Red Dot Finder and long-sightedness

[Voight-Kampff]

Hi All,

I'm just getting started on observing with a scope, having recently got a Celestron Astromaster 130eq. While some aspects of the build quality show where the corners have been cut it's generally quite pleasant to use and I've already found a few DSOs by star hopping.

What I'm finding difficult is getting located onto a starting point using the RDF. I am long-sighted and have to use glasses for reading, using a computer etc. What happens,  when trying to look through the RDF in the dark, if I don't use my glasses I can see the target star fine but the dots are very, very, blurred and I can't really see if they're properly lined up. If I put my glasses on I can see the dots better but I can't see the star!

Any hints on how to get around this, or should I start looking for an alternative finder? Do any long-sighted readers have any experience of using other types of finder?

Cheers,

Pete

[Dave In Vermont]

I need glasses for reading and seeing anything up to a bit over a meter away. Anything up close is a blur without them. I use RACI's and straight-thru 8 or 9 X 50mm finder-scopes. I don't need glasses for this - or my telescopes - as the focuser compensates for the focus of my eyes. But I can see where a red-dot finder would present problems.

Have you tried a regular finder-scope without your glasses? It should be fine. I can also follow a green-laser in the sky, but I never use one for this. They can get you in trouble with other astronomers and passing aircraft.

Good luck,

Dave

[ronin]

You should not have a problem and you should actually find it "more comfortable".

It is the operation of an RDF that is I suspect that is messing it up.

I suspect that you are looking at the RDF, which with the RDF being not far away you find difficult/uncomfortable.

Do not look at the RDF, look at the sky beyond/behind the RDF.

What then happens (should) is that the optics of the RDF creates a red dot in your vision, which should indicate where the finder (and scope) is pointed at.

Ideally the RDF should just be a simple glass plate, however there are a lot of bits around it and people tend to look at the assembly, and optically the red dot is not created at the assembly it is created at infinity so you need to look at infinity to see/get a dot.

The setup is a bit like looking through a window at home, you do not look at the glass you look at the object beyond the glass. Just an RDF has so many bits people end up looking at the bits. Sit at home, look out a window, then look at the window frame, then back at whatever outside. With an RDF you look at the object then swing the red dot into position - without quite looking at the RDF.

[Astro Imp]

Ronin has given a very good explanation of the way red dot finders work, however one thing to bare in mind is they are used with both eyes open. You adjust the scope so the two images(the star and red dot) become one.

HTH and good luck.

[Voight-Kampff]

Thanks for the tips.

I realised a couple of nights back I should be using both eyes - it's proving quite hard to get my eyes to ignore the finder and just look through it, given you've confirmed that's what I should be doing I'll just have to persist with 'til I master the knack of making the dot pop out to infinity.

What makes it tricky is the Celestron finder is pretty cheap and plastic, and stars appear considerably dimmer when viewed through it.

Am I right in thinking Telrads work the same way, you keep both eyes open?

Pete

[bambuko]

... they are used with both eyes open....

Well... not always.

Unless both your eyes are perfect, majority of us will be "more" (shortsighted or longsighted) in one eye than the other.

I am so shortsighted in the left eye, that when using either Rigel or Telrad only my right eye is usable.

Wearing corrective glasses is no use, because  whilst I can see stars through them, they make red dot/circle so blurred as to be un-usable.

So I am using only one eye with my unit power finders.

I am not the only one with this problem either...

[sheeprug]

Hi All,

I'm just getting started on observing with a scope, having recently got a Celestron Astromaster 130eq. While some aspects of the build quality show where the corners have been cut it's generally quite pleasant to use and I've already found a few DSOs by star hopping.

What I'm finding difficult is getting located onto a starting point using the RDF. I am long-sighted and have to use glasses for reading, using a computer etc. What happens,  when trying to look through the RDF in the dark, if I don't use my glasses I can see the target star fine but the dots are very, very, blurred and I can't really see if they're properly lined up. If I put my glasses on I can see the dots better but I can't see the star!

Any hints on how to get around this, or should I start looking for an alternative finder? Do any long-sighted readers have any experience of using other types of finder?

Cheers,

Pete

Hi Pete,

I use a Telrad - in fact I don't have a finderscope (yet).   I'm also very short sighted and now in my late 50s I can't read anything closer than about 6 feet without reading glasses either!     Thus I'm surprized you have problems with a RDF, because I find it works very well.     To be honest, this had puzzled me a bit, because I'd have expected problems, butI'd vaguely come the concluson that since with a reflector sight you're effectively looking at infinity there was no near focusing involved.  I do have to be careful about setting the brightness, but apart from that, no problems.

Do some RDFs work in a different way to a Telrad?

SR 

[bambuko]

... since with a reflector sight you're effectively looking at infinity there was no near focusing involved....

Stars are at infinity, but your red dot/circle/square etc, etc is very close to your eye, and you have to able to see both stars and red circle to be able to use unit power finders.

Like I said I have both Telrad and Rigel (and used other red dot contraptions) and what works for you might not necessarily work for others.

You didn't specify whether you are using glasses with your Telrad?

Are you using bi-focals or vari-focals?

Knowing this might help us understand why it works for you?

[Charic]

At the opposite end, I'm short sighted, and need my  glasses to keep my HGV/PCV licences current, but rarely wear them for anything else except fishing  and  to sharpen the Bulls-Eye that my Telrad produces, otherwise I see six  or is it 9 concentric rings?

I don't wear glasses by choice when using the eyepieces on my telescope or when using binoculars,  and yet  I see more Stars with my prescription glasses just when looking up. Perhaps I should learn to overcome their use and master the fine division that varifocals have.

What a strange hobby to have when so many of us are  either  short or long sighted, and our eyesight is paramount to image perfection  ?

[bambuko]

I thought you would join us in this discussion, sooner or later Charic

[sheeprug]

Stars are at infinity, but your red dot/circle/square etc, etc is very close to your eye, and you have to able to see both stars and red circle to be able to use unit power finders.

Like I said I have both Telrad and Rigel (and used other red dot contraptions) and what works for you might not necessarily work for others.

You didn't specify whether you are using glasses with your Telrad?

Are you using bi-focals or vari-focals?

Knowing this might help us understand why it works for you?

No, no tricks - I just wear my normal 'distance' glasses.   I assure you cannot focus on the Telrad body itself, but the red dot sight is sharp because it appears to be at infinity - that's how reflector sights work if you read the entry on wikipedia. There's no need to re-focus your eyes.   But like I said, I'd not really gven it very much thought and I no expert in optics anyway!

SR

[bambuko]

So I guess, you are also using the same normal 'distance' glasses when observing through telescope? or are you one of the on/off/on types?

I don't wear glasses when observing.

[Charic]

I thought you would join us in this discussion, sooner or later Charic

And I thought you were away spending your royalties on the new shorty Telrad design?

What, no patent? Lol

[sheeprug]

So I guess, you are also using the same normal 'distance' glasses when observing through telescope? or are you one of the on/off/on types?

I don't wear glasses when observing.

Well I've never described myself as 'on/off', but yes.  Aim the [removed word] with specs on so I can see the little buggers, then take the specs off to look at the buggers in more detail.

Having worn glasses for the last 50 years with a perscription in the -8 to -10 diopter range, and the last 7 years not being able to focus close, you tend to be come quite adept at finding a combination that works.    

The far sight bit does take some getting used to.  What happens is, without really thinking about it, you start sliding your regualr glasses down your nose when you need to focus more closely.  Then you get to a point when your nose ins't long enough and you take them off completely and view near things without glasses. Unfortunately in my case that means you read books from a distance of about 4 inches and everybody else starts making 'funny' comments about 'grandad'.  Eventually you let the kids select some trendyish frames from 'goggles4you' and get some reading glasses.  These then become your normal pair because it's so liberating being able to see properly again.     

SR.

[bambuko]

Mine is nowhere near as bad as yours and easily compensated for, using telescope and normal eyepieces/focuser.

I tried initially using glasses, but found it near impossible without trampling them, strangling myself with glass cord, or simply loosing them in confusion and darkness

Luckily for me, one of my eyes is "more" than other so I simply adopted one eyed technique  

It simply means that I have to be on the correct side of telescope to use the more usable eye ...

[Stub Mandrel]

I thought the optics in these finders create a parallel ray or cylinders of light, so the eye perceives them as being at infinity?

Even someone with perfect vision can't focus at 2 feet and infinity at the same time...

[Voight-Kampff]

I thought the optics in these finders create a parallel ray or cylinders of light, so the eye perceives them as being at infinity?

Even someone with perfect vision can't focus at 2 feet and infinity at the same time...

Having had a good look at my Celestron RDF while lining it up again today, I'm now convinced it doesn't have much in common with a Telrad or reflector sight type setup. It's just a couple of leds lighting up the edges of holes drilled in the plastic discs, with really coarse adjustment to line up the circles. I think I'll be investing in a different finder quite soon.

[Dave In Vermont]

Presbyopia, hardening of the lens of the eye, is a thing everyone can look foreward ( ) to around age 40. But my vision situation is quite rare: Monocular-vision. This is hereditary and comes down the family line of my mother. What this boils down to is I use one eye as the focus, and the other eye as the range-finder. This shifts off depending on which direction I'm looking at a given moment. I have full depth-perception (which confuses many people), but in public-school here all the kids are given a seeing-test.You look into a machine with one eye - you should see a red ball. Then the other eye - you should see a picnic-table. Then with both eyes and you should see rhe red ball on the picnic-table. Not me, though. So they took out a huge box full of different sets of glasses to try on and see if I see better in their machine. Nope.

The upshot was my being told to walk home with a note, which told my parents that I was completely blind and could not go to their schools in the town! Yes - send the blind-kid to walk down the busy streets with a note stating I was blind. GOOD School!

I'm hardly blind, as you may have noted, but I have the ability to choose which eye I use and which one knows how far away the target is. I'm way beyond marksman level with guns. As is my entire family as my dad's family are also genetically predisposed to excellence at things requiring great hand-eye coordination. Now while I can shoot the wings off a gnat at 100 meters, I can't see 3-D movies at the cinema. And a red-dot finder is right out as well!

So if you have perfect 20/20 vision today and you're 30-something or younger, and one day your newspaper is out-of-focus, don't flip-out. Chances are heavily in your favor of having presbyopia. Go try on some reading-glasses to get the right strength for your eyes. Mine requires 2½ X. Also take into account that you will need to focus your telescope to a different focal-point than you did before. So if you're used to focusing and most people will find your image quite sharp when they gaze into your eyepiece - not so anymore you old geezer!

Be seeing you,

Dave