Advice Regarding Semi-Apo Filters

[pulsar]

I have just recently purchased a Skywatcher Startravel 150mm refractor.

I have been reading about how to combat the chromatic aberrations common to achromatic refractors.

In one article I was reading it states that you can stack the Baader Neodymium Moon and Skyglow fliter with the Fringe killer filter. And thereby acheive the same results as if you purchased the Semi-Apo version also produced by Baader.

I already have the Neodymium filter and was wondering if it would be worth following the above route or just go for the Semi-Apo?

I would appreciate any good advice about this issue or indeed any advice on how to get the best performance from a Startravel 150.

Thanks.

[Capricorn]

A semi apo filter operates by reducing the light at the edges of the lens, so a cheaper method is to stop the lens down from 150 to 120 with a piece of card with a hole in it.

Ultimately an achro produces CA and that is it.

You cannot correct it, all you can do is to hopefully remove light from that part of the lens that produces the most and that is the lens edges. This should reduce it but as a consequence it reduces the amount of light through the lens so your f/5 becomes say f/7 but the aberations are still there such as SA the CA is reduced but not eliminated. So cut a hole in some card it does the same.

[12dstring]

The semi-Apo filter actually works by filtering out the violet fringes around stars caused by achromatic lenses, in the same way the Baader fringe-killer does.

There's so more info here: http://www.firstlightoptics.com/achromat-semi-apo-filters/baader-semi-apo-filter.html

It does mention the semi-Apo has a Neodymium substrate, so it's possible it's pretty similar to the Neodymium+fringe-killer, although the quote on there suggests the semi-Apo has less colour-shift and better CA reduction.

[pulsar]

A semi apo filter operates by reducing the light at the edges of the lens, so a cheaper method is to stop the lens down from 150 to 120 with a piece of card with a hole in it.

Ultimately an achro produces CA and that is it.

You cannot correct it, all you can do is to hopefully remove light from that part of the lens that produces the most and that is the lens edges. This should reduce it but as a consequence it reduces the amount of light through the lens so your f/5 becomes say f/7 but the aberations are still there such as SA the CA is reduced but not eliminated. So cut a hole in some card it does the same.

Thanks for the info there useful to know that and that explains why the objective lens cap centre is removeable too. I jsut had first light in my new refractor and Jupiter was stunning lots of detail there but just want to get rid of some of that blueish haze around the Planets. I also expect some CA on the moon if the clouds that have rolled in go away. We'll see but thanks for that.

[pulsar]

The semi-Apo filter actually works by filtering out the violet fringes around stars caused by achromatic lenses, in the same way the Baader fringe-killer does.

There's so more info here: http://www.firstligh...apo-filter.html

It does mention the semi-Apo has a Neodymium substrate, so it's possible it's pretty similar to the Neodymium+fringe-killer, although the quote on there suggests the semi-Apo has less colour-shift and better CA reduction.

Cheers appreciated I read this as this was where I purchased the scope from. They are really good people to deal with Steve rocks.

Like I explained in the reply to the last posting I found it helpful that I now know what the hole in my frac's lens cap is for.

But I don't think that the Baader semi-apo filter would reduce much of the light just adjust the wavelengths slightly.

I am still tempted by it though as my neodymium one is fantastic when used on my LX90 SCT or even my Skymax 127.

I also will be using this telescope for public observations and would appreciate any more information on these filters or how to get best performance from the telescope.

Thanks

[John]

I think these scopes are designed for, and best used for, low to medium power viewing of deep sky objects, ie: star clusters, nebulae and galaxies.

Even with a filter to reduce the chromatic aberration (CA), they will not excel as high power planetary / lunar observation instruments.

A UHC filter will help increase the contrast of nebulae somewhat - a 2" one used with a low power, wide field eyepiece will show lovely view of the Veil Nebula in Cygnus

In the past such scopes were referred to as "rich field" scopes and they do really well at that.

I have used a William Optics VR-1 filter with a Skywatcher 150mm F/8 refractor and it did reduce the CA across and around the moon and Jupiter by around 50% but it was still pretty visible. An F/5 150mm will produce a lot more CA which even the best filter is going to struggle to reduce to a level where the scope becomes a good high power instrument I feel.

Reducing the aperture of the scope will reduce the visible CA of course but also reduces resolution and contrast. If the aperture in the dust cap is 60mm then you will get the contrast and resolution of a 60mm scope from your 150mm, but much less CA. No "free lunches" I'm afraid

[pulsar]

Thanks for the input there John I know there's no free lunches and that's why I have several telescopes. Different tools for different jobs.

I have just had first light with this OTA and was quite impressed with how much detail I could discern in Jupiters Cloudtops I was treated to multiple banding and the GRS and I think the scope performed very well indeed considering the short focal length of 750mm. And I normally use my SCT with 2000mm FL for planetary observing or my little 5 inch Maksutov Skymax with 1500mm FL. And yet this scope impressed with it's delivered view of Jupiter. Of course open clusters were stunning M45 was resplendant with stars and M35 was very nicely resolved with even a little space shuttle in there. I ws going for M44 when my plans were scuppered by the clouds rolling in and my temperature hit dew point too. Which ended my first light test. but I had real good fun with the telescope.

I have measured the aperture in the lens cap and it's 120mm but I am thinking even if these Semi-Apo filters cut only 50% of the CA and I think ican live with that compromise that's still an improvement and improvements can't be a bad thing. Can they?

[dweller25]

I went through all the options with my achro trying to reduce CA.

Save your money - the filters may reduce CA by 5% at best.

The best filter I found was actually a light yellow filter - much cheaper than the specialist anti CA filters and gave the same amount of CA reduction.

I do not like CA so eventually sold the achro.....

HTH

[ollypenrice]

This is never going to be a good imaging scope, though it would do best in narrowband I guess. If you do decide to stop down you can buy handy compas cutters for the job.

remember to cut the outer circle first. Cutting the inner one first is like sawing through the branch you're sitting on!

Olly

[John]

....I have measured the aperture in the lens cap and it's 120mm but I am thinking even if these Semi-Apo filters cut only 50% of the CA and I think ican live with that compromise that's still an improvement and improvements can't be a bad thing. Can they? .....

The William Optics VR-1 is the most "aggressive" CA reducing filter but does introduce some other traits that I was not too keen on. The Baader Semi-Apo is a bit milder but most seem to agree that it's the best compromise.

If you stop the scope down to a 120mm it will still be an F/6.25 achro so CA will be prominent.

I'm like David, I don't really like seeing CA in the view if I can avoid it so I use ED refractors and Newtonians now. An F/15 refractor would be aesthetically lovely but I don't have the mounts for one.

The best CA reducers I've ever used were Aries Chromacors which work wonders with F/8-ish achromats by actually bringing the CA to focus rather than filtering it out. Expensive and rare though so not an option for most.

[pulsar]

Thanks John.

I kinda knew when I decided to opt for this refractor what I was getting into. I am primarily a visual observer but do have interest in learning more about imaging. But I also appreciate the differences between telescope types. I love my LX90 SCT for planetary and very close study of the lunar surface. My Skymax Maksutov is perfect for lugging to my star parties. But a nice large aperture refractor that still remains portable has always been on my list for the beautiful crisp wide views it provides. And I just did not have the inclination to spend thousands of pounds to do that.

The Startravel I think will fill a niche in my collection quite nicely. And I am still leaning towards the semi-apo filter.

[Patrice Lanteigne]

The best CA reducers I've ever used were Aries Chromacors which work wonders with F/8-ish achromats by actually bringing the CA to focus rather than filtering it out. Expensive and rare though so not an option for most.

Rare is right! In five years I've yet to see one of the neutral ones for sale on the Canadian Astro Buy and Sell. I've seen a couple of the corrected ones on astromart for well over a thousand dollars. Rumour is that Istar might soon be putting a similar product on the market. Would love for ES to make one for their AR152, but I'm not holding my breath.

As for CA in achromats, true that large fast achromats will have CA, how much is too much is very much a matter of personal tastes. some folks don't like to see any while others don't mind it too much. I have a 6" f/6.5 achromat and while true that it does have chromatic aberration (Purple Haze - Hendrix, anyone?) I don't find overly objectionable. I have put a Baader Semi Apo on it and while it does do well enough there is still some CA left. I bought this scope primarily as a wide field and DSO chaser (which it does to excellence) so I now only use the Baader filter on the 8-24 zoom occasionally and when I want to split really close binaries. I mostly use either an ED100 or 6SE for those (I might replace the 6SE with a 180Mak for binaries and globulars), but the AR152 can split closer ones when seeing is good. The moon and the planets are left to the ED100.

Large fast achromats are somewhat specialized instruments and need some getting used to, however the views through then can be spellbinding. I can only imagine what the view would be like through a 6 or 7 inch triplet! (Anybody want an arm and a leg, or a kidney perhaps?)

[pulsar]

I have to admit I took the scope to my local society's meeting in support of BBC Stargazing Live. And I asked my friend Simon who runs the widescreen centre here in London. And he recommended the use of the Semi-Apo filter. We had over 300 people at the meeting including the BAA and over 100 people observed through the telescope. I was really chuffed with the responses that I received about the Startravel that I know I made the right decision for myself in aquiring one.

I noticed that as I "gently" bumped up the magnification that the CA grew less and less. Jupiter was showing an amazing amount of detail, heavy multiple banding with colour between the main belts. The Great Orion Nebula was stunning and star clusters were just beautiful. I found myself comparing the performance to equalling my 5 inch Skymax Maksutov it trully is a fine telescope that has been well made with a very well figured objective lens.

[pulsar]

I have thought it out for a couple of weeks and I have decided to go for the Semi-Apo filter. I have discussed this with a few members of my astronomy society and they recommend them.

So when I have tested it I will review it's performance.

[pulsar]

The 2 inch Semi-Apo filter arrived this morning in spotless condition. Thanks FLO now all we need is some clear skies for a bit and I can test it out. And then review it's performance.

[rwilkey]

I think you have made the right decision, I have heard from a friend at our local club that the semi-apo is the best one.

[jabberwocky]

I have the baader semi apo in my ST102, makes a very minimal improvement, not worth the money it cost. Short fast achros are bad for ca, may work better with a slower scope with less ca

[pulsar]

Sorry for not posting on this thread in a while but I have been very busy as of late. But I have to admit that the Semi Apo filter does indeed reduce the CA by quite a significant amount. And also it works perfectly as a Mars, Jupiter and Saturn filter. I feel it really helps to tease out some of the finer detail that I was struggling to resolve before applying the filter. CA seems to be reduced by at least 15-20% and when you apply the magnification the filter almost disappears in use.

I would recommend aquiring one of these filters if you get hold of a Startravel 150mm. I also use this filter in my LX90 SCT and again when used on planetary bodies makes a great amount of difference. For example Jupiter is just jaw dropping I can clearly see much more detail in the cloud belts and it really does bring out the Great Red Spot a treat. Baader make no false claims when they state that if you have never seen the GRS before with this filter you will. I eagerly await this appariton of Mars so I can relish in what this amazing piece of optical improvent will show me now. It also makes for a real nice Lunar filter it just takes the edge off of the glare.