102mm Apo v 120mm achro for 'green light' solar

[Mr Spock]

Today I decided to try out my venerable Helios 120mm achro for solar. It's an excellent scope, just plagued by CA and that dreaded primitive focuser. I figured on solar, using a continuum filter, the CA would vanish and I'd be left with a decent view.

It's not bad, but, the 102mm is sharper and more detailed. A lot of this is due to the better contrast of the 102mm, with dark areas darker and light areas lighter. In the end, optically and mechanically, they just aren't in the same league.

So, the 120 is going back into the corner of the cupboard where it came from. Probably retired for good.

[ONIKKINEN]

Interesting result, but maybe not that surprising given that expensive apochromat lenses are probably cared to better than inexpensive achromat lenses are.

The idea of a cheap achromat for some purposes is alluring but if the lenses are lemons and the focuser is plastic its not worth it. I wonder if there is a model of achromat scope out there with good lenses, but just with CA. Vixen or one of the longer bresser ones maybe? Would be interesting to compare a good achromat and a good apochromat in this way and my gut says it would be near impossible to figure out which was the 300€ scope and which the 1000€ one.

[Louis D]

It could have been spherical aberration and poor glass quality/polish in the 120 killing detail and contrast.  Well figured, high quality achromats of 120mm can be quite expensive just for the objective and cell.

[Stu]

I have a Celestron Omni 120 XLT in the attic; should try a side by side with the Tak FC100DC. The Omni has aspheric figuring to the lenses I believe, which reduces spherical aberration so it should be decent enough. The Tak has given me some amazing views though so would be an interesting comparison.

[ollypenrice]

I have the 150 version branded Bresser. Although it isn't bad, it is, like yours, in retirement. Maybe when our grandchildren are old enough...

Olly

[Gonariu]

In 1999 I bought the Konusuper 120 with which in 2003 I had made some beautiful observations of Mars even if for something I was not convinced (a slight lateral smear observing the planets, especially Venus). A couple of years ago I noticed a chip on the edge of the achromatic doublet of a few centimeters (less than half an inch), so I put a 90 mm black cardboard diaphragm in it. In doing so it has become fantastic, excellent views of the moon at 333X and of Mars in the opposition two years ago at 200X-333X. I have never had nor have I ever tried an apochromat (or ED apochromat), last year I preferred to buy the Nexstar 8 SE, an instrument that ultimately shows more for the same cost. It may be that one day I will be overcome by the temptation of apochromats, perhaps a Takahashi, now it would be wasted as I am little more than a "curious about the sky".

[Gonariu]

22 minutes ago, ollypenrice said:

Ho la versione 150 marchiata Bresser. Anche se non è male, è, come il tuo, in pensione. Forse quando i nostri nipoti saranno abbastanza grandi...

Olly

Is it a 150/1200 achromatic? I have read some nice reviews on the achromats 150/1200.

[ollypenrice]

3 hours ago, Gonariu said:

Is it a 150/1200 achromatic? I have read some nice reviews on the achromats 150/1200.

Yes. It gives a decent star test and, from memory, enjoyable views. I used it in the early days as a second string instruments for guests of our astro-gite but the business evolved more into imaging and we now have a 14 inch Meade for visual so the 150/1200 went into the loft. It's worth peanuts and really wasn't bad.

Olly

[Paz]

When I got a 102mm f7 apo I compared it to to a 120mm f5 achromat and I found that on white light solar they were broadly in the same league as each other when using a green solar continuum filter, I guess the aperture of the 120 compensating a bit for it's disadvantages in other areas and the green filter taking CA out of the equation.

[Gonariu]

For some months I have been looking at the Sun with an achromatic 80/400 and, to the indispensable full aperture solar filter, I add a green W58 filter (which is clearly not enough on its own to avoid burning the retina). With this filter I saw that both there is no trace of chromatism and that the contrast is much better.

[Louis D]

31 minutes ago, Gonariu said:

For some months I have been looking at the Sun with an achromatic 80/400 and, to the indispensable full aperture solar filter, I add a green W58 filter (which is clearly not enough on its own to avoid burning the retina). With this filter I saw that both there is no trace of chromatism and that the contrast is much better.

Try a #56 green.  I preferred the lighter green image of it over the darker #58 for solar viewing.

[Gonariu]

I have it. Soon I go up to the condominium terrace and I will follow your suggestion.

Edited July 11, 2022 by Gonariu

[Stu]

10 hours ago, Paz said:

When I got a 102mm f7 apo I compared it to to a 120mm f5 achromat and I found that on white light solar they were broadly in the same league as each other when using a green solar continuum filter, I guess the aperture of the 120 compensating a bit for it's disadvantages in other areas and the green filter taking CA out of the equation.

What sort of magnification were you using? I think at lowish power that would likely be the case, but up the power and the apo would likely show more fine detail due to lack of spherical abberation.

[Paz]

2 hours ago, Stu said:

What sort of magnification were you using? I think at lowish power that would likely be the case, but up the power and the apo would likely show more fine detail due to lack of spherical abberation.

I would generally go up to a 6mm SLV so 100x with the 120mm, and usually less magnification than that.

[IB20]

Interesting. In comparisons between my 3” Achromat & APO in Continuum filter WL solar, the biggest noticeable differences were ability of the APO to take higher magnifications but also faculae seemed to pop more.

[Stu]

46 minutes ago, Paz said:

I would generally go up to a 6mm SLV so 100x with the 120mm, and usually less magnification than that.

Under good to excellent conditions I take the 4” apo up to x200. At these levels, you really start to see the granulation cells as cells and the fine detail is amazing. The seeing does need to be good though. I always use binoviewers these days as otherwise the floaters get too intrusive.

[Highburymark]

53 minutes ago, Stu said:

Under good to excellent conditions I take the 4” apo up to x200. At these levels, you really start to see the granulation cells as cells and the fine detail is amazing. The seeing does need to be good though. I always use binoviewers these days as otherwise the floaters get too intrusive.

Agree. Maybe it’s my ageing eyes, but I don’t get a great deal out of white light observing under 150x. 200x is a good day, but ideally, and seeing only allows this rarely, I push for 250x and higher. Penumbra and granulation detail only reveals itself to my eyes at these powers, so obviously the optics need to be capable of delivering it. But it requires a lot of patience. And I only get the WL scope out if I already know that seeing is excellent in Ha.

[Nigella Bryant]

With achromatic lenses there will be spherical aberration. I use a Bresser 100mm stopped down to 80mm and that makes things much better. In fact I'd suggest always stopping achromatic lenses down 20mm. I'm contemplating getting a 150mm achromatic purely for solar stopped down to 130mm. Image's from the stopped down Bresser with CaK filter.

 

Edited July 12, 2022 by Nigella Bryant