Which Filter for 4" f/5 Achromatic Refractor?

[Chris]

Hi all, I've ordered this short achromat as a grab n go, and I'm looking for filter advice please. 

https://www.firstlightoptics.com/sky-watcher-starquest/sky-watcher-starquest-102r-f49-achromatic-refractor-telescope.html

Alas I can't afford the dream 4" Apo and an obsy build, so I was wondering which is the best filter I can use to try and undo some of the chromatic aberration without making the view appear too 'funky' and unnatural? 

Looking at the good old FLO site there's a number of Baader filters which all seem to be variations on a minus violet filter.

Which one would people recommend?

https://www.firstlightoptics.com/achromat-semi-apo-filters.html

Or any other?

Thanks  

[Graham Darke]

I’ve owned the 102 and 150mm versions of these scopes and never found the CA to be a problem for visual use only, provided the magnification was kept below 50x which is easy to accomplish given the short focal lengths. I found those filters to only give a marginal effect, if noticeable at all on bright targets, which is where you see the CA the most. If you want to observe bright objects at higher magnification then one trick I found was to put the cap back on and remove the small small cap from the middle ( the one you use for stopping the aperture down for solar). It reduces the aperture of course but the CA disappears. 

[Chris]

25 minutes ago, Graham Darke said:

I’ve owned the 102 and 150mm versions of these scopes and never found the CA to be a problem for visual use only, provided the magnification was kept below 50x which is easy to accomplish given the short focal lengths. I found those filters to only give a marginal effect, if noticeable at all on bright targets, which is where you see the CA the most. If you want to observe bright objects at higher magnification then one trick I found was to put the cap back on and remove the small small cap from the middle ( the one you use for stopping the aperture down for solar). It reduces the aperture of course but the CA disappears. 

Cool thanks, I've owned a number of achro's but never a ST102 so I was trying to gauge the CA. In summary then, an aperture mask is probably a better bet than any of the minus violet filters, Cheers

I guess I could stop it down to 80mm f/6.25 thus still keeping some image brightness.

Edited May 10, 2020 by Lockie

[John]

I had a Celestron branded ST102 F/5 recently for a while. I was surprised that the CA was not too bad at all at low to medium power. Stopping down helps of course if you don't mind the loss of light grasp and resolution.

When I had a 150mm F/8 achro I tried a William Optics MV1 filter and that seemed to reduce the CA around the moons limb by about 50%.

Interestingly, the CA that a 150mm F/8 generates should be around the same as a 102mm F/5 according to optical theory. A lot of folks live with the 150mm F/8's without filtering.

 

[Graham Darke]

Yeah, the really strong point of these short achromats is their low power wide field performance, where I’ve never found CA objectionable. 

 

[ScouseSpaceCadet]

As a Startravel 120 owner, I was lucky to pick up a new unopened 2" Baader Semi Apo filter for half price last year and it's stayed in the diagonal ever since. There's still some minimal fringing on bright objects at higher mags, but it's barely noticeable.

[Alkaid]

I bought all three Baader filters 2nd hand recently, to see which would benefit both my F5 ST102 and F10 Evostar 90. I needed a lockdown writing project!

I’m halfway through writing a review complete with terrestrial and lunar shots, should be ready soon...

[Bobby1970]

I have owned the ST102 and 120. I had a baader semi apo filter in the diagonal and found it did improve things somewhat. It won't turn your scope into a "semi apo" but I thought it was a worthwhile addition for a relatively cheap scope. 

[Alkaid]

I’ll be entirely honest, the Semi Apo was the one I would have just gone for, but my own finding whilst doing the review in progress  in comparing all three Baader filters is that it was actually the worst. More on this soon (although all this is my own humble opinion!)

[Chris]

4 hours ago, John said:

I had a Celestron branded ST102 F/5 recently for a while. I was surprised that the CA was not too bad at all at low to medium power. Stopping down helps of course if you don't mind the loss of light grasp and resolution.

When I had a 150mm F/8 achro I tried a William Optics MV1 filter and that seemed to reduce the CA around the moons limb by about 50%.

Interestingly, the CA that a 150mm F/8 generates should be around the same as a 102mm F/5 according to optical theory. A lot of folks live with the 150mm F/8's without filtering.

 

Thanks John, it's interesting that the 102 f5 should show similar levels of CA as the 150 f8 as the later is a scope I've always been curious about and intended to try at some point. I've used the ST120 before and thought the CA was a little too much without intervention so I'm looking forward to playing with the 102.

[Chris]

3 hours ago, ScouseSpaceCadet said:

As a Startravel 120 owner, I was lucky to pick up a new unopened 2" Baader Semi Apo filter for half price last year and it's stayed in the diagonal ever since. There's still some minimal fringing on bright objects at higher mags, but it's barely noticeable.

 

2 hours ago, Bobby1970 said:

I have owned the ST102 and 120. I had a baader semi apo filter in the diagonal and found it did improve things somewhat. It won't turn your scope into a "semi apo" but I thought it was a worthwhile addition for a relatively cheap scope. 

 

1 hour ago, Alkaid said:

I’ll be entirely honest, the Semi Apo was the one I would have just gone for, but my own finding whilst doing the review in progress  in comparing all three Baader filters is that it was actually the worst. More on this soon (although all this is my own humble opinion!)

Ah very interesting, maybe this is more of a subjective rather than a quantitative thing then. Maybe I should try and do my own shoot out then! I'll add this test to my list of youtube video ideas. 

Thanks

 

[jetstream]

5 hours ago, Lockie said:

Which one would people recommend?

what are you going to look at primarily? the article said the contrast booster removes most all CA but reduces transmission and adds a yellow cast. It would be good for planets IMHO but my natural no experience guess would be to use the semi APO

[Chris]

26 minutes ago, jetstream said:

what are you going to look at primarily?

A bit of everything which is why I'm after something which will improve the CA without making the view too un natural. It's not the ideal scope for a do it all scope I know, but needed something I can use as part of the obsy setup once built and big scope installed. The ST102 can be piggybacked as a super finder guide scope.  

[ScouseSpaceCadet]

There's definitely a degree of subjectivity & difference in perception whatever the numbers say. It will be interesting to read the latest comparisons

The past reviews I've read suggested the semi apo filter was the softest, with more remaining CA than the contrast booster, but less light loss and no colour casting. Without having tried the others, that rings true. It would be interesting to see how the semi apo fairs on a longer focal length achro, for instance a 90mm f10 or 102mm f9.8.

 

 

Edited May 10, 2020 by ScouseSpaceCadet

[Alkaid]

I promise to complete my little write up soonish, but will now probably need to await the next moon, probably from 1st quarter onwards so that I can take comparison pics through each filter. I have already found some interesting things with both the F5 and F10 fracs (they each prefer a different filter) but don’t want to spill all until I’ve got the images for you. Hope you guys will take a read when ready and don’t mind waiting....

[Chris]

6 minutes ago, Alkaid said:

I promise to complete my little write up soonish, but will now probably need to await the next moon, probably from 1st quarter onwards so that I can take comparison pics through each filter. I have already found some interesting things with both the F5 and F10 fracs (they each prefer a different filter) but don’t want to spill all until I’ve got the images for you. Hope you guys will take a read when ready and don’t mind waiting....

I would be interested in reading your thoughts. No rush 

[John]

2 hours ago, Lockie said:

Thanks John, it's interesting that the 102 f5 should show similar levels of CA as the 150 f8 as the later is a scope I've always been curious about and intended to try at some point. I've used the ST120 before and thought the CA was a little too much without intervention so I'm looking forward to playing with the 102.

I find this table useful. From my experience with achromats I would say that it is borne out in the real world:

[Chris]

A very useful chart John. I remember I used to have this on my drive somewhere. Thanks for the reminder

[Alkaid]

3 hours ago, Lockie said:

 

 

Ah very interesting, maybe this is more of a subjective rather than a quantitative thing then. Maybe I should try and do my own shoot out then! I'll add this test to my list of youtube video ideas. 

Thanks

 

Excellent! It was the lack of YouTube comparisons that led me down this path too. I won’t do a vid myself but will watch yours with great interest should you get around to the project.  I’ll put my own SGL review up soon.  I am comparing the Semi Apo, Fringe Killer and Contrast Booster.

Edited May 10, 2020 by Alkaid

[John]

When I've owned achromats I've not been that bothered by the CA that they produce - its one of the characteristics that you would expect from the design.

I've been more bothered by the levels of SA (spherical aberration) that are often possessed by these scopes. IMHO that constrains the performance more than CA levels and can't be filtered out.

 

 

[mikeDnight]

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.

Edited May 10, 2020 by mikeDnight

[Chris]

55 minutes ago, Alkaid said:

Excellent! It was the lack of YouTube comparisons that led me down this path too. I won’t do a vid myself but will watch yours with great interest should you get around to the project.  I’ll put my own SGL review up soon.  I am comparing the Semi Apo, Fringe Killer and Contrast Booster.

I hope so, it would be a shame to have my hands on a short achro and not give it a go. I'll look out for these filters second hand and start experimenting with them as and when I find them. Might be a little while as I've just started working on two video series- An up cycle obsy build series, and a £200 to £2k  telescope series, starting with my choice of astrographs. 

My channel is here if you want to keep an eye out?

https://www.youtube.com/channel/UCBTXZYuFWQ6lx51L4GeY0Lw?view_as=subscriber

I'll keep my eyes peeled for your review! but no pressure  

 

[Chris]

49 minutes ago, John said:

When I've owned achromats I've not been that bothered by the CA that they produce - its one of the characteristics that you would expect from the design.

I've been more bothered by the levels of SA (spherical aberration) that are often possessed by these scopes. IMHO that constrains the performance more than CA levels and can't be filtered out.

 

 

You're absolutely right John, I once had a Synta 102 f10 with bad SA and you just couldn't get a sharp image. This is a worry at this price point and focal ratio, I just hope I'm lucky enough to get a good one. If not there probably isn't much I can do to correct it. CA is something I can control to some extent hence the thread, but I should should probably check that the SA is reasonably controlled first. If I remember rightly you just check for any difference between the fresnel rings in and outside focus, but is there any thing specific to look out for?

[Chris]

47 minutes 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 specialist being wide star fields, ideal as a comet seeker.

True, but if it's your only setup for a while wouldn't a filter or stopping down help with those higher mags? I probably should have got a 130p with my sensible hat on, but I really really miss refractors. It must be two years since I've owned one and it's my favourite telescope design!

Edited May 10, 2020 by Lockie

[John]

16 minutes ago, Lockie said:

... If I remember rightly you just check for any difference between the fresnel rings in and outside focus, but is there any thing specific to look out for?

You need good seeing for these to be clear:

In broader terms, scopes that have a reasonable degree of under or over correction seem not to be able to support higher magnifications.