Which Filter for 4" f/5 Achromatic Refractor?

[GazOC]

1 hour ago, mikeDnight said:

To be honest, I've always felt that trying to filter out CA detracts from the cleanness of the view. CA is the nature of the beast and not a fault, so it's best to view the 4" F5 achromat as a specialist instrument. It's specialism being wide star fields, ideal as a comet seeker.

I agree, Mike. I've owned all the f5 Skywatcher refractors at one time or another, tried most of the filter options and the best views for me are the unfiltered ones 

 

The only time I've been really dissatisfied with a short refractor was when it was my only scope because it does have very distinct strengths and weaknesses. Match it up with a reasonable planetary/ lunar scope and enjoy it for what it is 

 

(Money allowing, of course!)

Edited May 10, 2020 by GazOC
Typo

[Chris]

That's another handy chart, I'm hoping for diffraction limited, I'll compare the rings to this chart when both the scope and good seeing arrive. If it can support 80-100x that would do me fine I reckon. 

[RodAstro]

Hi Chris

A nice scope you have ordered.

I also have one as my finder scope and a grab and go scope.

My recommendation after trying many filters is, use a light yellow staked with a polarizing filter this gives the most natural colour, for me anyway. These two filters are cheap and also useful in your kit for ever anyway.

Don't use them on widefield views as all a filter does is remove light and that is counter to that sort of viewing, I just use them for planets and the moon.

My other thing to say is all scopes have their problems no matter how much you spend, I feel to many people spend most of their time looking at the telescope and not the object and seem to have little interest in astronomy but more in optics, much easier to fulfill that in the daytime with cameras, but then your other half might see what you have spent all your money on🤪🤣

 

Cheers Rod 

[Chris]

2 minutes ago, RodAstro said:

Hi Chris

A nice scope you have ordered.

I also have one as my finder scope and a grab and go scope.

My recommendation after trying many filters is, use a light yellow staked with a polarizing filter this gives the most natural colour, for me anyway. These two filters are cheap and also useful in your kit for ever anyway.

Don't use them on widefield views as all a filter does is remove light and that is counter to that sort of viewing, I just use them for planets and the moon.

My other thing to say is all scopes have their problems no matter how much you spend, I feel to many people spend most of their time looking at the telescope and not the object and seem to have little interest in astronomy but more in optics, much easier to fulfill that in the daytime with cameras, but then your other half might see what you have spent all your money on🤪🤣

 

Cheers Rod 

Thanks for the filter hack Rod. I'm personally into both astronomy and optics including how they are made (I occasionally do a bit of mirror grinding) I think both things certainly complement each other because you can get more of a wow factor looking at say Jupiter through say a C9.25 Edge HD compared to a 130p, but I know what you're saying. I guess it's like cars to some extent. People like nice things and it does rain alot so why not have a scope that looks the part too 🤪

[mikeDnight]

2 hours ago, Lockie said:

 is there any thing specific to look out for?

If you have a Ronchi eyepiece you can aim your scope at a bright star, alter the focus until you have five lines or less, and if the lines are straight you've got a well figured lens. If there is any appreciable curvature to the lines you're either over or under corrected and will suffer from SA. A bug comb held at the open draw tube can also act as a Ronchi grating to give a good indication of lens figure too! Find a bright star, remove the eyepiece, then hold the comb flat against the draw tube rack the focuser in or out until the light from the star floods the field. You'll se the black Ronchi lines that will give you a good idea of the figure. If the lines are wedge shaped, you're out of colimation.  

I've used a number of well figured Chinese achromat's, but I've also seen a lot that are sub standard. If the lens has SA then I'd return the scope as its not fit for purpose.  Even in a short achromat the star images should be sharp, although there will be some CA around bright stars. But if the scope has SA its star images will have an annoying blur affecting the diffraction rings as well as contrast and definition. 

 

[Chris]

12 minutes ago, mikeDnight said:

If you have a Ronchi eyepiece you can aim your scope at a bright star, alter the focus until you have five lines or less, and if the lines are straight you've got a well figured lens. If there is any appreciable curvature to the lines you're either over or under corrected and will suffer from SA. A bug comb held at the open draw tube can also act as a Ronchi grating to give a good indication of lens figure too! Find a bright star, remove the eyepiece, then hold the comb flat against the draw tube rack the focuser in or out until the light from the star floods the field. You'll se the black Ronchi lines that will give you a good idea of the figure. If the lines are wedge shaped, you're out of colimation.  

I've used a number of well figured Chinese achromat's, but I've also seen a lot that are sub standard. If the lens has SA then I'd return the scope as its not fit for purpose.  Even in a short achromat the star images should be sharp, although there will be some CA around bright stars. But if the scope has SA its star images will have an annoying blur affecting the diffraction rings as well as contrast and definition. 

 

Thanks Mike, great advice there! A Ronchi eyepiece sounds like something I would enjoy owning but it's good to know there is a cheaper way with the kids nit comb  I'll go look at the price and if reasonable I think I'll order one. Sounds like it might be a good long term investment for someone who has historically gone through a shed load of scope lol

Edited May 10, 2020 by Lockie

[barkis]

I own a Long Perng 150mm. f8 Frac.  
Sure it's CA is poor, but the lunar detail is so good, I can make the aberration 
disappear by just not thinking about it.
Ron.

[Chris]

53 minutes ago, barkis said:

I own a Long Perng 150mm. f8 Frac.  
Sure it's CA is poor, but the lunar detail is so good, I can make the aberration 
disappear by just not thinking about it.
Ron.

You probably don't need to do this if you can tune the CA out, but have you ever tried stopping it down to 4" f12? I reckon that would be pretty nice on the moon!

[Chris]

I've just ordered this to check SA correction:

https://www.firstlightoptics.com/specialist/gerd-neumann-ronchi-eyepiece.html

I'm also wondering if I can use it to test the mirror I'm very slowly grinding (Ronchi gram?). I really need to get back to that project soon.

Edited May 11, 2020 by Lockie

[barkis]

1 hour ago, Lockie said:

I've just ordered this to check SA correction:

https://www.firstlightoptics.com/specialist/gerd-neumann-ronchi-eyepiece.html

I'm also wondering if I can use it to test the mirror I'm very slowly grinding (Ronchi gram?). I really need to get back to that project soon.

What size blank are you working Lockie, are you still in the polishing stages?
Ron.

[barkis]

1 hour ago, Lockie said:

You probably don't need to do this if you can tune the CA out, but have you ever tried stopping it down to 4" f12? I reckon that would be pretty nice on the moon!

I've cut a couple of diaphragms for fun's sake, but I enjoy the full aperture on 
the moon. I've used it a number of times on Doubles which I enjoy a great deal too.
This scope is no Rolls Royce, but I'm very satisfied with what I get out of it.
It's quite a heavy lump, but the EQ6 is more than a match for it 😀.
I must add, I have not used it since I had my cataracts sorted, so aperture reduction may
more advantageous now.

Edited May 11, 2020 by barkis

[Chris]

8 minutes ago, barkis said:

What size blank are you working Lockie, are you still in the polishing stages?
Ron.

It's an 8.75" blank @ about f/6.3. It's been shelved for about a year because of a house move and me making a lot more Astronomy YouTube videos but it's always at the back of my mind so really want to finish the project at some point. 

I'm a bit rusty with where I am with it, but I think I had just finished the fine grinding and needed to make some kind of testing apparatus (foucault/ronchi?) I had bought a metal microscope platform with micrometer movements on x and y axis, but that's as far as I had got with making a tester. 

[barkis]

58 minutes ago, Lockie said:

It's an 8.75" blank @ about f/6.3. It's been shelved for about a year because of a house move and me making a lot more Astronomy YouTube videos but it's always at the back of my mind so really want to finish the project at some point. 

I'm a bit rusty with where I am with it, but I think I had just finished the fine grinding and needed to make some kind of testing apparatus (foucault/ronchi?) I had bought a metal microscope platform with micrometer movements on x and y axis, but that's as far as I had got with making a tester. 

I made a number of Newt. Mirrors way back in my younger days. I just got addicted to the process, and enjoyed so much.   I worked with NE Howard's Telescope Making book, and my testing and figuring was done with the aid of a home made Foucault tester, and Coudre masks.  My first project was also with a 8.75  Mirror kit, bought from a Charles Frank shop in Glasgow.

It was a long task, as many mistakes were made but as with any other task, mistakes are how you learn.  It also was an F6, which is a good photo visual instrument.

SGL has quite a number of guys who have made their own mirrors, some are in the very large category too, so we all wish you well on your project, and should you need any help or advice, you will get a good response here I'm sure.

Ron.

 

 

[Chris]

3 hours ago, barkis said:

I made a number of Newt. Mirrors way back in my younger days. I just got addicted to the process, and enjoyed so much.   I worked with NE Howard's Telescope Making book, and my testing and figuring was done with the aid of a home made Foucault tester, and Coudre masks.  My first project was also with a 8.75  Mirror kit, bought from a Charles Frank shop in Glasgow.

It was a long task, as many mistakes were made but as with any other task, mistakes are how you learn.  It also was an F6, which is a good photo visual instrument.

SGL has quite a number of guys who have made their own mirrors, some are in the very large category too, so we all wish you well on your project, and should you need any help or advice, you will get a good response here I'm sure.

Ron.

 

 

Ron, You've helped me a ton already!  this is my grind thread......it's been dormant for a while, but I made a good start thanks to you lot! Looking at the date I last touched the mirror in spring of 2018! Where does the time go?

 

 

Edited May 11, 2020 by Lockie

[barkis]

@Lockie I'm pleased you are re motivated. It would be a great shame not to finish the task,
especially as you have already made substantial progress.
The Sagitta may be shallower at f6.7, the figuring may seem an easier proposition, the only
other consequence, is the shadows under the knife edge when figuring will be a bit fainter that a faster mirror.
Nothing daunting  though.

My biggest headache proved to be the diffraction round the edges of the Coudre mask apertures,
the can be quite bright .  I resorted to the Everest method at times. That method uses fine pins or headless nails
pinned at the positions of the various mirror zones (ie)  30%, 50%, 71%, and the edge, the very crucial bit.      

Don't get worried about the dreaded turned edge, the mirror makers nightmare. With care, easily avoided.
I take it you are working with the aid of a book on mirror making. I still have my  Howard book if you 
wish to borrow it. I sold my Texereau one, which is a more in depth one.
Ron.

 

Edited May 11, 2020 by barkis

[Chris]

49 minutes ago, barkis said:

@Lockie I'm pleased you are re motivated. It would be a great shame not to finish the task,
especially as you have already made substantial progress.
The Sagitta may be shallower at f6.7, the figuring may seem an easier proposition, the only
other consequence, is the shadows under the knife edge when figuring will be a bit fainter that a faster mirror.
Nothing daunting  though.

My biggest headache proved to be the diffraction round the edges of the Coudre mask apertures,
the can be quite bright .  I resorted to the Everest method at times. That method uses fine pins or headless nails
pinned at the positions of the various mirror zones (ie)  30%, 50%, 71%, and the edge, the very crucial bit.      

Don't get worried about the dreaded turned edge, the mirror makers nightmare. With care, easily avoided.
I take it you are working with the aid of a book on mirror making. I still have my  Howard book if you 
wish to borrow it. I sold my Texereau one, which is a more in depth one.
Ron.

 

Hi Ron, I will get there for sure, I just want to build the obsy first (If the mirror could end up in the Obsy that would be nice.) From memory I think it's turning out to be closer to f6.3 (f6.7 was my initial target though) I was working from a combination of picking peoples brains and the internet, but I do appreciate the book offer  I do have a book on how to grind lenses for a refractor which I bought when my eyes got too big for my stomach lol

Your input is always appreciated Ron, thanks. 

[barkis]

1 hour ago, Lockie said:

Hi Ron, I will get there for sure, I just want to build the obsy first (If the mirror could end up in the Obsy that would be nice.) From memory I think it's turning out to be closer to f6.3 (f6.7 was my initial target though) I was working from a combination of picking peoples brains and the internet, but I do appreciate the book offer  I do have a book on how to grind lenses for a refractor which I bought when my eyes got too big for my stomach lol

Your input is always appreciated Ron, thanks. 

I went back to your original thread Chris. You got some good input there for sure, and you can certain on plenty of encouragement on your progress. You will get there, and we'll all be waiting for your first light post.   I won't wish you luck, you don't need it, just plain old determination diluted with a sensible approach.

Ron.