What kind of lens do I have

[Moonlit]

As I recently got my first telescope, a used old Helios 120mm f/8.3 refractor and I'm quite annoyed by the chromatic aberration, I'm trying to figure out whether I'd be more pleased with a smaller aperture doublet (they're expensive, I can get a Sky-Watcher 80ED for my budget) or just sticking with my Helios and perhaps get a secondary Newtonian or Schmidt-Cassegrain for visual observation.

I only have the EPs that came with the scope when it was new: 1.25" diagonal, 2x Barlow, 1.25" 10mm and 20mm, but I'm planning on getting a 2" PanaView 32mm (70 FoV) and perhaps a 1.25" Doppler UltraWide LER 9mm.

 
The CA is the worst, and I'm having trouble even using the 10mm EP as my eye has to be very close and in the exact right position to get a good view. The 20mm is much more forgiving. 

 

 

If I knew what kind of lens I'm looking through, I'd know more about what I should look for. 

So does anyone have an informed guess as to:

- What kind of coating does this lens have?

- Is it just a single lens or a doublet?

- Is this lens pretty good for its kind?

- Why I don't seem to be able to even focus the picture when I use the 10mm + Barlow - might just be that I'm inexperienced and that this is normal.
 

The scopes sticker says: "TELESCOPE D=120mm F=1000mm COATED OPTICS MADE IN CHINA"

[FracTak]

There is a good review of your scope here. It is an achromatic doublet, classic crown and flint glass which at f8.3 will show a fair amount of chromatic abberation, that’s just what is to be expected with that aperture, focal ratio and glass type.

 

http://scopeviews.co.uk/SW120Evostar.htm

 

another here:

 

https://stargazerslounge.com/topic/145468-helios-evostar-120mm-f83-first-light-quick-review/

 

Not or sure about the focus issue with the Barlow. Have a try on the Moon which is easy to find and focus on and see how you go. That combination would give x200, so you need to be centred on a star, planet or the Moon to know whether you are focused or not.

[Moonlit]

On 9/13/2022 at 7:32 PM, FracTak said:

There is a good review of your scope here. It is an achromatic doublet, classic crown and flint glass which at f8.3 will show a fair amount of chromatic abberation, that’s just what is to be expected with that aperture, focal ratio and glass type.

Thanks! I discovered that using the 40mm aperture (removing the small lid on the lens cover) removed most of the CA, which is logical when I think about it. I'll have to see if it helps with focusing too. 
The focusing is apparently just the way it is - the focuser is rack&pinion with no fine adjustment so I just have to be careful. I've heard of Bahtinov filters, that'll come later. On reflectors you can apparently use "double vision" to focus if you have two holes placed opposite one another, I dunno if that trick works on refractors. 

[Nightspore]

 

You might find this useful. There is an old trick to ameliorate cyan fringing on achromats.

 

 

You can use a yellow filter, often a Wratten #8.

 

 

Baader have some specialist dedicated 'fringe killing' filters. Although they're not cheap.

 

 

A light yellow filter stacked with a 'light pollution' filter will work for excessive CA. The LP filter takes out some of the sodium band and naturalises the image slightly.

 

 

 

 

[FracTak]

21 hours ago, Moonlit said:

I discovered that using the 40mm aperture (removing the small lid on the lens cover) removed most of the CA, which is logical when I think about it.

 

Yes, it will certainly reduce the CA by reducing aperture and making the focal ratio much longer. Do bear in mind that it will also reduce the resolution detail) you see through the scope quite dramatically as resolution scales with aperture.

[Moonlit]

Yesterday was actually clear and I used the scope for a few hours and I can't honestly say I was annoyed by the CA even once. It might have contributed to some glare when trying to focus on Saturn. The Moon wasn't up and that's where I've been annoyed by CA earlier.

 

So if I could go several hours without annoyance I shouldn't think I need any filters for it but I much appreciate the tips on CA filters! I did, however, realize that I need a UHC filter for viewing nebulas - I found where the Pacman nebula (NGC 281) was and saw the area in great detail (with the 20mm Plössl that came with the scope), but no nebulosity.

Today I received my new 2" Panaview 32mm 70deg. EP and the daytime test was awesome! Such a big picture, I can't wait to train it on NGC 281 again tonight.
I will probably order a 2" UHC filter for that EP - Thought about a O-III filter but I expect it removes too much light for visual observation.

[Nightspore]

With a narrowband UHC you're probably going to need around 4-6mm of exit pupil for visual. So around a 30-35mm eyepiece at least. Although you may get away with a 3mm exit pupil on a good night. I find I can get as narrow as 1.5mm with a broadband UHC. Personally (for visual) I actually prefer broad and narrowband OIII filters. With a 32mm eyepiece a narrowband OIII should work with your scope.

Edited September 19, 2022 by Nightspore

[Moonlit]

31 minutes ago, Nightspore said:

With a narrowband UHC you're probably going to need around 4-6mm of exit pupil for visual. So around a 30-35mm eyepiece at least. Although you may get away with a 3mm exit pupil on a good night. I find I can get as narrow as 1.5mm with a broadband UHC. Personally (for visual) I actually prefer broad and narrowband OIII filters. With a 32mm eyepiece a narrowband OIII should work with your scope.

Hmm. I know what exit pupil is (width of the "light beam" from the EP?), and quick googling suggests that: exit pupil(mm) = focal length of EP(32mm) divided by F-number of scope(8.3), so exit pupil should be 32/8.3 = 3.9mm with my new EP. 

The filter I was thinking of ordering is this: http://skywatcher.com/product/uhc-filter-2/
It allows Hb(486nm) in addition to the 496/501 O-III wavelengths. FWHM should be around ~22nm according to the graph.

That seems to fit your recommendation, I think?

[Nightspore]

4 minutes ago, Moonlit said:

Hmm. I know what exit pupil is (width of the "light beam" from the EP?), and quick googling suggests that: exit pupil(mm) = focal length of EP(32mm) divided by F-number of scope(8.3), so exit pupil should be 32/8.3 = 3.9mm with my new EP. 

The filter I was thinking of ordering is this: http://skywatcher.com/product/uhc-filter-2/
It allows Hb(486nm) in addition to the 496/501 O-III wavelengths. FWHM should be around ~22nm according to the graph.

That seems to fit your recommendation, I think?

 

I think that filter is the same as this filter. If it is narrowband you should be able to use a 3.9mm exit pupil (32mm eyepiece). If conditions allow it. You might find this link useful.