Refractor or Reflector?

[Ratlet]

I seem to remember @Chris and yourself @vlaiv having some really interesting discussions about filter use to reduce chromatic aberration in fast scopes.

Hmmm.  I might take the ST80 out for its first spin.

[John]

25 minutes ago, Ratlet said:

I seem to remember @Chris and yourself @vlaiv having some really interesting discussions about filter use to reduce chromatic aberration in fast scopes...

 

The drawback with filtering of course is that it removes light that ought to be contributing to the contrast and sharpness of the image.

 

 

Edited June 1, 2023 by John

[Louis D]

16 minutes ago, John said:

The drawback with filtering of course is that it removes light that ought to be contributing to the contrast and sharpness of the image.

Of course, the drawback of not filtering is that it leaves unfocused light flooding across the image reducing contrast and sharpness.

For instance, I was out looking at Venus in my ST80 last night comparing various filters, including a few new ones.  Without violet and far red filtration, it was difficult to discern its phase.  With both in place, it was quite clear what phase it was in.  However, Venus is sort of the acid test for a fast achromat, thus making the filtering challenge that much more fun for me to research.

I have reflectors, a Mak, an ED, and an APO I could have used, but I was interested in finding out how much can be teased out of a fast achromat by properly filtering unfocused wavelengths.  I'm not going to put a wanted ad on CN to try and secure a used Chromacor for the price of a nice APO to fix the chromatic focus of a fast achromat.  What would be the point of that given my scope collection?  Instead, I want to be able to make recommendations to beginners on how to get the most out of their fast achromats since they are so commonly bought by them as starter scopes.

You are right that filtering out certain wavelengths for low contrast objects will make it more difficult, if not impossible, to see certain features.  In those cases, it may take using a series of complementary filters to tease out various features, refocusing for each filter.

[John]

1 hour ago, Louis D said:

Of course, the drawback of not filtering is that it leaves unfocused light flooding across the image reducing contrast and sharpness.

For instance, I was out looking at Venus in my ST80 last night comparing various filters, including a few new ones.  Without violet and far red filtration, it was difficult to discern its phase.  With both in place, it was quite clear what phase it was in.  However, Venus is sort of the acid test for a fast achromat, thus making the filtering challenge that much more fun for me to research.

I have reflectors, a Mak, an ED, and an APO I could have used, but I was interested in finding out how much can be teased out of a fast achromat by properly filtering unfocused wavelengths.  I'm not going to put a wanted ad on CN to try and secure a used Chromacor for the price of a nice APO to fix the chromatic focus of a fast achromat.  What would be the point of that given my scope collection?  Instead, I want to be able to make recommendations to beginners on how to get the most out of their fast achromats since they are so commonly bought by them as starter scopes.

You are right that filtering out certain wavelengths for low contrast objects will make it more difficult, if not impossible, to see certain features.  In those cases, it may take using a series of complementary filters to tease out various features, refocusing for each filter.

or use a newtonian ?

you can get several of those for the price of a chromacor 🙂

[neil phillips]

12 minutes ago, John said:

or use a newtonian ?

you can get several of those for the price of a chromacor 🙂

Haven't followed the entire discussion. But the softening effect on the image caused by unfocussed light, and or under and over correction of cheaper achromat lenses. Can not be over stated. I've seen first hand the varying degree of this problem on lunar images with different mass produced achromats. It varied quite a bit. Some worse than others. But all suffered to some degree. The sharpest achromat i had recently was a F12.8 70 mm skywatcher and it was indeed very sharp.

  For me personally if i ever did get another achro. It would certainly be the longest FL i could get. Preferably F15.

These softening effects that manifest with cheap achro lenses. Show in lunar images as a rather milky misty type of appearance similar to what one might expect when a lens is dewed over. Compared to sharp Newtonian optics. It can be off putting.

Having said all this the Bresser 127l i was using, was a fun scope to use. Yes it did have a slight softening to its images. But wasn't too obtrusive. It was still very enjoyable to use. With many of the benefits of a large quality refractor at a fraction of the price. For those who can not afford large premium refractors. You certainly do get to have a little taste of what its like to own a large refractor. As a fun scope they are really rather good i found.

False color. When imaging in mono, on lunar and solar wasn't a problem. And it did well for the money i paid. 

But really ultimately for lunar and planetary guys a 6" F8  Newtonian of reasonable optical quality will be a somewhat sharper proposition. With the best color correction possible.

Better than any achromat no matter its quality.

As a side note my 8" Classical Cassegrain is such a sharp performer.  It wouldn't surprise me if it matched or likely out performed a 5" premium refractor of many thousands of pounds. If anyone wants to test that theory send me a 5" premium refractor and i will pit it against my (7.3) CC lunar imaging showdown.  Now of course i realize there's much more to a 5" premium refractor than just lunar imaging. I am only suggesting For certain purposes. Under  certain conditions, Much cheaper telescopes can sometimes outperform much more expensive ones. But almost always will be mirror based of lager aperture, especially with good optics. As another example my 12" Newtonian can outperform my CC Even though i rate its optical quality less highly. You cant get away from the laws of physics unfortunately. All of this is of course seeing dependent too. 

 I am probably going a bit off topic now. So back to the virtues of achromats and filtering to reduce CA and minimize damaging effects of out of focus light on imaging or viewing. I liked my Baarder 495 it did seem to sharpen up the image. And without too much of a transmission loss. But louis is the one whos doing the testing so will know much more than me about any of that

Edited June 1, 2023 by neil phillips

[Louis D]

19 minutes ago, neil phillips said:

 I am probably going a bit off topic now. So back to the virtues of achromats and filtering to reduce CA and minimize damaging effects of out of focus light on imaging or viewing. I liked my Baarder 495 it did seem to sharpen up the image. And without too much of a transmission loss. But your the one whos doing the testing so will know much more than me about any of that

I've found that cutting all violet and blue as with a true Wratten #8 or K2 filter (same as a Baader 495) gets you 95% of the way to the sharpest image possible in fast achromats on all but Venus.  Venus really needs a deep red cut filter like a cyan filter to get all the way there because the violet/blue cut filter only gets you about 80% of the way on Venus.  The unfocused deep red is really strong on Venus once you filter out the violet/blue.

I don't know why, but the other planets, the moon, the sun, and Sirius really don't improve all that much with an additional red cut filter like Venus does.  Dimmer objects often don't seem to improve at all with violet/blue cut filtering.  If anything, they sometimes get too dim to observe.

Basically, try various filtering options on each object with an achromat to see which yields the best view to your eye.  It may not be the same as when imaging the object.

[Louis D]

37 minutes ago, John said:

or use a newtonian ?

you can get several of those for the price of a chromacor 🙂

I actually bought both a used 6" f/6 KUO achromat and a used 6" f/5 GSO Newtonian and have compared them side by side on the same objects on the same night on my DSV-2B mount.  The Newtonian with a GSO CC walked all over the refractor with a TSFLAT2 except for the spider diffraction spikes on all objects I observed.  The lack of CA and SA really tipped the scales in the Newt's favor.  It also weighs about half that of the refractor, making mounting much more stable.  Crazily enough, the Newt cost 1/3 the price of the achromat!  I would struggle to recommend the fast refractor over the fast Newtonian aperture for aperture.

I'll have to find a tight double sometime to see if the refractor's lack of a central obstruction yields a better split or if the CA and SA negates that benefit.

[neil phillips]

33 minutes ago, Louis D said:

I've found that cutting all violet and blue as with a true Wratten #8 or K2 filter (same as a Baader 495) gets you 95% of the way to the sharpest image possible in fast achromats on all but Venus.  Venus really needs a deep red cut filter like a cyan filter to get all the way there because the violet/blue cut filter only gets you about 80% of the way on Venus.  The unfocused deep red is really strong on Venus once you filter out the violet/blue.

I don't know why, but the other planets, the moon, the sun, and Sirius really don't improve all that much with an additional red cut filter like Venus does.  Dimmer objects often don't seem to improve at all with violet/blue cut filtering.  If anything, they sometimes get too dim to observe.

Basically, try various filtering options on each object with an achromat to see which yields the best view to your eye.  It may not be the same as when imaging the object.

I only got the 495 after what Neil English was finding using one. Seems like you agree. I was surprised how much the elimination of out of focus blue light using it. Tightened up the image. But from what ive seen seen with numerous achromats over the years, under and over correction of mass produced Chinese lenses, can be quite damaging to there optical performance, If i ever get another achromat again (i sold 4 a while back)  would likely be a f15 scopetech As i may enjoy trying that..  But EDs are coming down in price. So for not a lot more. might be a better proposition. But i come from a time where long white refractors. Just look so darn cool. Sucker for old school maybe ?

Edited June 1, 2023 by neil phillips

[neil phillips]

38 minutes ago, Louis D said:

I actually bought both a used 6" f/6 KUO achromat and a used 6" f/5 GSO Newtonian and have compared them side by side on the same objects on the same night on my DSV-2B mount.  The Newtonian with a GSO CC walked all over the refractor with a TSFLAT2 except for the spider diffraction spikes on all objects I observed.  The lack of CA and SA really tipped the scales in the Newt's favor.  It also weighs about half that of the refractor, making mounting much more stable.  Crazily enough, the Newt cost 1/3 the price of the achromat!  I would struggle to recommend the fast refractor over the fast Newtonian aperture for aperture.

I'll have to find a tight double sometime to see if the refractor's lack of a central obstruction yields a better split or if the CA and SA negates that benefit.

All good points. I was wondering about double stars halfway through reading. But to up the anti throw in a 6 F8 and for double stars i will bet it will do well ( hypothetical of course ) As you didn't mention owning one  Price still much cheaper

Edited June 1, 2023 by neil phillips

[John]

1 hour ago, neil phillips said:

..... from what ive seen seen with numerous achromats over the years, under and over correction of mass produced Chinese lenses, can be quite damaging to there optical performance....

That is pretty much my experience as well. If the CA is commensurate with the aperture and focal ratio then I'm OK with it but SA just undermines the performance to an extent that is frustrating.

 

 

[neil phillips]

10 hours ago, John said:

That is pretty much my experience as well. If the CA is commensurate with the aperture and focal ratio then I'm OK with it but SA just undermines the performance to an extent that is frustrating.

 

 

Not so easy to figure good ones. As we would like. Agreed

[Louis D]

3 hours ago, neil phillips said:

Not so easy to figure good ones. As we would like. Agreed

And yet the Far Eastern optical shops have been cranking out consistently well figure parabolic mirrors for some time now.  Why has this level of optical prowess eluded their refractive optics figuring lines of business?

[neil phillips]

19 minutes ago, Louis D said:

And yet the Far Eastern optical shops have been cranking out consistently well figure parabolic mirrors for some time now.  Why has this level of optical prowess eluded their refractive optics figuring lines of business?

I think the situation appears to be somewhat variable. But generally tolerances could be better with many of the lenses I've had my hands on. But to answer your question. Could it be attention to detail has been somewhat better with the mainstay of sales, namely Newtonians. With achro refractors being relegated to a sufficient  level of performance type attitude. That many customers will just accept. It also creates a good distinction between that and the somewhat better ED lines. It is strange. when you see the reputation Mak cassegrains seem to garner Though. With a much more consistent reputation amongst armatures. Could it be there is little incentive or presumed need to improve the achro lines ?

Edited June 2, 2023 by neil phillips

[John]

32 minutes ago, Louis D said:

And yet the Far Eastern optical shops have been cranking out consistently well figure parabolic mirrors for some time now.  Why has this level of optical prowess eluded their refractive optics figuring lines of business?

4 lens surfaces to figure, polish, coat, space and mount VS a single surface ?

The slower chinese achromats I've had have generally been quite decent to be fair but there is quite a bit of variability, eg: I've had two of the 127mm F/9.4's - the Bresser branded one was good but the Meade branded one was very mediocre.

The chinese ED doublets that I've had have been much more consistently good. I have the feeling that they simply take more care when more expensive glass is being used and of course they charge quite a bit more for the unit.

 

Edited June 2, 2023 by John

[neil phillips]

Just now, John said:

4 lens surfaces to figure, polish, coat, space and mount VS a single surface ?

The slower chinese achromats I've had have generally been quite decent to be fair but there is quite a bit of variability, eg: I've had two of the 127mm F/9.4's - the Bresser branded one was good but the Meade branded one was very mediocre.

 

 

Strange that i found the same distinction with the bresser 127l and a 80mm Meade i had. But 4 surfaces is another good point

[Gonariu]

A few days ago I saw Venus with the acromatic 80/400 (the Konus Vista) putting X174 and the Contrast Booster filter, the planet was contrasted but there was a few of blue halo, other time seeing Venus at X100 the CA is less. Instead looking at Moon at X174 and the Contrast filter it was very nice. I would like very much to buy a long acromatic as 90/1300 (Vixen Polaris 90 L or Kenko) or, above all, a 108/1600!!! 

[RT65CB-SWL]

On 01/06/2023 at 19:17, vlaiv said:

There is something seriously wrong with that scope

I know the images are pretty much c-r-a-p, but £20.00GBP secondhand for a tube that is mainly ‘thin’ aluminium with a plastic R&P, I am not going to let it get me down or beaten. Trying to focus with one hand and smartphone on other hand ain’t easy.

Edited June 2, 2023 by RT65CB-SWL