I think I finally saw the airy discs!

[Buqibu]

A few days ago I made a post asking for advice because I could not see the airy discs in my 4 inch achro. Some people said it might be the seeing, others said maybe I wasn't magnifying enough. Well last night I was getting some unusally clear views of the planets, so I decided to try and see the airy discs again, maybe the seeing would permit. I tried and....nothing. The same smudgy shape I had  been describing. I decided as a last resort to remove my diagonal, maybe it was the cause. So I did. Put in the barlow plus Celestron Zoom, at 165x. After a few minutes adjusting my position (sore neck, sitting on the floor, you know the deal)...there they were. These 3 or 4 rings around Polaris. The star itself looked like a disc. I was very pleased, having solved this "issue" that had been bothering me for a while. Not that it matters that much, its a very simple thing, all telescopes do it, but maybe that was the thing that bothered me. Anyway, back on went the diagonal, the optical imperfections didnt outweigh its convinience😁. Thanks for the help! Just one question, the rings seemed to be only appearing in the lower part of the star, was this due to the seeing or maybe astigmatism or smth like that? Thanks

[vlaiv]

If diagonal is preventing you from seeing airy disk and rings - change diagonal ASAP!

[John]

If the diffraction rings only appear on one side of the star and / or do not appear concentric around the central airy disk then that suggests to me that there is some sort of collimation error in the optics. It could be the diagonal (if in use), the focuser or the objective lens tilt.

You should see something like these images inside the focus point, at focus and outside of the focus point:

 

 

[Buqibu]

1 hour ago, John said:

If the diffraction rings only appear on one side of the star and / or do not appear concentric around the central airy disk then that suggests to me that there is some sort of collimation error in the optics. It could be the diagonal (if in use), the focuser or the objective lens tilt.

You should see something like these images inside the focus point, at focus and outside of the focus point:

 

 

Yeah, might be. Though because its a refractor I'd rather not tinker with it and risk messing it up. The views are otherwise quite sharp so its okay

[John]

7 minutes ago, Buqibu said:

Yeah, might be. Though because its a refractor I'd rather not tinker with it and risk messing it up. The views are otherwise quite sharp so its okay

As long as you are happy with the views then that is all that matters.

 

[Alan64]

12 hours ago, Buqibu said:

A few days ago I made a post asking for advice because I could not see the airy discs in my 4 inch achro. Some people said it might be the seeing, others said maybe I wasn't magnifying enough. Well last night I was getting some unusally clear views of the planets, so I decided to try and see the airy discs again, maybe the seeing would permit. I tried and....nothing. The same smudgy shape I had  been describing. I decided as a last resort to remove my diagonal, maybe it was the cause. So I did. Put in the barlow plus Celestron Zoom, at 165x. After a few minutes adjusting my position (sore neck, sitting on the floor, you know the deal)...there they were. These 3 or 4 rings around Polaris. The star itself looked like a disc. I was very pleased, having solved this "issue" that had been bothering me for a while. Not that it matters that much, its a very simple thing, all telescopes do it, but maybe that was the thing that bothered me. Anyway, back on went the diagonal, the optical imperfections didnt outweigh its convinience😁. Thanks for the help! Just one question, the rings seemed to be only appearing in the lower part of the star, was this due to the seeing or maybe astigmatism or smth like that? Thanks

A diagonal will never improve an image.  It can only degrade an image, if poorly-designed and manufactured.  The ideal is not to have a diagonal affecting an image at all, as though it hasn't even been placed into the telescope; invisible.  

Are you still using the diagonal that came with the telescope?  This is the one I'm talking about.  I have one too, and that came with my Celestron "AstroMaster" refractor...

If so, that is an Amici, erect-image diagonal.  It is primarily for use during the day, for land-based objects; birds in trees, ships at sea, that sort of thing.  For use at night, however, a star diagonal is preferred.  The word "star" says it all.  This is the same Amici, and compared to my Star...

Note the larger aperture of the Star, and for an object's light to pass through.  That is especially beneficial when using a low-power eyepiece, like a 32mm Plossl.  If you don't have one of those, such will provide the lowest power and widest view of the night sky.

There are two types of star-diagonals in the marketplace for your refractor.  One uses a glass-prism, like the Amici, but a star-type prism instead.  The other uses a mirror.  Your telescope is at f/6.5.  You can make use of either, a prism or a mirror.  A mirror will not introduce additional false-colour, but it is not as durable, and over the years.  

If you're located in the UK...

Prism... https://www.rothervalleyoptics.co.uk/celestron-90-degree-star-diagonal-125.html

Mirror... https://www.rothervalleyoptics.co.uk/antares-90-star-diagonal-125.html

If elsewhere, you should have enough information here to find one there.  In my opinion, this is the best 1.25" star-mirror diagonal on the planet...

https://www.firstlightoptics.com/diagonals/tele-vue-enhanced-aluminium-90-diagonals.html

This is an example of a better star-prism... https://www.firstlightoptics.com/diagonals/baader-t-2-prism-star-diagonal-32mm.html

Edited August 6, 2021 by Alan64

[Buqibu]

2 hours ago, Alan64 said:

A diagonal will never improve an image.  It can only degrade an image, if poorly-designed and manufactured.  The ideal is not to have a diagonal affecting an image at all, as though it hasn't even been placed into the telescope; invisible.  

Are you still using the diagonal that came with the telescope?  This is the one I'm talking about.  I have one too, and that came with my Celestron "AstroMaster" refractor...

If so, that is an Amici, erect-image diagonal.  It is primarily for use during the day, for land-based objects; birds in trees, ships at sea, that sort of thing.  For use at night, however, a star diagonal is preferred.  The word "star" says it all.  This is the same Amici, and compared to my Star...

Note the larger aperture of the Star, and for an object's light to pass through.  That is especially beneficial when using a low-power eyepiece, like a 32mm Plossl.  If you don't have one of those, such will provide the lowest power and widest view of the night sky.

There are two types of star-diagonals in the marketplace for your refractor.  One uses a glass-prism, like the Amici, but a star-type prism instead.  The other uses a mirror.  Your telescope is at f/6.5.  You can make use of either, a prism or a mirror.  A mirror will not introduce additional false-colour, but it is not as durable, and over the years.  

If you're located in the UK...

Prism... https://www.rothervalleyoptics.co.uk/celestron-90-degree-star-diagonal-125.html

Mirror... https://www.rothervalleyoptics.co.uk/antares-90-star-diagonal-125.html

If elsewhere, you should have enough information here to find one there.  In my opinion, this is the best 1.25" star-mirror diagonal on the planet...

https://www.firstlightoptics.com/diagonals/tele-vue-enhanced-aluminium-90-diagonals.html

This is an example of a better star-prism... https://www.firstlightoptics.com/diagonals/baader-t-2-prism-star-diagonal-32mm.html

Thank you! That's exactly the diagonal it has, I have an astromaster 102. Guess they have to make some compromises to make it suitable for terrestrial observing. Will check these out🙂

[Alan64]

4 hours ago, Buqibu said:

Thank you! That's exactly the diagonal it has, I have an astromaster 102. Guess they have to make some compromises to make it suitable for terrestrial observing. Will check these out🙂

Well, you can use an Amici-diagonal at night, there's no law against it, but there's one thing about said diagonal that you might find very interesting.  There is such a thing called the "Amici line"...

Have you seen an illuminated line going through Jupiter like that?  Venus will exhibit that, too.  Saturn and Mars might, if at opposition, but then may not.  It's the brightness of the object that brings out that line.

Edited August 6, 2021 by Alan64

[Buqibu]

1 hour ago, Alan64 said:

Well, you can use an Amici-diagonal at night, there's no law against it, but there's one thing about said diagonal that you might find very interesting.  There is such a thing called the "Amici line"...

Have you seen an illuminated line going through Jupiter like that?  Venus will exhibit that, too.  Saturn and Mars might, if at opposition, but then may not.  It's the brightness of the object that brings out that line.

I can't say I've seen that line, but I have noticed then when out of focus there is this line that divides the circle in two. This makes it so I can never achieve perfect focus in point objects like stars, for planets its not a big deal.

[Sunshine]

This must be thorn in every amateurs side, we all want to reassure ourselves that our scope which we paid hard earned money for is doing what it should. There’s no avoiding it, now that you have seen the elusive airy disc just remember that not every night will you be able to. Enjoy your scope, if the views look great then rest assured your scope is working.

[John]

1 minute ago, Sunshine said:

This must be thorn in every amateurs side, we all want to reassure ourselves that our scope which we paid hard earned money for is doing what it should. There’s no avoiding it, now that you have seen the elusive airy disc just remember that not every night will you be able to. Enjoy your scope, if the views look great then rest assured your scope is working.

One additional challenge is that the core optics of many / most scopes are good but those of the low cost accessories that are provided with them are somewhat weaker and do not allow the core optics to deliver to their full potential.

The problem for the manufacturers / retailers is that including a decent quality diagonal and eyepieces would add a substantial amount to the purchase price of the scope which might well make it look uncompetitive in the market place, to a newcomer to the hobby.

 

 

[Buqibu]

Absolutely, getting obsessed over details and optics only achieves frustration, I almost forgot why I even got into astro in the first place, to learn and relax in the beauty of the night sky, not to do optical figure estemations🤣🤣. Thanks for all the help 

[Alan64]

There are means and ways of improving your entry-level achromat, and to where it would rival the contrast of a fine apochromat; jet-black skies almost all of the way up to an object's edges, with the object itself standing out from said background in high-relief.

[Alan64]

What, no question as to how?

You should have a look through my achromats.

[Buqibu]

20 hours ago, Alan64 said:

What, no question as to how?

You should have a look through my achromats.

Sorry! Haven't had time to check back on the forum, indeed how would an achromat deliver views like an apo?

 

[Alan64]

6 hours ago, Buqibu said:

Sorry! Haven't had time to check back on the forum, indeed how would an achromat deliver views like an apo?

 

That's okay.  I'm glad you asked...

The way you do that is to deaden the entire inside of the telescope, from front to back, and to all sources of stray-light, whether natural(Moon), or artificial(porch, street and passing-automobile lights).  This is done with deep- and deepest-black, matte paint(spray-can or just a can), and flocking.  It's merely arts & crafts type work; for examples...

You can stick with just spray-cans, and spray from the can(where applicable), or spray paint into a plastic or metal container; a plastic lid from a jar, or a plastic container from a fruit or pudding cup.  They must be washed, of course, beforehand.  I lay a plastic sandwich-bag over the container, then a metal jar lid over that, and to keep the paint moist.  I do this between each short session of painting.  To thin the paint, if necessary, do not use paint-thinner, but better quality mineral-spirits.  Just a few drops into the container will re-moisten the paint.  As you can see in the image above, I use different sizes of artist-type brushes to apply the paint, and by hand.  I get those brushes cheap from my local super-department store.

Here, I've painted the edge of this doublet-lens all round...

Note the before-image, bottom-left.

The goal there is to make the lens(es) as invisible as possible, to prevent stray light from being reflected.  Ideally, we would not have anything between our eyes and the objects we're observing, but, unfortunately, our eyes, our pupils specifically, are very weak and make for poor telescopes, by themselves.  They can't even magnify; 1x is it.  That's why we all love telescopes.

Now, when painting the edges of lenses, invariably, unavoidably, some paint is going to get on the actual surface of the lens, there at the edge, all round, top and bottom.  You then take cotton-swabs, lightly dampened with 91% isopropyl-alcohol, then wipe the excess off away from the lens, but only one wipe per tip.  You can rotate the tip halfway round for a second wipe, but only if you get good at it.  You have to take care, however, not to remove any paint from the frosted, vertical edge all round.  It may sound difficult, but it isn't really.  You can source a scrap lens, just one, for practicing beforehand.  

I also paint the insides of the telescope tubes.  Before and after...

The telescopes that come from overseas, including yours, and practically all of my own, are painted on the inside, but the paint is lighter, tending towards grey, rather than a deep-black.  I call it "Synta grey".  Synta made your telescope, and several of my own as well.  Look at this before-image of my 6"/150mm f/5 Newtonian, without and with flash...

..."Synta grey".  It's such a lovely shade of grey, yet far removed from a deep-black.  There's some orange in that as well; rust, gads.

I also paint the screw-tips on the inside; everything that's glossy or shiny, and that could reflect stray-light.  This includes inside the draw-tubes of the focussers...

Also, in the case of refractors, the inside of the focusser housing...

Then, there is flocking.  Flocking is like very low pile carpeting, but for telescopes, and some accessories as well.  This is flocking...

Note how deep the black is; deeper than vast majority of black paints.  It is sold in rolls, usually, and it's self-adhesive.  You must wash the inside of a telescope tube first, dry it thoroughly, then gloss it throughout with a clear-gloss enamel...

The stuff I use smells to the high heavens above.  It's best to spray it outdoors, of course.  After the enamel cures and hardens, you cut the flocking into strips, and apply...

Within that Newtonian, there are only two things that reflect light: the primary-mirror, and the secondary mirror.  

This is the one and only instance where I flocked a focusser's draw-tube...

Here's an instance where I made a special flocked shield for a focusser's housing...

There was no need to paint the inside of the housing, behind that shield, in that instance.

In the end, you want the interior of your telescope...

...dead, and like the inside of Dracula's casket when it's closed, tight.

[Alan64]

In addition, blackening and flocking will not make up for the lack of the special lenses used in apochromats.  Such will not correct false-color, either, nor will it make the images sharper, but it can make the images a bit easier to see, stand out.  You will notice a difference in the views, rich contrast, but only if you blacken and/or flock the telescope throughout.

[Buqibu]

44 minutes ago, Alan64 said:

That's okay.  I'm glad you asked...

The way you do that is to deaden the entire inside of the telescope, from front to back, and to all sources of stray-light, whether natural(Moon), or artificial(porch, street and passing-automobile lights).  This is done with deep- and deepest-black, matte paint(spray-can or just a can), and flocking.  It's merely arts & crafts type work; for examples...

You can stick with just spray-cans, and spray from the can(where applicable), or spray paint into a plastic or metal container; a plastic lid from a jar, or a plastic container from a fruit or pudding cup.  They must be washed, of course, beforehand.  I lay a plastic sandwich-bag over the container, then a metal jar lid over that, and to keep the paint moist.  I do this between each short session of painting.  To thin the paint, if necessary, do not use paint-thinner, but better quality mineral-spirits.  Just a few drops into the container will re-moisten the paint.  As you can see in the image above, I use different sizes of artist-type brushes to apply the paint, and by hand.  I get those brushes cheap from my local super-department store.

Here, I've painted the edge of this doublet-lens all round...

Note the before-image, bottom-left.

The goal there is to make the lens(es) as invisible as possible, to prevent stray light from being reflected.  Ideally, we would not have anything between our eyes and the objects we're observing, but, unfortunately, our eyes, our pupils specifically, are very weak and make for poor telescopes, by themselves.  They can't even magnify; 1x is it.  That's why we all love telescopes.

Now, when painting the edges of lenses, invariably, unavoidably, some paint is going to get on the actual surface of the lens, there at the edge, all round, top and bottom.  You then take cotton-swabs, lightly dampened with 91% isopropyl-alcohol, then wipe the excess off away from the lens, but only one wipe per tip.  You can rotate the tip halfway round for a second wipe, but only if you get good at it.  You have to take care, however, not to remove any paint from the frosted, vertical edge all round.  It may sound difficult, but it isn't really.  You can source a scrap lens, just one, for practicing beforehand.  

I also paint the insides of the telescope tubes.  Before and after...

The telescopes that come from overseas, including yours, and practically all of my own, are painted on the inside, but the paint is lighter, tending towards grey, rather than a deep-black.  I call it "Synta grey".  Synta made your telescope, and several of my own as well.  Look at this before-image of my 6"/150mm f/5 Newtonian, without and with flash...

..."Synta grey".  It's such a lovely shade of grey, yet far removed from a deep-black.  There's some orange in that as well; rust, gads.

I also paint the screw-tips on the inside; everything that's glossy or shiny, and that could reflect stray-light.  This includes inside the draw-tubes of the focussers...

Also, in the case of refractors, the inside of the focusser housing...

Then, there is flocking.  Flocking is like very low pile carpeting, but for telescopes, and some accessories as well.  This is flocking...

Note how deep the black is; deeper than vast majority of black paints.  It is sold in rolls, usually, and it's self-adhesive.  You must wash the inside of a telescope tube first, dry it thoroughly, then gloss it throughout with a clear-gloss enamel...

The stuff I use smells to the high heavens above.  It's best to spray it outdoors, of course.  After the enamel cures and hardens, you cut the flocking into strips, and apply...

Within that Newtonian, there are only two things that reflect light: the primary-mirror, and the secondary mirror.  

This is the one and only instance where I flocked a focusser's draw-tube...

Here's an instance where I made a special flocked shield for a focusser's housing...

There was no need to paint the inside of the housing, behind that shield, in that instance.

In the end, you want the interior of your telescope...

...dead, and like the inside of Dracula's casket when it's closed, tight.

Wow, thanks for the long and detailed response. I should try this in the future, looks great for improving contrast. Cheers!

[Alan64]

If you reside in the U.S., Surplus Shed has lenses, cheap, cheaper than cheap, and onto which to practice your painting techniques...

https://www.surplusshed.com/search_lenses.php

They're cheap enough to get two, a plano-concave, and a double-convex.