58 replies to this topic

[ChuckS]

I am about 8 months into astrophotography (with no background in astronomy prior) and have made progress. My current rig is Losmandy GM811G, Redcat 51, and Nikon modded DSLR.  I am learning PixInsight and NINA. Definitely, my skills are improving. I am now starting to think about a longer focal length refractor - probably in the 100mm, f/7 range. I want something that will be a long-term keeper. I have been shooting mainly nebulae, galaxies and star clusters, and these will remain my primary targets for the foreseeable future.

 

I don't really know how to evaluate quality - what to look for. I know Strehl is a metric a lot of people use as a measure of optical quality, and that at some point, the seeing conditions override the resolution the telescope could provide (is a Strehl of 95+ meaningful in most skies in So Cal?). I also assume that, all things being equal, larger objective lens mean better resolution. Then there are things like focusers, reducers, flatteners. 

 

I don't even really know what exactly to ask. Maybe: how does one with limited background assess the quality of a telescope for astrophotography. What is critical, what is just marketing fluff? Someone mentioned to me, 'get the highest quality optics you can afford?' How would I assess that? I assume refractor because it's what I know, but is this where the 'highest quality optics' is found? 

 

Thanks a lot. 

 

 

[Jethro7]

Hello Chuck,

 If you stay within the Premium brands you will do well. FWIW, It is hard to beat a Takahashi TSA 120 F/7.9 triplet APO. Your GM811G  will handle it.

Takahashi's are probably the most accessable of all the premium brands. Others like Astrophyics, APM LZOS and TEC are awesome  scopes but the lead time for delivery is in years or in question, for new.

 

Takahashi TSA 120

https://www.takahash...-telescope.html

 

HAPPY SKIES AND KEEP LOOKING UP Jethro

Edited by Jethro7, 23 June 2022 - 02:03 AM.

[Avgvstvs]

Sometimes highest quality is not where you expected.

Some doublets and triplets are poor, individual lenses do show variations.

My advice is to get it on the optical bench and just get it tested by a professional.

I had to when my AP performed poorly and I was expecting gold.

Edited by Avgvstvs, 23 June 2022 - 02:13 AM.

[db2005]

It's an excellent question and I wish there was an easy answer.

 

One reason is that many key aspects of optical quality are difficult to quantify in a way that is accessible to prospective buyers. Consequently, ad copy is often littered with loaded marketing terms like "diffraction limited", "ED", "SD", "Lanthanum", "FPL-53", "FPL-51", "FPL-55", "Fluorite" and I am already forgetting some.

 

But in reality it takes actual experience, good eyes, good seeing and patience to judge whether optics are actually good. To name some important aspects that contribute to optical quality: spherical correction, accuracy of lens figure and polish, color correction (CA correction), correction of spherochromatism (another frequently overlooked aspect of color correction), control of light scatter, control of internal reflections, quality of coatings, light transmission. A great refractor will render razor-sharp, contrasty views of moon and planets, and render stars as pinpoints with no astigmatism and show text-book perfect round airy discs of stars when magnification is high enough. The problem is, unless you have enough experience at the eyepiece and are mentally able to compare the view with that from other telescopes, how will you "know" what excellent optics really look like? I've owned several scopes over the years, but it wasn't until my 5th scope, the Vixen SD81S, that I finally realized what truly great optics look like. This is another good reason to pursue opportunities to get as many looks through other people's scopes as you possibly can. If at all possible, it's wise to go to star-parties, public observatories and ask astronomy-friends for observing sessions and advice.

 

Since the proliferation of fluoride-glasses (such as FPL-53 and its siblings) in the last couple of decades there has been much attention to color correction of refractors, perhaps because it is easy for observers to understand that less false color is better, and more false color is worse. Yes, that's true, but... it is quite common to see ED refractors with pretty good control of false color and then fall short in other aspects, such as poor spherical correction, poor control of spherochromatism or outright rough or astigmatic optics. In my opinion there has been too much focus on glass types and too little focus on optical quality (quality of lens figure and polish = quality of workmanship), to the extent that it is not unheard of for inexpensive run-of-the mill EDs to be soundly outperformed by a good achromat costing less.

 

The easy answer, if there is one: In my experience, the most reliable predictor of a refractor's performance is its price tag.

 

And slow optics are more likely to be of good quality than fast optics. Slow optics are much easier to make and are more tolerant of small manufacturing defects.

Edited by db2005, 23 June 2022 - 07:42 AM.

[noisejammer]

My take is that a few things matter.

 

1. Is the field flat enough for the sensor you have? If not, is a flattener available? What about a reducer?

 

2. Is the spot size over all colours small enough so you don't get bloat in stars? This is particularly important when using a one-shot colour camera like a DSLR.

 

3. How well is the inside of the scope baffled / blackened?

 

4. Does the image go to mush if you decide to try near infrared? (This would need a different camera but it can be done.)

 

5. Is the focuser stable enough so that you can easily achieve focus without introducing asymmetry to the optical train?

 

6. Can the focuser be motorised so that you can focus the scope without shaking the mount?

 

7. What's the tube made of? If this is not thermally stable, it may cause focus shifts as the night temperatures fall.

 

8. What do accessories like rings and a finder cost? Can you save a lot of moolah using off-brand equivalents?

 

9. Does it come with a transport case?

 

10. Can you afford it?

 

As has been commented, triplet offerings by Tak, LZOS, A-P and a few others tick most of these boxes. You should expect to wait several months to several years ... premium scopes are built in small batches and makers very seldom build more than they have immediate orders for.

 

Within limits, I submit that a high Strehl ratio (say 0.95 or higher) is most important when the scope is to be used for both imaging and observing. When imaging, you can get away with a scope that isn't even diffraction limited.

[james7ca]

I agree with Jethro7. I'd start by looking at one of the Takahashi refractors. Most of the other premium brands are almost impossible to get unless you'd be willing to go to the used market (where it can still be difficult but at least possible in a time frame of a month or two -- depending on your luck and what you are looking for).

 

Stellarvue seems to be an up and coming manufacturer and they apparently are now making their own optics right here in the U.S. (a definite rarity). However, while they do have scopes for immediate delivery I've yet to see many results from their new designs. I have one of their older 80mm APOs and it's a fairly nice scope with a good Feather Touch focuser but the optics were imported from an overseas third-party (most likely China, meaning it was likely based upon a fairly generic set of optics).

 

That said, all manufacturers make some "lemons" so be ready to test and evaluate so that you can either send it back or get warranty service.

[Jon Isaacs]

Within limits, I submit that a high Strehl ratio (say 0.95 or higher) is most important when the scope is to be used for both imaging and observing. When imaging, you can get away with a scope that isn't even diffraction limited.

 

 

One has to distinguish between imaging and visual.  As Bruce suggests, for imaging, there are other factors that are more important than near perfect on-axis optics. That is what Strehl measures, on-axis.  Nothing about field curvature or even color correction since interferometers are use lasers that measure only at a single color.

 

Photography is not pushing the theoretical limits of the optics.  Viewing Jupiter at 280x centered in an eyepiece requires high quality but photography is very different.  

 

Jon

[james7ca]

...Photography is not pushing the theoretical limits of the optics.  Viewing Jupiter at 280x centered in an eyepiece requires high quality but photography is very different.  

 

Jon

Jon, I'm not sure I agree with that. IMO when working near critical sampling you are going to see more potential problems in the image quality (i.e. with photography) than you will ever see with your naked eye. Cameras are just more sensitive to color and can record details much fainter than can be seen with the eye. In fact, this is one reason why I believe that there are two classes of APOs. One I call a visual APO and the other is a photographic APO. The photographic APO needs much better color correction and that's why some ED scopes appear to be color free visually but not so when used for photography.

 

The fact that you can record more detail on the planets in a processed image than you can see with the naked eye just shows how much performance can be lost when viewing a planet with an eyepiece (at any magnification). Plus, when using a telescope visually you are testing both the scope and the observer's eyesight and most people don't have perfect 20/20 vision. In the latter case what is to be judged, the scope or the person's eyesight?

 

That said, you can certainly take bad pictures with a good telescope and a careful observer with good eyesight will see more than a typical novice. Plus, there is a certain grandeur in seeing an object with your own eyes (at the eyepiece) that is probably lost when simply doing imaging. But, I think the detail and clarity that you can get in a good image is just as inspiring as a good view through a telescope (and easier to achieve with a camera given the light pollution that most of us have to deal with).

 

I don't think there is any question about this fact, photography is just more demanding in terms of optical performance than is visual work with a telescope.

Edited by james7ca, 23 June 2022 - 06:11 AM.

[Jon Isaacs]

Jon, I'm not sure I agree with that. IMO when working near critical sampling you are going to see more potential problems in the image quality (i.e. with photography) than you will ever see with your naked eye. Cameras are just more sensitive to color and can record details much fainter than can be seen with the eye. In fact, this is one reason why I believe that there are two classes of APOs. One I call a visual APO and the other is a photographic APO. The photographic APO needs much better color correction and that's why some ED scopes appear to be color free visually but not so when used for photography.

 

 

I am discussing Strehl ratio as a measure of optical quality.  One does not buy a 4 inch refractor for planetary imaging.  Long exposure tracking, wide field.. In your images, are you resolving or even trying to resolve 1.3 arc-second double stars across the field?  Visually, only the very center of the field matters, high magnifications of that very center, a high Strehl matters.  

 

I get it that astro-photography requires very good color correction, the camera captures a much broader spectrum than the human eye.  But as Bruce said, diffraction limited optics are not required because astrophotography with a small refractor is about the entire image and not just the center where the optical quality is the highest.

 

Daniel Mounsey has pointed this out repeatedly.. 

 

Planetary imaging relies heavily on processing and very short exposures, it's not about great optics that have excellent contrast and resolution for it's aperture, a C-14... not a 4 inch apo.

 

Jon

[bobhen]

Imaging Refractors...

 

1. Price is a very good indicator of quality
2. Triplets are considered better for imaging, as they can be made both more color free and faster than doublets. However, not all triplets are of superb quality. See point number one
3. High quality triplets use excellent glass with special properties that reduces chromatic aberration 
4. High quality triplets for imaging have focusers that are large and robust and smooth
5. Refractor makers that have been around for a couple of decades is also a good indicator of quality
6. Strehl: There are many ways to measure Sthrel so “be careful” if using “only that” as a guide to quality. Do you know what the differences are in how Strehl is measured? There are polychromatic (all colors) and single wavelength (individual color) measurements. As the telescope maker R F Royce says, “generally” to be considered really high quality, a Strehl above .96 is preferred.
7. Flattners are desired to sharpen the images at the corners. If just starting out, one can of course crop the images or wait to purchase a flattener later.
8. Any scope design can be made to really high quality. See point number one.
9. For imaging, both image scale (for small objects like galaxies) and wide fields (for large objects like sprawling nebulas) are desirable but one usually cannot get both in one telescope. Refractors are “usually” for wide fields and compound scope with their longer focal length and larger apertures are “usually” for small objects that need image scale. Even a refractor of 100mm will show galaxy detail just not nearly as detailed as say a 10" SCT.  

 

Here is a list of really superb triplet apochromatic (color free) refractor makers that have been producing extremely high quality refractors with very high Strehl ratios, and for decades…

1. Astro-Physics
2. TEC: Telescope Engineering Company
3. CFF (Europe)
4. Takahashi: only the TSA and TOA models. Takahashi gets their lenses from Canon
5. APM: only the scopes with LZOS Russian lenses
6. Tele Vue: only their 101 and 127 Petzval design “4 element” refractors

 

If budget is a consideration, here are some other makers to consider…

 

1. Stellarvu: although they are producing some nice refractors, they have not been doing that nearly as long as the above.
2. TS: Telescope Service: German retailer selling mostly Chinese sourced refractors. They have an extremely wide and varied selection.
3. William Optics: Chinese sourced refractors

 

You should take some time to do a little book-learning and research, but any scope from the top 6 makers listed would be a really superb choice and a lifetime telescope. If your budget is tighter, then the other makers will also make you very happy.

 

Moving up to a refractor in the 100mm to 140mm size should make you happy and not overwhelm your mount. Of course, any of the scopes above will also be a superb visual instrument as well.

 

Hope some of this helps.

 

Bob

Edited by bobhen, 23 June 2022 - 06:42 AM.

[james7ca]

I am discussing Strehl ratio as a measure of optical quality.  One does not buy a 4 inch refractor for planetary imaging.  Long exposure tracking, wide field.. In your images, are you resolving or even trying to resolve 1.3 arc-second double stars across the field?  Visually, only the very center of the field matters, high magnifications of that very center, a high Strehl matters.  

 

I get it that astro-photography requires very good color correction, the camera captures a much broader spectrum than the human eye.  But as Bruce said, diffraction limited optics are not required because astrophotography with a small refractor is about the entire image and not just the center where the optical quality is the highest.

 

Daniel Mounsey has pointed this out repeatedly.. 

 

Planetary imaging relies heavily on processing and very short exposures, it's not about great optics that have excellent contrast and resolution for it's aperture, a C-14... not a 4 inch apo.

 

Jon

I don't know about 1.3 arc second double stars but I've clearly resolved/imaged a 0.9 arc second double star using my NP127is with a split that was better than Dawes' limit would suggest. I've also imaged planets and the moon/sun with scopes as small as 90mm (or even smaller on the moon and sun). And just recently I clearly imaged a 1.6 arc second double star with a my C90 (shown __HERE__ ) along with a decent split on that same double using a 72mm refractor. I've also imaged double stars that many reported that they could never see with larger scopes (but there YMMV, since seeing is so critical).

 

As for diffraction limited optics I'd agree that you don't absolutely need that for imaging, but most often the better the optics the better the image. However, you are certainly correct that there aren't many scopes that are diffraction limited over a large field.

 

In fact, from an optical design and cost perspective it's probably unreasonable to expect diffraction limited performance over a large field. There is an old optician's axiom that there is a certain limit to how much total information you can produce or record from an optical system. So, you can either have high resolution over a small field or a wider field with less resolution per unit area (such that the total infomation content is basically the same in both cases).

 

This is perhaps not as true as it once was, given the advances that have been made in the types of optical glasses that are available and the use of computer aided design, but it's still probably not far from being correct.

 

So, for visual you MIGHT be satisfied with a small field that is diffraction limited only in its center (and not so great elsewhere) while for SOME forms of photography you probably want a wider field with decent average performance over that larger area. But, that doesn't mean that you can't record those defects with photography or that you can just ignore (or not see) those defects because you are imaging. Given an adequate image scale (which you may or may not apply) you are definitely going to see those defects in your image if you look for them.

 

In any case, this is getting away for the OP's intent so I don't see more value in continuing along this line.

Edited by james7ca, 23 June 2022 - 09:04 AM.

[james7ca]

Here is a list of really superb triplet apochromatic (color free) refractor makers that have been producing extremely high quality refractors with very high Strehl ratios, and for decades…

1. Astro-Physics
2. TEC: Telescope Engineering Company
3. CFF (Europe)
4. Takahashi: only the TSA and TOA models. Takahashi gets their lenses from Canon
5. APM: only the scopes with LZOS Russian lenses
6. Tele Vue: only their 101 and 127 Petzval design “4 element” refractors

 

I don't think I would put Tele Vue on that list because while they've been in the business for years they just aren't in the same league as those other manufacturers (in terms of the optics, not necessarily for the mechanics). Plus, I don't think Tele Vue is really that serious about imaging. They'd certainly like to be considered for imaging but that's really not their main driver. For example, Takahashi is still actively updating their scopes while Tele Vue's Nagler/Petzval design was basically created back in the film days of imaging. They did do a modest update for their imaging system models (the NP127is and NP101is) but I believe that effort was begun almost ten years ago.

 

That said, an NP127 or NP101 can be a really good performer for visual work, but if you want superior imaging then you may be able to find better or more cost effective solutions. And note that I say this as someone who has had reasonable success imaging with my own NP127is (but maybe my sample of that scope wasn't really as good as they get).

 

To be truthful, however, there are probably lots of scopes that are being produced today that exceed in capabilities what the average user will ever be able to accomplish with them. What I mean is that the tools (the scopes) are probably "better" than what the average workman will make of them and that's probably also true with Tele Vue's NP127is or NP101is (here's looking at you james7ca   ).

[rgsalinger]

I bought an ASKAR 107PHQ for 2500USD. Here's a blown up screen shot of an QHY268 APS-C using Aberration Inspector in Pixinsight. This is a 4" F 7.5 telescope and, as you can see is a superb performer. Unless you really feel that you can't get a nice picture in one night with this scope, I would strongly recommend it over the more expensive brands. With the advent of the high QE back illuminated Sony chips, you really don't need an F 5 telescope to come home with a very nice picture. 

 

 

Here's an example of what I was abble to craft with 8 hours of integration time using that very scope. 

 

Here's an example using the scope with less than 4 hours of integration time. 

 

 

I think that people are still unaware that the Chinese telescope makers have come a long way in terms of quality offerings at lower prices. I'm just hoping that they come up with an equivalent wide field telescope sometime soon. 

 

Of course an AP, Tak, TV and maybe even the latest SV scopes have better mechanics but if you don't want to spend that kind of money, these new ASKAR scopes are just wonderful. 

 

Rgrds-Ross

[briansalomon1]

I am about 8 months into astrophotography (with no background in astronomy prior) and have made progress. My current rig is Losmandy GM811G, Redcat 51, and Nikon modded DSLR.  I am learning PixInsight and NINA. Definitely, my skills are improving. I am now starting to think about a longer focal length refractor - probably in the 100mm, f/7 range. I want something that will be a long-term keeper. I have been shooting mainly nebulae, galaxies and star clusters, and these will remain my primary targets for the foreseeable future.

 

I don't really know how to evaluate quality - what to look for. I know Strehl is a metric a lot of people use as a measure of optical quality, and that at some point, the seeing conditions override the resolution the telescope could provide (is a Strehl of 95+ meaningful in most skies in So Cal?). I also assume that, all things being equal, larger objective lens mean better resolution. Then there are things like focusers, reducers, flatteners. 

 

I don't even really know what exactly to ask. Maybe: how does one with limited background assess the quality of a telescope for astrophotography. What is critical, what is just marketing fluff? Someone mentioned to me, 'get the highest quality optics you can afford?' How would I assess that? I assume refractor because it's what I know, but is this where the 'highest quality optics' is found? 

 

Thanks a lot. 

I've done very little astrophotography, just a few images in H-alpha, a few shots of the Moon, so I don't have much to add to that aspect of your question but I have been using TeleVue refractors for 25 years and have owned and looked through many other scopes over the years.

 

I know that it's very rare, even way out in Death Valley, to get an evening of "perfect" seeing. Processing can help select the best frames but can't improve seeing. So when we buy very high end telescopes we're buying something that we can only really use to it's full extent on certain nights when the seeing is very good, and we typically had to take a long drive to get to a dark site.

 

It's also true that an excellent all-around telescope like NP101 won't be the absolute best at everything. You could end up with many very expensive telescopes that are rarely ever used to their full capabilities.

 

Not knowing astrophotography very well at all, I'm going out on a limb here, but it seems you'd be wanting either wide field views or higher magnifications because there really aren't any telescopes that can deliver world-class performance for both.

 

100mm f7 is not a wide field telescope, but I'll speak to what I know. For larger Nebulae and star clusters I would think NP101is or NP127is with their great optics, focuser and OTA would be telescopes you could keep the rest of your life and be completely satisfied with. Takahashi is making a Petzval model that appears to be trying to compete with TeleVue https://optcorp.com/...g-ota-telescope.

 

Takahashi has been famous for making world class telescopes with a triplet lens assembly, and manufacturers don't always succeed when they try a new design so I would read all the reviews of the Takahashi Petzval before I'd buy one, but I'd be really surprised if they got it wrong.

 

In general, Astro Physics, TeleVue, Takahashi are brands people have trusted for decades. Those telescopes are keepers.

 

[rgsalinger]

Here's a thing about AP versus visual when it comes to scope selection. With AP, the ability of the telecsope to present round stars with a large chip is the exact same thing as having a shorter focal length. The scope I'm recommending produced this result with a full frame chip. This has become even more critical with the small pixel back illuminated cameras. 

 

 

I don't have the QHY600 any more, long story but eventually I'll get another full frame camera to use with this scope. So, it's actually an excellent wide field scope with a  2.75° x 1.83° field of view. 

 

Rgrds-Ross

[KTAZ]

Stellarvue SVX102T. Highest quality APO triplet 102mm F7. Produced right here in the USA.

 

https://www.stellarvue.com/svx102t/

 

Give them a call and talk to them. They offer fantastic personal service both before and after the sale.

[Jon Isaacs]

In any case, this is getting away for the OP's intent so I don't see more value in continuing along this line.

 

 

That was my point. You don't buy a 4 inch refractor to image double stars center in the field. 

 

Jon

[ChuckS]

Guys! Wow! I’m not even half way thru reading this amazing set of responses. This is an great info source of detailed ways to consider this or at least get me pointed in the right direction. So, truly, thanks.

One basis question. Since price seems to be a pretty reliable indicator of quality, for a 100mm refractor, what’s’s the threshold price? I don’t mind paying for quality I can’t yet appreciate, but I also don’t need to impress anyone with a status telescope. So is, say, $3500 sufficient?

[25585]

Hello Chuck,

 If you stay within the Premium brands you will do well. FWIW, It is hard to beat a Takahashi TSA 120 F/7.9 triplet APO. Your GM811G  will handle it.

Takahashi's are probably the most accessable of all the premium brands. Others like Astrophyics, APM LZOS and TEC are awesome  scopes but the lead time for delivery is in years or in question, for new.

 

Takahashi TSA 120

https://www.takahash...-telescope.html

 

HAPPY SKIES AND KEEP LOOKING UP Jethro

LZOS is effectively orphaned now, being Russian & so sanctioned. 

[Jethro7]

LZOS is effectively orphaned now, being Russian & so sanctioned.

Hello 25585
Yep, LZOS products are in question. There are other avenues to move products. Sometimes necessity finds away.

Happy skies and keep looking up Jethro

[Oyaji]

A couple of random observations:

 

1.  Astrobin is your friend.  Whatever scope you are considering buying, if you go to Astrobin you can search on that scope and see some pics taken with that scope.  True, people don't post their dogs (and we all have them) on Astrobin, but to me that's a plus, for these pics will show you what these scopes can do at their very best.  So, I just searched on my refractor TSA120, and the search returned over 1800 pics!  A search on the Stellarvue SVX102T returned over 300 pics to look at.

 

2.  Some of the discussion above is Ford vs. Chevy vs. Cadillac vs. BMW.  People do become attached to their scopes and fiercely loyal to their brands--especially CN Tak owners (the Takitis! thread on CN has been going on since 2014 and has over 7,689 posts as of this morning) .  In my case, you would have to pry my Taks or my Vixen from my cold, dead hands!  And yet, when I look at other scopes on Astrobin costing a lot less, I can't say that the Tak images are demonstrably superior.  But they are consistently good.  

 

3.  One reason I bought my two Taks--or perhaps, a justification for paying the price of a Tak--was that I figured the chance of getting a bad sample was remote.  Also, I figured that whenever I produced a subpar image, I wouldn't be able to blame it on the scope; it would be up to me to figure out what else I did wrong.  Indeed, this has turned out to be true.  But I suppose it would be true of other high quality scopes being recommended here.

 

4.  Another friend is the field of view calculator at  http://astronomy.too.../field_of_view/ .

 

     When deciding on your second, keeper scope, think about what kind of camera you may be attaching to it in the future.  Many if not most imagers eventually trade in their DSLR's for dedicated astrocameras, if for no other reason than convenience and the advantages of cooling.  The camera you choose may affect the focal length of the scope you choose.  The FOV calculator will help you with that process.

 

5.  Lastly, keep in mind that if you make a mistake in the scope you purchase, you can always re-sell it on CN Classified or on Astromart, usually at 70% to 80% of what you paid.  So, don't worry; be happy!

Edited by Oyaji, 23 June 2022 - 11:29 AM.

[ChuckS]

Sometimes highest quality is not where you expected.

Some doublets and triplets are poor, individual lenses do show variations.

My advice is to get it on the optical bench and just get it tested by a professional.

I had to when my AP performed poorly and I was expecting gold.

You do this after the purchase, I assume. If it's not great, then what?

[Oyaji]

You do this after the purchase, I assume. If it's not great, then what?

Finding an optical professional to test a refractor telescope is easier said than done.  I wouldn't even know where to start.  And IMHO it's unnecessary.  When you look at your pics, you either have tight, round stars edge to edge, or you don't.  As the poet said,  "It doesn't take a weatherman to tell which way the wind blows."  

[briansalomon1]

Here's a thing about AP versus visual when it comes to scope selection. With AP, the ability of the telecsope to present round stars with a large chip is the exact same thing as having a shorter focal length.

ASKAR_full_frame.JPG

 

Rgrds-Ross

Oh, I think I get it. A larger chip would be similar to using a longer focal length eyepiece then?
 

[gnowellsct]

I am discussing Strehl ratio as a measure of optical quality.  One does not buy a 4 inch refractor for planetary imaging.  Long exposure tracking, wide field.. In your images, are you resolving or even trying to resolve 1.3 arc-second double stars across the field?  Visually, only the very center of the field matters, high magnifications of that very center, a high Strehl matters.  

 

 

 

Jon

Oh goodness we have seen it done and the results posted right here.  In the refractor forum no option is off the table.  GN