Fluorite vs Aperture

[LewisM]

Which would win in your opinion - a fluorite 102mm of a VERY high quality brand, or a 127mm ED of a good Chinese brand. The 102 is f/9, the 127 is f/7.5 (and comes with a further FR etc)

My ONLY application is astrophotography, with OCCASIONAL visual.

Just a real bugger deciding right now!

[wavelandscott (Scott)]

Not sure for AstroPhotography but for visual size matters...

I suspect that for AP that the more colourfree the better but since I am not an AP person my advice is not worth much...

[LewisM]

Size is not so important IMHO, but QUALITY of optics is.

I am using an ED80 for widefield astro, and wanted a better deeper longer focal for those pesky DEEP little things

I am seriously swaying FL102, but...

[MattT]

This is what you want.http://www.istar-optical.com/istar_017.htm
Only problem might be the price You'll never need another scope after this one.
Cheers Matt

[bojan]

Originally Posted by MattT

This is what you want.http://www.istar-optical.com/istar_017.htm
Only problem might be the price You'll never need another scope after this one.
Cheers Matt

Oh yes, you will.. a bigger one

[LewisM]

Aperture schmaperture.

Fluorite vs ED... smaller FL vs larger ED... pros/cons....

[gbeal]

Both are important, but I don't think there is sufficient difference between 102 and 127mm, I have had both.
If you want aperture, and for DSO imaging, then look at a reflector, a newt like the 10" f4. I know you've been into reflector-land and come back, but if aperture is what you want, then the newt is the scope.
Short wide-field stuff you have sorted with the 80mm. If you want the decision between the 102 and the 127, AND the 80 is staying as part of the package, go the ED127. Big scope though, nose heavy too.
Gary

[ausastronomer (John Bambury)]

Originally Posted by LewisM

Which would win in your opinion - a fluorite 102mm of a VERY high quality brand, or a 127mm ED of a good Chinese brand. The 102 is f/9, the 127 is f/7.5 (and comes with a further FR etc)

My ONLY application is astrophotography, with OCCASIONAL visual.

Just a real bugger deciding right now!

Hi Lewis,

I know nothing about astrophotography, but I know quite a bit about the Vixen FL102S and I assume that is the 4"/F9 Flourite you are considering.

The Vixen FL102S is a very high quality flourite doublet, notwithstanding it is a 10 + year old optical design. It is optically the equal of scopes like the Takahashi FS102 and at least as good as the Televue 102, which are from the same generation. The Tak FS102 is F8 and the TV102 is F8.6. It's about as good as you will get out of a doublet APO. At F9 its fairly slow by todays APO standards and because it is a touch slower, colour correction is excellent for a doublet.

One big advantage of the Vixen FL102S is that the tube is smaller and lighter than most other 4" refractors, making it easy to handle, transport and mount. One negative of the Vixen FL102S is that the standard Vixen focuser is a bit of a lemon and most owners upgrade these to an aftermarket focuser like a Feathertouch or a Moonlight.

I am guessing you could pick up a 2nd hand Vixen FL102S and install a new focuser for well under $2,000; if the focuser hasn't already been upgraded. The 127ED APO is going to cost you $2k with a Chinese focuser still on it. It will also be a bigger scope phsically. Hard to know how they would stack up optically but with any optics out of China the good ones are pretty good, but there is also a chance of a lemon slipping through the cracks.

With the Vixen "it will just work right". One of my very favourite 4" scopes in fact.

Cheers,
John B

[Poita (Peter)]

If it is the Vixen FL102S then I would go that way.
Otherwise, if the 'very' high quality brand doesn't have any graphs or data from lens tests, then I would let it go.

If you want a high quality refractor for decent money 2nd hand, then look for a Lomo or similar. You get a lens test report with those and at least know what you are getting.

The cheap 127 scopes I have seen have been patchy, some are great performers, some not so much.

Your photography, is is going to be using an OSC or mono camera? Automated or are you happy to refocus each colour manually and use a filter drawer or similar? What are your skies like?

[Kunama]

102 = 81.67cm2 @ F9 a classic scope
127 = 126.6cm2 or 1.55x the light gathering area @ F7.5

For enthusiasts probably the FL102S, but the 127ED would be a good AP scope but needs at least a NEQ6 sized mount.

[gregbradley]

Neither.

You'd be better off saving a bit more and getting a 2nd hand Takhashi FSQ106N or if the budget will stretch a 2nd hand FSQ106ED.
You'd never outgrow those and you would not take a loss when you do the inevitable upgrade with the other 2.

Astrophotography is very demanding of the optics and neither of those you mention will meet high end needs.

F9 is too slow. Doublets are not APO but semi APO and have chromatic aberration fluorite or otherwise. F9 is an attempt to hide that CA. But it may show in images as magenta or blue haloed stars. It would take a lot longer to get an image and F9 is pushing 102mm of aperture too much. The chromatic and spherical aberration will reduce sharpness.

F7.5 is a bit slow but OK. Others can comment but I imagine the focuser will be inadequate for astrophotography. 127mm is good aperture though but you aren't going to get any galaxies except for the few very brightest.

FSQ's are F5 and quadruplets.

The 106N has 2 fluorite lenses and the ED model has ED instead.

106N has better colour rendition than 106ED and a stronger focuser which does not flex. It is the better bang for buck. But it does have slight vignetting on bright stars near the perimeter (they have a dark bar going through like a worm hole!). A minor flaw. No scope is perfect.
106ED colour rendition can be improved by flocking the interior of the scope as it gets a green bias from inside the tube from something (the black paint or the lens coatings?

F5 and 106mm seems a sweet spot for 4 inch refractors in my opinion. Every 2nd top image you see is from FSQ106ED. FSQ106ED though has been plagued at times with focuser flex with heavy cameras. There are several models to try to cope with this. I am not sure they ever have fully. I had one that had virtually no flex. I was lucky it seems from the number of posts about this topic. 106N has a stronger simpler focuser with a proper focus lock that works.

Basically 4 inch scopes work best as widefield imaging machines. 4 inches is not a lot of light gathering power so they work best in this zone. Super high quality lenses in the 4 inch range could be pushed a bit more but you are now pushing things a bit and the job is done better by larger aperture - either a refractor (very expensive) or a mirrored scope of varous designs.

Greg.

[LewisM]

It is a Vixen FL102, and it has had a Feathertouch installed. I have decided to go with it, and arranged all details.

Thanks for the help guys. Will pot some pics through it when I get it. It even has a dew heater permanently installed in the lens shade too - good thing, some nights around here have been positively RANK for dew (though, being at home, I keep a fan blowing over the scope always, and so far NIL dew)

[cometcatcher (Kevin)]

What's wrong with your present SW ED100? I haven't seen you max it out yet.

[Peter.M]

Lewis I didnt want to say anything but I feel compelled to now. You have heard that a lesser scope on a fantastic mount will be better than a fantastic scope on a lesser mount. In the last month I have seen images from 3 scopes from you. If the images you are producing are not up to the quality you want you need to work on it more yourself. Fantastic equipment does not make a great image, it has to do with the persons experience at capture and processing too. I have seen people with tens of thousands of dollars of equipment produce what I would consider bad images, and I have seen people with modest setups produce great images. Greg makes a good point that the tak fsq is a magnificent wide field scope but if you spent the 5-6 grand on one and your images still didnt meet your expectations what would you upgrade too?

What I am trying to say (and please take this constructively) is that you should get the maximum out of the gear you have night after night and then when your sure that you cant get any more out of that setup you should upgrade. If you keep going how you are now you may just end up spending a fortune on gear.

[LewisM]

Good points, so I guess I should state a few things.

I am happy with what I have now in terms of depth, field etc, I just want a bit better rendition of colours, sharpness etc. So,not planning going above 102 or so.

I dislike reflectors, mainly because of the diffraction spikes.Diffraction spikes to me are introducting an artificial "flaw" that is not there in reality. My goal is to reproduce the splendours of the heavens as accurately as is possible, without the ugliness of diffraction spikes.

I tried vaneless sytems, like Maksutovs, and disliked the system. Just never good enough for me. I tried Vixen's VC200L catadioptric, but again, vanes creating diffraction spikes. And I dislike Schmidts for some reason - probably the Maksutov notion.

So, it is refractor or nothing to me. I considered the William Optics smll tube quituplet, but too wide field for the DSO's I want. My budget right ow is $2200, and I don't think a Tak will fall into my lap for that, darn it.

So, for now, I will want to upgrade basically what I already have - from a Skywatcher ED100 - that I am VERY happy with - to a Vixen FL102 - if that is a bad decision, then let me know.

Thank again for the advice all.

[Kunama]

Originally Posted by LewisM

Good points, so I guess I should state a few things.


So, for now, I will want to upgrade basically what I already have - from a Skywatcher ED100 - that I am VERY happy with - to a Vixen FL102 -

Thank again for the advice all.

Hey Lewis,
How much difference in actual image quality do you expect from a change from an ED100 to a FL102, I hope it will be noticeable.
From what I have read and heard the Vixen is a nice scope and hopefully you will be happy with it. (If you're not happy with it, I call dibs )

[gregbradley]

Its hard to comment on the Vixen as it usually gets great responses as a visual scope but not many images posted using one.

I suppose a google search or perhaps cloudy nights forum or somewhere else for images using one.

If you want my opinion I would not expect much difference between 100mm SW ED doublet and Vixen FL102 photographically. It will be subtle and the biggest difference will be the F9 of the Vixen versus the lower F ratio of the SW. So it will be less widefield.

I personally love fluorite lenses as they have a tiny improvement over ED glass (less light scatter - if you shine a green laser through the lens it will only show the mating element not the fluorite lens, but ED glass will scatter a small amount and you will see all lenses).

If you want more sharpness that often is more an issue with tracking. Most refractors are sharp. How accurate is your tracking and autoguiding?

If the errors in the tracking create elongated stars that also means the object you are imaging is also smeared and will lose sharpness.

Its tough advice as scopes are way more exotic than boring mounts but at the end of the day the mount is the more important piece of gear.

Greg.

[LewisM]

Taking all this on board.... and now I am deciding on a Televue 101... quad-apo should be decent I assume

Tracking - pretty much immaculate, even in 20 min subs (no trailing evident). guess I got lucky with a PERFECT NEQ6 and a darned good autoguider (and went against everyone's recommendation for refresh rate LOL). Zenith can be a little tiny "jump" - not trail, but you can see the guider over-compensated. But I think a LOT of mounts have zenith complications.

So, narrowing this dow - TV 101 quadruplet, Vixen FL102 doublet or a Northgroup 127 Triplet.

[Kunama]

If you are thinking of an AP scope to keep for the long term and are happy with the 'reach' of the 4" 545mm , then maybe go for the TV 101.
I guess that would be at the upper limit of your current budget.

(or you could just use your current SW ED till you buy the FSQ 106 that I see in your future, by doing this you will better off in the long run)

[Larryp (Laurie)]

Originally Posted by LewisM

Taking all this on board.... and now I am deciding on a Televue 101... quad-apo should be decent I assume

Tracking - pretty much immaculate, even in 20 min subs (no trailing evident). guess I got lucky with a PERFECT NEQ6 and a darned good autoguider (and went against everyone's recommendation for refresh rate LOL). Zenith can be a little tiny "jump" - not trail, but you can see the guider over-compensated. But I think a LOT of mounts have zenith complications.

So, narrowing this dow - TV 101 quadruplet, Vixen FL102 doublet or a Northgroup 127 Triplet.

North Group make a 102mm triplet as well, and it is way within your budget even after you put a Moonlite focuser on it if you so wish.