree with Jon Issacs,

the Starblast is too big & heavy, to use as a finder scope. It would be a chore to try to mount and balance on a telescope.

 

Also, it will not give a correct image. I would go crazy trying to do a mental flip of the image, in the dob and trying to match it to a star chart! Use a finder scope that gives a correct image. It will save you a lot of time & frustration.



#5 Jon Isaacs

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Posted 02 August 2021 - 09:23 AM

You be better off using a small Apo refractor something like the William optics redcat.

 

For a finder, an achromat is better because it's significantly lighter and you don't need the color correction for a finder.  And cheaper too. 

 

Jon 


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#6 Cotts

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Posted 02 August 2021 - 09:24 AM

I advocate for the tiny Newtonian as a finder for a large newtonian if you can successfully counter-balance it..

 

Pros

- if you have both the scopes rotated so that the eyepice axes  are parallel, the orientation of the views in both scopes will be identical - no other kind of finder scope has this capability*.

- the view is identical to the sky or an atlas but just rotated..  Rotate the atlas the same way and you have a direct, un-flipped mental-visual path from atlas to finder to main scope. 

 

Cons.  Optical quality and coma.  But, let's be practical here, you're using a low power just to see some stars over a 3º-ish field.  Does it really matter if the finder is not diffraction limited?  I doubt it.

 

*the RA CI refractor finder gives an image that is rotated 180º from the view  in the main Newtonian so you have to 'unrotate' the view in your brain as you go back and forth between finder and main.... this is non-trivial if you are digging for a faint object.

 

The views in the sky have West being 90º clockwise from North.   So, too the views in a Newtonian telescope, finder or 'main' and so, too with the RA CI finder.   All of these can be matched to an atlas just by rotating the atlas.

 

An "ordinary" refractor-diagonal like the RedCat has the great disadvantage of having West being counterclockwise from North and no amount of fiddling with your atlas will ever match the view in the finder.  (If your atlas is printed on one side of the page you can view from the unprinted side with a light shining through to get the 'flipped' view...but most atlases are printed on both sides of the page.....)  Sky SAfari and other digital atlases have a simple flip/rotate function which is nice to relate the atlas view with the finder view.  But if you then have to mentally flip or rotate the view in the finder to that in the main scope you are not much further ahead..

 

I will be putting a 3-inch Celestron Newt on my new 20" dob. 

 

Dave


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#7 Cotts

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Posted 02 August 2021 - 09:29 AM

   I agree with Jon Issacs,

the Starblast is too big & heavy, to use as a finder scope. It would be a chore to try to mount and balance on a telescope.

 

Also, it will not give a correct image. I would go crazy trying to do a mental flip of the image, in the dob and trying to match it to a star chart! Use a finder scope that gives a correct image. It will save you a lot of time & frustration.

This is entirely the opposite of reality.  A Newt finder and a Newt 'main scope' will have identical image orientations  assuming the eyepieces are parallel.  Rotate your atlas and all three will be identical... see my detailed post #6.

 

Dave
 


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#8 Jon Isaacs

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Posted 02 August 2021 - 09:38 AM

Pros

- if you have both the scopes rotated so that the eyepice axes  are parallel, the orientation of the views in both scopes will be identical - no other kind of finder scope has this capability*.

- the view is identical to the sky or an atlas but just rotated..  Rotate the atlas the same way and you have a direct, un-flipped mental-visual path from atlas to finder to main scope.

 

The views in the sky have West being 90º clockwise from North.   So, too the views in a Newtonian telescope, finder or 'main' and so, too with the RA CI finder.   All of these can be matched to an atlas just by rotating the atlas.

 

 

The problem with a Newtonian as a finder is that the view cannot be easily aligned with the sky.  It's always rotated.  

 

With an RACI finder and Sky Safari, the naked eye view of the sky, the chart view and the RACI finder view can all be aligned, no rotation except for the finder cross hairs.  

 

Sky Safari is a game changer because it can provide correctly aligned images of the sky.

 

The narrow field of view is an issue as well. With Sky Safari, I can put an object in the field of view at 200-300x in the main scope using a Sky Safari and a 50mm RACI finder at 10x with a 6.4 degree TFoV.  

 

How often have you used a Newtonian as a finder?  How did you mount it so it could be easily aligned?  Was it mounted near the eyepiece so you could work back and forth from the finder to the main eyepiece?

 

Jon



#9 Jon Isaacs

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Posted 02 August 2021 - 09:40 AM

This is entirely the opposite of reality.  A Newt finder and a Newt 'main scope' will have identical image orientations  assuming the eyepieces are parallel.  Rotate your atlas and all three will be identical... see my detailed post #6.

 

Dave
 

 

So just how do you rotate the sky?

 

The Newt and the Newt finder may have the same image orientations but they will both be rotated compared to the sky.  I much prefer having the sky, the Sky Safari Charts, the RACI finder cross hairs and the scope all aligned motions of the scope.  It's all alt-az, up down. right left. 

 

I have cross hairs on the finder, cross hairs on the Sky Safari chart, I work between them and can use stars anywhere in the field for alignment. Compared to using a paper chart, it definitely feels like cheating.. 

 

Jon



#10 Cotts

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Posted 02 August 2021 - 09:57 AM

So just how do you rotate the sky?

 

The Newt and the Newt finder may have the same image orientations but they will both be rotated compared to the sky.  

 

Jon

Rotate the ATLAS to match the view in the finder.  That's what we all do.  With a Newt finder on a Newt main scope all three will then be the same orientation no matter where you have the scope pointed.. 

 

Mental exercise:  Your dob is pointed halfway up the sky and due east. The focuser on your dob is set up to be 30º above the horizontal for easy viewing and is on the right side of the tube (from behind).  You are sitting upright on your stool.  Q: Where is North in the eyepiece view?    Repeat for a bunch of other parts of the sky.  (Bonus points for viewing just below Polaris!).   North and the direction of drift can be just about anywhere in your seated view through the eyepiece...

 

My point is that we all rotate our atlases already to match the sky in whatever  direction our dob is pointing.  I just think that once you get the atlas/finder the same orientation it is nice to not have to do a 180º rotation to get the view in the main scope to match the first two.. 

 

Dave

 


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#11 Jon Isaacs

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Posted 02 August 2021 - 10:48 AM

Rotate the ATLAS to match the view in the finder.  That's what we all do.  With a Newt finder on a Newt main scope all three will then be the same orientation no matter where you have the scope pointed.. 

 

Mental exercise:  Your dob is pointed halfway up the sky and due east. The focuser on your dob is set up to be 30º above the horizontal for easy viewing and is on the right side of the tube (from behind).  You are sitting upright on your stool.  Q: Where is North in the eyepiece view?    Repeat for a bunch of other parts of the sky.  (Bonus points for viewing just below Polaris!).   North and the direction of drift can be just about anywhere in your seated view through the eyepiece...

 

My point is that we all rotate our atlases already to match the sky in whatever  direction our dob is pointing.  I just think that once you get the atlas/finder the same orientation it is nice to not have to do a 180º rotation to get the view in the main scope to match the first two.. 

 

Dave

 

Dave:

 

We don't all rotate our atlas's.  I am not using a paper atlas, I am using Sky Safari, my sky view is permanently aligned to the alt-az axes.  I am not using north-south, east-west.  When I move the scope on in the azimuth direction, the view in the finder moves in the azimuth direction, oriented by the cross hairs.

 

SkySafari has many capabilities that a paper chart does not have.  When I am using the magnifying finder, I choose a view that is appropriate for a 10x50 finder with cross hairs and a 6.4 degree TFoV, the cross hairs, the finder field, the star magnitudes are all chosen to match what I see in the finder.  Since Sky Safari is providing me with the cross hairs, it's relatively straightforward to simply match the view in the finder to the Sky Safari view. 

 

I can use stars anywhere in the field to align the finder cross hairs so putting objects in a small field of view of the main eyepiece is very doable.  With the 22 inch, hunting down galaxies, I will often use the 10mm Ethos as the finder eyepiece.  That's about 1/3 of a degree at 280X.  This is simulation I made of finder view on the way to centering the target.  

 

cross hairs pointing.jpg

 

With a paper chart, you only have the one chart magnification so maybe aligning them makes sense.  But when I move from the finder to the main eyepiece, I zoom in so that now I am looking at a star field that matches what I see in the main eyepiece.  I can adjust the magnitudes of the stars and galaxies/DSOs to match the what I am seeing.  

 

This is very different than using a paper atlas, I don't really see any advantage having the finder view and the main scope view aligned in rotation because they are so very different.  When I am working in the main eyepiece, I am looking at a chart that is appropriate for the main eyepiece.  It's relatively easy mentally rotate the image because the two are well matched. 

 

And as I asked, how are you mounting and aligning your Newtonian finder?  Is this something you use every time you observe?  Have you tried an electronic chart like SkySafari?  I switched over from paper to electronic charts more than 20 years ago.. 

 

Jon



#12 Gregrox

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Posted 02 August 2021 - 10:52 AM

   I agree with Jon Issacs,

the Starblast is too big & heavy, to use as a finder scope. It would be a chore to try to mount and balance on a telescope.

 

Also, it will not give a correct image. I would go crazy trying to do a mental flip of the image, in the dob and trying to match it to a star chart! Use a finder scope that gives a correct image. It will save you a lot of time & frustration.

A newtonian finderscope and a newtonian telescope can be set up to have almost the exact same image orientation, and that is one where you hold the finder chart upside down or at some angle. I don't really find that I have a problem holding star charts at an angle because, unlike the mirrored view in a refractor or cassegrain with a mirror diagonal, you can get an exact match just by looking. Whenever I have to deal with mirrored views, I need to take a picture or screenshot of the map or app i'm using, open it in my white-light-only phone's image editor, and mirror the image. Some right angle finderscopes have this problem, annoyingly including those which come with skywatcher flextube telescopes.

 

I have seen a Celestron FirstScope used as a finder. It takes the very low powers well enough that you can get over the poor optics. A SkyScanner or Zhumell Z100 could be used if you want a little higher optical quality, but still at a much lighter weight than a starblast OTA. I'm pretty sure I've seen a starblast or a similar ATM thing used as the finder on a very large ATM dobsonian.



#13 Cotts

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Posted 02 August 2021 - 11:39 AM

I have Sky Safari on my iPad.  It is especially excellent for use with SCT/Mak/Refractors and their 90 degree diagonal with the resulting mirrored view. I use the 'horizontal flip' mode.   I do rotate the iPad on the table beside me so that patterns of field stars in triangles, quads etc look the same as in the eyepiece....(it helps to lock the auto-rotate in the iPad...)  Finding obscure, faint double stars becomes a cinch.

 

I'm looking forward to having a large dob to try this with. 

 

The My little 76mm Celestron First scope and rings with 26mmTelevue Plossl (757g +208g +208g) =1173g = 2.6 pounds.... Compare with Orion CT80 80mm Compact Refractor Telescope Optical Tube at 3.25 pounds with no rings,

Celestron 93781 Illuminated RACI Finder Scope at 2 pounds with rings but only 50mm...,

WO Redcat with no rings and no diagonal weighs 4 pounds and may not even reach focus with a diagonal...

 

Dave



#14 rob1986

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Posted 02 August 2021 - 11:40 AM

The problem with a Newtonian as a finder is that the view cannot be easily aligned with the sky.  It's always rotated.  

 

With an RACI finder and Sky Safari, the naked eye view of the sky, the chart view and the RACI finder view can all be aligned, no rotation except for the finder cross hairs.  

 

Sky Safari is a game changer because it can provide correctly aligned images of the sky.

 

The narrow field of view is an issue as well. With Sky Safari, I can put an object in the field of view at 200-300x in the main scope using a Sky Safari and a 50mm RACI finder at 10x with a 6.4 degree TFoV.  

 

How often have you used a Newtonian as a finder?  How did you mount it so it could be easily aligned?  Was it mounted near the eyepiece so you could work back and forth from the finder to the main eyepiece?

 

Jon

I take it your RG had one



#15 Jon Isaacs

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Posted 02 August 2021 - 12:59 PM

I have Sky Safari on my iPad.  It is especially excellent for use with SCT/Mak/Refractors and their 90 degree diagonal with the resulting mirrored view. I use the 'horizontal flip' mode.   I do rotate the iPad on the table beside me so that patterns of field stars in triangles, quads etc look the same as in the eyepiece....(it helps to lock the auto-rotate in the iPad...)  Finding obscure, faint double stars becomes a cinch.

 

I'm looking forward to having a large dob to try this with. 

 

The My little 76mm Celestron First scope and rings with 26mmTelevue Plossl (757g +208g +208g) =1173g = 2.6 pounds.... Compare with Orion CT80 80mm Compact Refractor Telescope Optical Tube at 3.25 pounds with no rings,

Celestron 93781 Illuminated RACI Finder Scope at 2 pounds with rings but only 50mm...,

WO Redcat with no rings and no diagonal weighs 4 pounds and may not even reach focus with a diagonal...

 

Dave

 

Dave:

 

I built this for my 16 inch back in 2007. It was based on a 76 mm Tasco rocket scope. I think the focal length was 280 mm. 

 

4261997-Meade 16 inch Superfinder.jpg

 

It worked to some extent but I quickly realized a 50 mm finder was more effective. A 3 inch Newtonian is not very efficient with light so even in the center of the field it's not much brighter than a 50 mm refractor and off axis it really is a mess and poorly illuminated. The field of view is more determined by the secondary size than the field stop.  Small, fast Newtonians are just not very good scopes in comparison to a small refractor finder.

 

At the time I was using straight through finder's, it wasn't until later that I began using RACI finder's.

 

Currently, my 50 mm SV RACI finder with rings weighs 550 grams. I'm using a 20 mm 68° Explore Scientific, it weighs 250 grams, the entire finder weighs 800 grams, 28 ounces.

 

StellarVue finder on 22 inch.jpg

 

The eyepiece is well corrected and provides a 6.4° field at 10x with 5 mm exit pupil. I have 70 mm and 80 mm finders. The 70 mm weighs 780 grams with a diagonal and rings, operates at a full 70 mm aperture and has excellent optics. With the 24 mm TV Widefield it provides a 4.5 degree field at 13x with a 5.3 mm exit pupil.

 

Its a great finder but I typically prefer the wider field of the 50 mm.

 

Both these finder's also serve as Rich Field Telescopes so I often find myself just enjoying the view through the finder. I'm also using my wide field refractors less as these little guys do the job.

 

I love my Newtonians but small aperture Newtonians are compromised compared to a refractor..

 

That's how it's worked for me as a dedicated 100% Starhopper who observes 150-180 nights a year..

 

Jon


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#16 rob1986

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Posted 02 August 2021 - 03:41 PM

Dave:

 

I built this for my 16 inch back in 2007. It was based on a 76 mm Tasco rocket scope. I think the focal length was 280 mm. 

 

 

 

It worked to some extent but I quickly realized a 50 mm finder was more effective. A 3 inch Newtonian is not very efficient with light so even in the center of the field it's not much brighter than a 50 mm refractor and off axis it really is a mess and poorly illuminated. The field of view is more determined by the secondary size than the field stop.  Small, fast Newtonians are just not very good scopes in comparison to a small refractor finder.

 

At the time I was using straight through finder's, it wasn't until later that I began using RACI finder's.

 

Currently, my 50 mm SV RACI finder with rings weighs 550 grams. I'm using a 20 mm 68° Explore Scientific, it weighs 250 grams, the entire finder weighs 800 grams, 28 ounces.

 

 

 

The eyepiece is well corrected and provides a 6.4° field at 10x with 5 mm exit pupil. I have 70 mm and 80 mm finders. The 70 mm weighs 780 grams with a diagonal and rings, operates at a full 70 mm aperture and has excellent optics. With the 24 mm TV Widefield it provides a 4.5 degree field at 13x with a 5.3 mm exit pupil.

 

Its a great finder but I typically prefer the wider field of the 50 mm.

 

Both these finder's also serve as Rich Field Telescopes so I often find myself just enjoying the view through the finder. I'm also using my wide field refractors less as these little guys do the job.

 

I love my Newtonians but small aperture Newtonians are compromised compared to a refractor..

 

That's how it's worked for me as a dedicated 100% Starhopper who observes 150-180 nights a year..

 

Jon

some of it is probably misconceptions regarding how to use a finder. You're not expected to see the DSO in the finder, I think. The finder allows you to recognize the correct star pattern, to locate the DSO.

 

Correct?


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#17 Jon Isaacs

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Posted 02 August 2021 - 04:16 PM

some of it is probably misconceptions regarding how to use a finder. You're not expected to see the DSO in the finder, I think. The finder allows you to recognize the correct star pattern, to locate the DSO.

 

Correct?

 

That's the way I do it. It's nice when an object is visible in a finder but realistically, with a larger scope, that rarely happens for DSOs.

 

I know that some observers use Newtonians, I think Kevin R. does with his 25 inch but it's a 6 inch F/4 if I remember correctly and mounted to the mirror box. This makes working between the eyepiece and Newtonian finder a challenger. And if I'm not mistaken, Kevin uses DSCs.

 

If I were going to mount a larger finder, I'd keep the 50mm and mount a 102 mm or 120 mm F/5 Achromat to the mirror box.

 

What I actually do is occasionally mount a Telrad to my NP-101 and the match Telrad views when I find something interesting. A laser on the refractor could work but I avoid green lasers.

 

Lots of ways to star hop, there is no one right way. If Dave has found a small Newtonian works for him, that's great. It didn't work for me. 

 

Jon


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#18 rob1986

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Posted 02 August 2021 - 04:26 PM

That's the way I do it. It's nice when an object is visible in a finder but realistically, with a larger scope, that rarely happens for DSOs.

 

I know that some observers use Newtonians, I think Kevin R. does with his 25 inch but it's a 6 inch F/4 if I remember correctly and mounted to the mirror box. This makes working between the eyepiece and Newtonian finder a challenger. And if I'm not mistaken, Kevin uses DSCs.

 

If I were going to mount a larger finder, I'd keep the 50mm and mount a 102 mm or 120 mm F/5 Achromat to the mirror box.

 

What I actually do is occasionally mount a Telrad to my NP-101 and the match Telrad views when I find something interesting. A laser on the refractor could work but I avoid green lasers.

 

Lots of ways to star hop, there is no one right way. If Dave has found a small Newtonian works for him, that's great. It didn't work for me. 

 

Jon

been thinking of an ST80 with 2in focuser and a 56mm plossl with graduated cross-hairs and an incribed angle circle around it. the DSO wont get brighter with the oversized exit pupil, but the stars will. I'm sure a smaller FL ep could give excellent results too.



#19 HellsKitchen

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Posted 02 August 2021 - 06:04 PM

been thinking of an ST80 with 2in focuser and a 56mm plossl with graduated cross-hairs and an incribed angle circle around it. the DSO wont get brighter with the oversized exit pupil, but the stars will. I'm sure a smaller FL ep could give excellent results too.

 

That is an 11.4mm exit pupil. If you are anywhere light polluted, the view will be very washed out. You'll be losing a lot of light to the extent where you may aswell stick with a 50mm finder. Plus it would be an unnecessarily heavy combo. 

 

You would be much better served by something like an ES 24mm 68* if you were to use an ST80. You'd still get a very reasonable 4* TFOV with a darker sky background and far more contrast without any light loss. Pair it with a red dot finder or a telrad and you'd be golden. 


Edited by HellsKitchen, 02 August 2021 - 06:07 PM.


#20 rob1986

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Posted 02 August 2021 - 06:10 PM

That is an 11.4mm exit pupil. If you are anywhere light polluted, the view will be very washed out. You'll be losing a lot of light to the extent where you may aswell stick with a 50mm finder. Plus it would be an unnecessarily heavy combo. 

 

You would be much better served by something like an ES 24mm 68* if you were to use an ST80. You'd still get a very reasonable 4* TFOV with a darker sky background and far more contrast without any light loss. Pair it with a red dot finder or a telrad and you'd be golden. 

will it accept cross-hairs?



#21 HellsKitchen

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Posted 02 August 2021 - 06:15 PM

will it accept cross-hairs?

 

As far as I know, there are no 56mm plossls sold commercially with crosshairs, so I assumed you were going to make your own crosshairs...



#22 MitchAlsup

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Posted 02 August 2021 - 06:27 PM

any thoughts on using newts as finder scopes? 

I routinely chase down things like Stephan's Quintet with nothing more than Telrad or Rigel.

Then use a wide angle low power EP in the main optics.



#23 rob1986

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Posted 02 August 2021 - 06:36 PM

As far as I know, there are no 56mm plossls sold commercially with crosshairs, so I assumed you were going to make your own crosshairs...

ability to use cross hairs depends on where the focal plane is



#24 Keith Rivich

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Posted 02 August 2021 - 07:58 PM

I use a 6" f/3.9 newt as my "sit down" finder on my 25". Works great. I use a 2" 35mm eyepiece with a cross-haired red lit reticle. A Paracorr is a must. Also works great as a stand alone scope. With my 50mm University Optics eyepiece and an OIII filter I can fit the entire Veil Nebula in the eyepiece. Beautiful sight!

 

25-Annotated.jpg



#25 Cotts

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Posted 02 August 2021 - 10:40 PM

The best part of my plan is that i already have the 76mm mini newt, the rings, the eyepiece etc.  No cost to try it...

 

Dave


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