Stupid question re Finders

[Jasonb]

Hi there,

I'm a newbie and am getting a track record for asking stupid questions! Here's another one...

A big disadvantage with finders on Newtonians/Dobsons is how awkward it is to get perfectly behind the finder. So my stupid question is why does the finder have to be mounted at the front, near the eyepiece? Why can't it be mounted near the back, so you can get behind it as the tube wouldn't be in the way?

There's probably a really easy answer, but like I said, I'm a newbie!

Thanks in advance...

[Jiggy 67]

That’s actually quite a good question, I can’t think of any good reason why it can’t other than:

it’s close to the eyepiece making it easier to switch between the two

If it was at the bottom of the OTA it would be awkward when looking towards the zenith 

[AlexK]

What Jiggy said ^^^^
Many folks observing sitting in the chair high in the air. Jumping back and forth would be tiring.

But you can place your finder anywhere on the OTA where you feel it comfortable! It all depends on your scope, its mount range of freedom, your pointing/observing flow, and physical condition/parameters of your body.

E.g. on mine I had the Telrad (my only pointer) placed on the side for over a decade:

As that was super convenient (and even more healthy) with my quick TPM flow, when I've been using the telescope "Bazooka style" sighting through standing on my knees.

Now I'm using the QuInsight for pointing and have it placed a bit above the Eyepiece and all the way behind the rim of the OTA!
No actual image yet, but that allows to simply turn your head to the left from the eyepiece view and point to the next target.

As you can see I'm keeping my stock RACI finder in place, as I just need to stretch a bit to take a look through its eyepiece. Not using it for pointing though as that's counterproductive having the QuInsight aligned.



 

Edited December 4, 2020 by AlexK

[John]

As long as the finder is aligned accurately with the main scope it can be placed where you like.

It is useful to have the finder / finders close to the eyepiece though so that you can move your eye quickly from the finder to the eyepiece. Right angled finders are much easier to use on a dobsonian, and other scope designs for that matter.

On my 12 inch dobsonian I have my two finders close to each other and also close to the eyepiece:

 

[Jasonb]

Excellent answers, thanks! I can see myself ending up with a right angle one at some stage, it just seems easier.

Edited December 4, 2020 by Jasonb

[AlexK]

The right angle finder is actually harder to use. You need a lot of practice pointing looking into the ground instead of along the OTA into the starry sky (boring isn't it? ) and a good digital star chart capable of flipping the chart left to right (mirror) to match your finder FOV. If you really want to point tediously star-hopping with optical finder (e.g. heavy light pollution) I'd recommend to add some RDF for the initial direct pointing, otherwise it's a lot of confusion. Better look for a RACI finder (Right Angle Correct Image). You still looking into the ground with it, but at least it moves the field intuitively when you push/pull your Dob around.

OTOH, an optical finder is a PITA in my opinion for an average scope (up to 16" easily). After 30 years of fiddling with different finders and pointers, I'm settled on Telrad and TPM for the past 10+ years. Because as soon as you learn constellations to the extent of being able to identify every well visible star (after proper darkness adaptation there are just about 500 to ever care for) nothing could beat Telrad, even GoTo and the latest StarSense Explorer tech. You just move the reticle between stars naked eye to match its position on the chart and your target is in the main eyepiece in 5-6 seconds from the idea to try finding that dim nebula guaranteed every time. My RACI finder is just a mounted binoculars replacement.

Edited December 4, 2020 by AlexK

[wulfrun]

If you put a finder near the back end of a long-tube scope, it means that when pointing near the zenith the finder will be on the floor and somewhat problematic. Mount the scope higher and you'll need a ladder for a newtonian's eyepiece.

By the way, my idea of a stupid question is the one you are too embarrassed to ask the experts and hence pay the price. As a newbie myself I'd hope I don't fall into that trap.

[Tiny Clanger]

In answer to the original question : I think the shoe for the 'finder is usually placed at the front of the newt. tube because the simplest RDFs which are often bundled with 'scopes need some distance between eye and dot to use. On my heritage 150 ( 😀  there, I said it !) it absolutely has to be at the front because it's the only solid bit .

I bought a RACI  after two months or so of observing to get over the whole 'can see Messier object with binoculars, can't get the thing lined up for the telescope view' problem, as well as the annoyance of having to do weird crouching or interested budgie head tilting to use the thing . I have no trouble at all translating from newtonian eyepiece view to RACI  all-correct view or vice versa, and have the RDF there too as it is sometimes handy for targets I can see with the naked eye. . It never even crossed my mind that the disparity in orientation of views might be a problem, after all the RDF is always right way round, right way up ...

I bought a shoe (£5) from FLO for the RACI, and have experimented with where on the (half length heritage ) tube to put it : best place I've found by shifting it around (using sticky ads for a temporary fix) is by the balance point of the  'scope , which is convenient for balance as well as looking through. 

Now I just need to gather the courage to drill holes in my lovely little 'scope to mount the shoe properly ...

[Stu]

1 minute ago, wulfrun said:

By the way, my idea of a stupid question is the one you are too embarrassed to ask the experts and hence pay the price. As a newbie myself I'd hope I don't fall into that trap.

SGL doesn’t work like that, fortunately. There are no stupid questions, and you will get helpful comments regardless of what you ask, however basic. 👍

[wulfrun]

1 minute ago, Stu said:

SGL doesn’t work like that, fortunately. There are no stupid questions, and you will get helpful comments regardless of what you ask, however basic. 👍

Exactly what I meant but perhaps didn't express clearly. Like most forums I inhabit, SGL does have a lot of people at all levels who are happy to share their expertise. I meant the stupid question is the one you fail to ask because you though you'd look stupid.

[Stu]

3 minutes ago, wulfrun said:

Exactly what I meant but perhaps didn't express clearly. Like most forums I inhabit, SGL does have a lot of people at all levels who are happy to share their expertise. I meant the stupid question is the one you fail to ask because you though you'd look stupid.

Ah yes! I see what you mean now 🤣👍👍

[John]

38 minutes ago, AlexK said:

The right angle finder is actually harder to use....

 

I don't find this at all. I find them both ergonomically easier and the view through them relates directly with what my eyes see, what my star charts (non digital) show and what my red dot / illuminated reticule finder shows.

I now use RACI finders on all my scopes

The most important thing though is to find something that works for you and there will be variations on that person to person

 

 

 

[Stu]

1 minute ago, John said:

I don't find this at all. I find them both ergonomically easier and the view through them relates directly with what my eyes see, what my star charts (non digital) show and what my red dot / illuminated reticule finder shows.

I now use RACI finders on all my scopes

The most important thing though is to find something that works for you and there will be variations on that person to person

 

 

 

I’m the same John, although I think Alex was meaning that right angle finders are a little harder to get to the right area of sky in the first place, because you are not looking along the length of the scope. That why a Telrad/Rigel and RACI combo works so well I think.

[AlexK]

49 minutes ago, Tiny Clanger said:

In answer to the original question : I think the shoe for the 'finder is usually placed at the front of the newt. tube because the simplest RDFs which are often bundled with 'scopes need some distance between eye and dot to use.

A common misconception. All RDFs (except really exotic el-cheapo China plastic inventions) are using the light collimation principle utilizing a semi-transparent concave mirror (or a lens) as a collimator for that. In a nutshell, it is placing the lit reticle at infinity, where the stars are. You supposed to place your eye as close as you feel comfortable to avoid hunting for the reflex dot around. Sadly, several optical aberrations of a cheap mirror as well as possibly not ideal eye-sight of the observer coupled with fixed focal position of the reticle might ruin the collimator principle to the extent of the dot being not perfectly fixed in the sky as it supposed to. But in Telrad, for example, which is using the exact same principle, you can focus the reticle perfectly collimated, so you just bring your face to the mirror, notice the reticle and point with it as if it's really in the sky.

Edited December 4, 2020 by AlexK

[John]

4 minutes ago, AlexK said:

A common misconception. All RDFs (except really exotic el-cheapo China plastic inventions) are using the light collimation principle utilizing a semi-transparent concave mirror as a collimator for that....

I thought the Telrad used a flat mirror and a lens to focus the reticule onto the display window ?. This is from the Company Seven website:

 

 

 

1. Red L.E.D., 2. Telrad reticle, 3. Adjustable tilt flat mirror, 4. focusing lens,
5. display window inclined at 45 degrees shown with simulated projected reticle.

 

 

 

[John]

10 minutes ago, Stu said:

I’m the same John, although I think Alex was meaning that right angle finders are a little harder to get to the right area of sky in the first place, because you are not looking along the length of the scope. That why a Telrad/Rigel and RACI combo works so well I think.

Thanks Stu.

The RACI's do have an OTA though. Just ignore the prism at the end

 

[AlexK]

On 04/12/2020 at 11:17, Stu said:

I’m the same John, although I think Alex was meaning that right angle finders are a little harder to get to the right area of sky in the first place, because you are not looking along the length of the scope. That why a Telrad/Rigel and RACI combo works so well I think.

That's only partially correct  I've been also trying to steer from the RA optical finder idea the OP mentioned towards the RACI one and warn about the required mental equilibristics transitioning from the naked eye direct view to the RA and then to the Newtonian FOV, which is also upside down. Sure thing, for some people that might be a nobrainer, and definitely it's learnable, but I saw folks really struggling with that, especially in B1 sky when even a cheap RACI is full of stars enough to cause a nausea in unprepared minds ....

Edited December 5, 2020 by AlexK

[AlexK]

1 hour ago, John said:

I thought the Telrad used a flat mirror and a lens to focus the reticule onto the display window ?. This is from the Company Seven website:

 

 

 

1. Red L.E.D., 2. Telrad reticle, 3. Adjustable tilt flat mirror, 4. focusing lens,
5. display window inclined at 45 degrees shown with simulated projected reticle.

 

 

 

Sorry, for the generalization (corrected that post for clarity). I might be too far with that, there are rare (2-3 models I ever encountered) RDFs with the lens collimator indeed. However, Telrad and RDF are two very different pointing devices in terms of the supported pointing flow,  even though I know for sure that 99% of observers are using their Telrads as a trivial RDF all the time, because there is no "how to point with Telrad" instruction coming with it  Telrad is still better at that indeed, due to the reduced aberrations ruining the collimation away from the optical axis. The primary difference is that RDF has just a dot which is rather easy to collimate satisfactory on a cheap, while Telrad has a set of rings covering the much wider FOV, which is harder to maintain collimated thus it's equipped with robust collimation mechanism. Only recently we've got even wider rings pattern with the Telrad successor invention QuInsight. It's using a much higher quality two component collimator objective allowing to expand the 4 degrees Telrad reticle to whopping 16 degrees! Using it exclusively lately.

Re the image above: it's a bit misleading. #5 is not a projection display, it's just a light splitting mirror, making the collimated (parallel) beam from the lit reticle and the starlight coinciding in your eye optics as if both are coming from the same direction at infinity. And #4 is not a focusing lens, but a collimating lens (or simply "collimator"). See my Wiki link above for details of that physics.

Edited December 4, 2020 by AlexK

[Louis D]

Given my neck injuries, I can no longer use an RDF/Telrad/QuikFinder above about 30 degrees nowadays on a regular basis.  To work around this, I fitted a green laser sight to each scope which is by far the quickest and easiest way yet that I've found to put the scope on bright targets.  I then center the object in the main scope and do a Skeye app alignment on my cellphone attached to the scope at about 60 degrees to the sky so I can see the screen.  After aligning on several bright stars or solar system objects, I can use the app to locate objects objects close enough that they show up in my widest field eyepiece.  I can't star hop most nights because my skies have gotten so washed out that only alignment stars are still visible naked eye.

I know that laser sights are not popular outside the US for various legal reasons, but they have been catching on quite a bit here among advanced amateurs.  I don't recommend using them at outreach events because they attract little kids like flies who are then completely distracted by their presence.  As far as lasing an aircraft, I just keep my ears open for their distinctive sound and scan the sky before lighting up the laser.

Back to the OP's question, you generally want your finder or sighting device up high enough to be able to get behind it or off the the side of it as with right angle finders.  Put it at the back, and you'll be on the ground trying to use it.  If you're having trouble getting aligned with it because of the tube, put it on a 2" to 4" riser or stalk to get enough clearance for your head.

As far as Telrad vs QuikFinder, the former has zero parallax issues as your head bobs about while the latter shifts a bit relative to the sky.  I've read the QF circles are actually projected at about 6 feet instead of infinity.  The circles are also way easier to find in the Telrad than in the QF.  They're thicker, larger, and for some reason I've never understood, just plain easier to acquire.  Thus, I've been using the Telrad more than the QF, when not using the laser for various reasons, because I can only tolerate the pain of torquing my neck around to look through them for a few seconds at most.  Spending that time trying to locate the circles in the QF is a complete waste of precious neck torquing time for me.  In the Telrad, I can consistently find them almost instantly.  Still, both pale in comparison to the ease of use of a laser, though trying to torque my neck above 60 degrees can be painful in its own right to sight on a high object.  That's where Skeye comes in handy.  Align on lower stars/objects and use the app to navigate to higher objects.  It's not as accurate as my digital setting circles, but the user interface is way more intuitive.  I just need to get in the general vicinity, and I'm good to go from there.

[Jasonb]

Well, I have to say, I'm delighted I asked this question, reading the answers has been fascinating.

I should explain that when I said it sounds like a Right Angle Finder is the way to go, I actually meant a RACI, I'm just not knowledgeable enough yet to remember the difference between them all! The Telrad also sounds very interesting. In the end, I only have a 5" Reflector, so I don't want to be getting something that's too 'big' for the ota, but apart from that, I'm only starting and I'll work with what I have for a while and get used to it and its pros and cons before deciding if I need a change or not! 

Thanks for taking the time to reply everyone!

[John]

1 hour ago, Jasonb said:

Well, I have to say, I'm delighted I asked this question, reading the answers has been fascinating.

I should explain that when I said it sounds like a Right Angle Finder is the way to go, I actually meant a RACI, I'm just not knowledgeable enough yet to remember the difference between them all! The Telrad also sounds very interesting. In the end, I only have a 5" Reflector, so I don't want to be getting something that's too 'big' for the ota, but apart from that, I'm only starting and I'll work with what I have for a while and get used to it and its pros and cons before deciding if I need a change or not! 

Thanks for taking the time to reply everyone!

If and when you start looking, this is one way to tell if an optical finder is RA or RACI:

Edited December 5, 2020 by John

[Per_S]

I replaced the stock 6x30 finder with a 9x50 RACI finder, and added a Rigel Quickfinder to my Skywatcher 150PL. It's a vast improvement - recommended 🙂

[RobertI]

When I decided to upgrade the stock finder on my 150PL, I went for a "straight through correct image" finder rather than a RACI, mainly because I am used to the 'both eyes open' approach to viewing straight through. It works really well on the 150PL as I stand up to observe and the finder is at head height, so craning the neck to look through the straight through finder is no problem. However on my refractor, where I observe seated, it's a different story and getting low enough for the straight through finder is very awkward and I really need a RACI - possible early Christmas present!  

Edited December 5, 2020 by RobertI