Achromatic or Apo?

[frank32gld]

Hi,

I read in this forum about the problems with chromatic aberration (CA) and using an Apo design will be better to reduce the CA.

Does this mean the same for solar viewing?

I am thinking about a refractor as a first scope that will be used for visual (planets, moon and a bit of dso) and also solar viewing as a next step with the right filters ( prefer h-alpha at this moment) so I can use the same telescope for that.

Is it wise to buy an achromatic scope for that or better go directly for an Apo design like the SW80ed?

I do photography for a number of years now so then CA worries me a bit.

Are the solar filters on the front of a telescope that heavy that I have to get a bit more sturdier mount in the first place?

Thanks for your help!

Frank

Sent from my mobile phone.

[Alan64]

Hello,

The Vixen A80M 80mm f/11 achromat exhibits minimal CA, but as an achromat it requires a longer focal-length, therefore a longer tube, to keep the CA at a minimum...

http://www.vixenoptics.com/Vixen-A80M-Refractor-Telescope-p/2606.htm

An 80mm f/16 achromat exhibits virtually no CA, but the tube is considerably longer.

At the opposite end, an 80mm f/5 fast-achromat, such as the Orion ST80, exhbits quite a bit of CA when viewing brighter objects.

I've no experience in solar-observing with an achromat, however.

The Sky-Watcher Pro 80 ED, at f/7.5, would make for a great all-arounder, for both solar system and brighter deep-sky observations with only slight CA evident...

http://www.optcorp.com/expired/skywatcher-sizzling-summer-savings/sw-s11100-pro-80-ed-apo-refractor-telescope.html

This is an an example of a colour-free apochromat... http://www.optcorp.com/takahashi-fc-76ds-doublet-fluorite-apo-refractor-tfk0761.html

White-light solar filters are not heavy at all, especially this RG-Film filter for my 102mm apochromat...

http://www.thousandoaksoptical.com/solar.html

An H-Alpha system shouldn't weigh much more... http://www.thousandoaksoptical.com/halpha.html

Do you have an alt-azimuth or an equatorial mount?

[frank32gld]

Hello,

The Vixen A80M 80mm f/11 achromat exhibits minimal CA, but as an achromat it requires a longer focal-length, therefore a longer tube, to keep the CA at a minimum...

http://www.vixenoptics.com/Vixen-A80M-Refractor-Telescope-p/2606.htm

An 80mm f/16 achromat exhibits virtually no CA, but the tube is considerably longer.

At the opposite end, an 80mm f/5 fast-achromat, such as the Orion ST80, exhbits quite a bit of CA when viewing brighter objects.

I've no experience in solar-observing with an achromat, however.

The Sky-Watcher Pro 80 ED, at f/7.5, would make for a great all-arounder, for both solar system and brighter deep-sky observations with only slight CA evident...

http://www.optcorp.com/expired/skywatcher-sizzling-summer-savings/sw-s11100-pro-80-ed-apo-refractor-telescope.html

This is an an example of a colour-free apochromat... http://www.optcorp.com/takahashi-fc-76ds-doublet-fluorite-apo-refractor-tfk0761.html

White-light solar filters are not heavy at all, especially this RG-Film filter for my 102mm apochromat...

TO RG solar filter.jpg

http://www.thousandoaksoptical.com/solar.html

An H-Alpha system shouldn't weigh much more... http://www.thousandoaksoptical.com/halpha.html

Do you have an alt-azimuth or an equatorial mount?

Hi Alan,

I dont have a mount yet as i will buy my first telescope, thanks for your great reply and i hope i will get some information about the CA with solar viewing.

Thanks.

[Drop Of Sun]

Hi Frank,

CA should not be a problem with solar h-alpha because you are dealing with a very, very narrow range of wavelengths of light, which should come to focus at the same point. I have not used an achro for solar. I figure I might as well use an ED scope as I also use it as a general purpose scope as well. That's what I really like about doing solar with a regular refractor, you can use the scope for other things too unlike the dedicated h-alpha scopes.

[Dave In Vermont]

I use a ST80 with a front-end filter for solar-observing (visual only at present). This is a 80mm F/5 achromatic refractor. The Sun through my Thousand Oaks film filter presents the Sun as bright orange. Other manufacturer's, such as Baader in Germany, give a white Sun image. I've not encountered anything like CA when viewing the Sun through the F/5 visually. I have no idea what astrophotography would do though, but I doubt CA would be an issue.

Anyone else?

Dave

[Stu]

Hi Frank,

For solar white light observing, CA can be an issue as you are viewing all wavelengths. You can use a filter called a Continuum filter which just passes a narrow band in the green spectrum. This gets around any CA issues and enhances surface features such as granulation.

The options for white light would be either a front mounted filter, best option is Baader Astro Solar film ND5.0 which you can either buy as a ready made filter or just the film to make your own. The other, better option would be a Herschel Wedge, something like a Lunt 1.25" which gives excellent results.

Solar Ha observing is much more expensive! CA is not an issue here because you are filtering on a very narrow band of frequencies in order to show the prominences and other Ha features.

Options here are a front mounted filter with a blocking filter at the rear, or my suggestion would be a Daystar Quark which you can use in a normal refractor. You put it in the diagonal and add an eyepiece and away you go. It gives you Ha capability in scopes of significantly higher aperture for lower cost than a dedicated scope.

As Alan says, something like a Vixen 80M would give very nice performance for planetary observing and white light solar. It would not work with a Quark as the focal length is too long.

The suggestion of a Skywatcher 80ED is I think ideal. They have high quality optics without being overly expensive. Whilst you don't necessarily need an apo for Ha observing, I do think apo optics tend to be figured better than some of the cheaper achro's so you avoid other issues such as Spherical Abberation which reduces image quality. The 80ED would be great for white light solar with a film filter or Herschel Wedge. It would work very well with a Quark and because it has minimum CA and a well figured lens it would give nice planetary views as well.

[Moonshane]

I have used a Vixen 80m, a Skywatcher 120mm achro, a 120mm ed and a 80mm apo all with a lunt wedge and the views on all but the best seeing have been best in the 80mm apo. When seeing is great the 120 ed is far better then anything else I have used.

I do fall into the multi use camp and feel an 80mm ed would with a wedge would provide the best results and an excellent scope to use generally. You could also use it with a quark if you wanted ha views too. not cheap but then as a fellow sgler once said, if you buy quality you only cry once :0)

[Paul73]

The ED80 is a great little scope that shows exceptionally sharp stars and works well for solar white light. It also shows awesome wide field views.

However, it is only little. So in my experience, struggles with the fainter DSO and doesn't have the focal length for serious planetry observing.

In the telescope world. There is really no such scope that does everything really really well......

Question for those more experienced: Would the stretch to the ED100 make the difference for DSO viewing?

Paul

[frank32gld]

So far all of you thanks for your help.

I will going to read more about the different filters and the quark system.

Frank

Sent from my mobile phone.

[rwilkey]

Hi Frank, CA is not an issue in an achro or an apo as so much light (99.9%) is filtered out, so it's just not something that would occur.  I use the Baader film with my 100mm scope and find it works perfectly well.