Refractor choice?

[Chris]

7 hours ago, Dave1 said:

I've been seriously looking at ED's for the first time ever. I am curios to know how a 120Ed F7.5 fares against an F12-F15 127mm achro? Are they really that much better? I choose 127mm because that as close as I could get to a 120mm in old long focus refractors.

I've owned plenty of achromats and ED's, and just one triplet. ED's and Apo's control CA remarkably well, I'd say the ED80 f/7.5 shows a pinch less CA than the circle K 76mm f/16, well they both show close to none, but the f/16 won points for it's flatter field and depth of focus. ED's tend to have duel speed focusers to help with this. 

On the other hand the f/16 was wobbly to mount and you can mount an ED80 on almost anything. I think this is where ED's and triplets really win, they are just more practical than a high end long achromat. Long achromats are only more practical if on a budget in which case I think the 4" and above Bresser's are the ones to go for in that scenario.

For me nicely finished classic looking long achromats are things of the heart, and ED's are things of the head. 

[Dave1]

Thanks Lockie for the response.

Well guys I have started the process for doing a deal for a secondhand telescope. If the deal goes through I will do a reveal! 

Dave

[Chris]

We like surprises, Dave, good luck with the deal  

[Ben the Ignorant]

It's sad to say because of their value and immense achievements over two centuries, but achromats are never entirely focused.

[nightfisher]

3 hours ago, Ben the Ignorant said:

It's sad to say because of their value and immense achievements over two centuries, but achromats are never entirely focused.

Ben thats a bit of a broad statement to make, i guess you refer to the RGB colour spectrum, but i have used plenty of achromats and seen plenty of images that suggest focus is pretty darn good 

[Ben the Ignorant]

Yes, and I do own a small achromat that I like (I gave it an AZ-4 mount, so it can't complain it's disrespected), but a swath of the spectrum is always a bit out of focus, and after much agonizing about the price, the weight, the length, and the performance, I decided my future refractors will be apo's. Will have to pay at least three times the price but focus is focus, can't deny that.

I wish we were in the year 2080 so all refractors are apo's and affordable, nothing to debate and lament about. :-)

[Stu]

To be fair, judging all achros based upon the performance of an entry level fast scope is a tad harsh. A well figured long focal length achromatic can give excellent results with minimal secondary spectrum. They do of course have other challenges such as mounting them securely but still, many people do enjoy using them.

[Ben the Ignorant]

I'm not judging all achros based on one. Achros I've used and/or owned list as: cheap japanese 50mm f/8, cheap chinese 60mm f/13, russian 70mm f/7, Vixen 4-inch f/9 or f/10, Sky-Watcher 150mm f/8, Sky-Watcher 4-inch f/9 or f/10, William Optics 80mm f/6, vintage observatory-grade 150mm f/8, Celestron 102mm f/6.5, and the very best of all: Unitron 4-inch f/15, which gave me the best view of Mars ever, topped only by a (rare) Vixen 130mm doublet apo. Maybe I forgot a couple.

My little 80mm is not that fast, by the way, it's not the usual f/5 but the better f/7.5, and it was added radial screws in the cell in order to center the lenses optimally. It gives impeccable Airy disks when adjusted, but compared to my friends' 80mm f/7.5 apo doublets, the superior resolution was already apparent at low power. My own 80mm triplet does not even compare, no false color or optical defects can be spotted at 240x.

I'm not sure which apo's I will own, but TS' new 4-inch FPL-53 f/7 doublet and their 130mm f/7 FPL-53 triplet get a lot of ogling from me. A small Lacerta 72mm f/6 FPL-53 would tempt me too, if it was not that expensive for the diameter. But I'm financially sorry to admit only apo's will do for me now. What I would save on a long-focus achro would be spent on a larger and taller mount, and be lost to lack of convenience: storage space, eyepiece swing, weight, sensitivity to wind, etc.

I'm not trying to be opiniated or to stir up discussion, I am lamenting that I will have to spend big on smallish diameters in order to have efficiency: intense images from as compact a tube-and-mount combo as I can get. Having to spend more is not a reason for bragging in my case, I avoid that as much as I can (it's painful, :-) ouch!), but having all the light focused at the same spot is necessary. 

If eyepieces had the chromatic aberration (at the center) that achromatic objectives have, I would not be happy with that. Realizing this made me give up the compromise of achros, except for travel instruments and public viewing; my prized scopes stay with me, and they stay at home. Except for my friends at the observatory, of course. I would love to still be able to accept the compromise, but I compared too many telescopes, and I know too much about optical performance. Pray for me cause I'm gonna suffer financially. :-)

[F15Rules]

20 hours ago, Ben the Ignorant said:

It's sad to say because of their value and immense achievements over two centuries, but achromats are never entirely focused.

You have clearly never looked through a good quality long focus refractor!

Dave

[Chris]

22 minutes ago, F15Rules said:

You have clearly never looked through a good quality long focus refractor!

Dave

He does mention in his last post once owning a 4" f15 Unitron, Dave.

[Stu]

11 hours ago, Ben the Ignorant said:

I'm not judging all achros based on one. Achros I've used and/or owned list as: cheap japanese 50mm f/8, cheap chinese 60mm f/13, russian 70mm f/7, Vixen 4-inch f/9 or f/10, Sky-Watcher 150mm f/8, Sky-Watcher 4-inch f/9 or f/10, William Optics 80mm f/6, vintage observatory-grade 150mm f/8, Celestron 102mm f/6.5, and the very best of all: Unitron 4-inch f/15, which gave me the best view of Mars ever, topped only by a (rare) Vixen 130mm doublet apo. Maybe I forgot a couple.

My little 80mm is not that fast, by the way, it's not the usual f/5 but the better f/7.5, and it was added radial screws in the cell in order to center the lenses optimally. It gives impeccable Airy disks when adjusted, but compared to my friends' 80mm f/7.5 apo doublets, the superior resolution was already apparent at low power. My own 80mm triplet does not even compare, no false color or optical defects can be spotted at 240x.

I'm not sure which apo's I will own, but TS' new 4-inch FPL-53 f/7 doublet and their 130mm f/7 FPL-53 triplet get a lot of ogling from me. A small Lacerta 72mm f/6 FPL-53 would tempt me too, if it was not that expensive for the diameter. But I'm financially sorry to admit only apo's will do for me now. What I would save on a long-focus achro would be spent on a larger and taller mount, and be lost to lack of convenience: storage space, eyepiece swing, weight, sensitivity to wind, etc.

I'm not trying to be opiniated or to stir up discussion, I am lamenting that I will have to spend big on smallish diameters in order to have efficiency: intense images from as compact a tube-and-mount combo as I can get. Having to spend more is not a reason for bragging in my case, I avoid that as much as I can (it's painful, :-) ouch!), but having all the light focused at the same spot is necessary. 

If eyepieces had the chromatic aberration (at the center) that achromatic objectives have, I would not be happy with that. Realizing this made me give up the compromise of achros, except for travel instruments and public viewing; my prized scopes stay with me, and they stay at home. Except for my friends at the observatory, of course. I would love to still be able to accept the compromise, but I compared too many telescopes, and I know too much about optical performance. Pray for me cause I'm gonna suffer financially. :-)

Fair enough, I assumed you were talking about an ST80.

Whilst I have had some very nice views through long focal length refractors, I've reached similar conclusions to you, hence the FC100. It just covers so many bases that it probably replaces the need for two or three other scopes. No point in having a 102mm f11 for instance, or a smaller travel scope because it is airline portable. It is also lightweight enough for small altaz mounts.

I had a Carton 60mm f16.7 for a while. Lovely little scope, but I struggled with the small exit pupil at high powers. The scope could take it but my eyes couldn't. I'm sure a 4 or 5" f15, of 6" f12 would be lovely, if permanently mounted!

[F15Rules]

1 hour ago, Lockie said:

He does mention in his last post once owning a 4" f15 Unitron, Dave.

Hi Chris

Fair comment, and I'd actually not seen Ben's long response following his earlier statement that "achromats are never entirely focused", when I first replied- something which I know is not true, having used long fracs for years. He did later clarify this to say that he was referring to all wavelengths, which makes more sense, whereas I (and I suspect other readers) read it as meaning a long achromat is never truly sharply in focus, which definitely isn't the case.

I think we should remember that the OP specifically asked for advice about long achromats, and, while they definitely do present challenges, especially regarding a suitable mount, they can (at F13 and slower) present truly wonderful images with negligible chromatic aberration.

Interestingly, Ben said himself the the Unitron F15 gave him his best ever views of Mars: having owned one of these, it was "ok" optically, not outstanding, whereas my D&G 5"F15 was a truly outstanding optic- but MUCH harder to mount solidly!

I have nothing at all against apo's, using two high end doublets myself, for ease of use, but they are definitely harder to focus at high powers without a dual speed focuser as compared to a long achromat. I think all scope designs have compromises somewhere along the line, but all also have their strengths too??..

Dave

[Chris]

5 minutes ago, F15Rules said:

I think we should remember that the OP specifically asked for advice about long achromats, and, while they definitely do present challenges, especially regarding a suitable mount, they can (at F13 and slower) present truly wonderful images with negligible chromatic aberration.

Interestingly, Ben said himself the the Unitron F15 gave him his best ever views of Mars: having owned one of these, it was "ok" optically, not outstanding, whereas my D&G 5"F15 was a truly outstanding optic- but MUCH harder to mount solidly!

I have nothing at all against apo's, using two high end doublets myself, for ease of use, but they are definitely harder to focus at high powers without a dual speed focuser as compared to a long achromat. I think all scope designs have compromises somewhere along the line, but all also have their strengths too??..

Dave

Hey Dave, you don't need to tell me mate, I love the flat easy focus views of longs achro's, their length and thus mounting just isn't practical for me with my micro obsy/shed thing  

I think we got onto ED's and Apo's because the OP said this:

On 8/12/2017 at 21:49, Lockie said:

I've been seriously looking at ED's for the first time ever. I am curios to know how a 120Ed F7.5 fares against an F12-F15 127mm achro? Are they really that much better? I choose 127mm because that as close as I could get to a 120mm in old long focus refractors.

 

[Dave1]

Well I'm still relatively new to astronomy, but isn't the other advantage of long focal refractors, that they can take higher magnifications than they theoretically should be able for a given diameter. At least that is the gist I am getting from reading reviews and some of the comments alluded to by experts, including Neil English. Plus when we combine focal length, focal speed, and OTA diameter together, would you rather have a long focal length slow refractor that only just shows the planet and brings the focus of the observers eye only to that object, no other distractions. Or a short focal length refractor of the same diameter that can only reach the same magnifications but with much more in the field of view. I'd rather have a slow achro, simply because I would rather focus all the light and only have the planet I want to view in the reticule.

I see the light coming from a planet more of a laser beam in a slow achro, rather than a cone from a fast to medium speed refractor of either achro or opolar. I may not explained well what I mean. More focused light from the object I specifically want to view, therefore producing a better image of what I specifically want to view.

Dave

 

[Chris]

Hi Dave, you can get to high mag plus narrow FOV views with shorter scopes, you just use a shorter focal length eyepiece e.g. a 5mm instead of a 10mm, or a Barlow lens with the 10mm.

Maybe have a play around with 12Dstring below, and see what focal length plus eyepiece combos show you in terms of framing targets  

http://www.12dstring.me.uk/fovcalc.php

Interestingly, Neil English has stopped favoring Refractors now and has starting speaking very highly of Newtonian's and Dobs. I've got to admit my 6" f/8 is lovely on planets and stellar objects due to it's long focus and small central obstruction...but I digress.

 

[Ben the Ignorant]

2 hours ago, Lockie said:

He does mention in his last post once owning a 4" f15 Unitron, Dave.

Used it but didn't own it, the 1.5m tube required two persons to be piggybacked on our club 10-inch motorized newtonian. Not that the Unitron was that unwieldy, but the other (now decommissioned) scope was not very convenient.

[Stu]

54 minutes ago, Dave1 said:

Well I'm still relatively new to astronomy, but isn't the other advantage of long focal refractors, that they can take higher magnifications than they theoretically should be able for a given diameter. At least that is the gist I am getting from reading reviews and some of the comments alluded to by experts, including Neil English. Plus when we combine focal length, focal speed, and OTA diameter together, would you rather have a long focal length slow refractor that only just shows the planet and brings the focus of the observers eye only to that object, no other distractions. Or a short focal length refractor of the same diameter that can only reach the same magnifications but with much more in the field of view. I'd rather have a slow achro, simply because I would rather focus all the light and only have the planet I want to view in the reticule.

I see the light coming from a planet more of a laser beam in a slow achro, rather than a cone from a fast to medium speed refractor of either achro or opolar. I may not explained well what I mean. More focused light from the object I specifically want to view, therefore producing a better image of what I specifically want to view.

Dave

 

Dave, that is not necessarily the case. The field of view is determined by a number of factors, and when I'm viewing planets in my apo, it is just the planets that I'm viewing, you don't end up with a wider view you choose an eyepiece which suits the purpose.

For instance:

A 100mm f15 scope has a focal length of 1500mm

To reach x150 mag, you would use a 10mm eyepiece. Let's say that this has a 70 degree afov, the actual field would be 0.47 degrees with an exit pupil of 0.67mm

A 100mm f7.5 scope (to keep the maths simple) has a focal length of 750mm

To reach the same mag, you just need a 5mm, 70 degree afov eyepiece. That gives you x150, the same 0.47 degree field of view and 0.67mm exit pupil. No difference at all.

I do always use dual speed focusers on my scopes as I feel this is essential for reaching the best focu. It is especially useful for white light solar, but also very important for planetary observing, in fact any high power viewing.

 

[Ben the Ignorant]

2 hours ago, F15Rules said:

You have clearly never looked through a good quality long focus refractor!

Dave

Yes, but only a slice of the spectrum is focused while the rest is only very  close to being focused. You can actually focus an achromat on its green or blue or red spot, and sense a small difference in the focuser's knob position. I'm not bashing achromats, I'm just saying I've capitulated to the reality of the painfully expensive apo's doing a more complete job of focusing light in a tighter spot, which is what contrast and resolving power are about.

[Dave1]

44 minutes ago, Lockie said:

Hi Dave, you can get to high mag plus narrow FOV views with shorter scopes, you just use a shorter focal length eyepiece e.g. a 5mm instead of a 10mm, or a Barlow lens with the 10mm.

Maybe have a play around with 12Dstring below, and see what focal length plus eyepiece combos show you in terms of framing targets  

http://www.12dstring.me.uk/fovcalc.php

Interestingly, Neil English has stopped favoring Refractors now and has starting speaking very highly of Newtonian's and Dobs. I've got to admit my 6" f/8 is lovely on planets and stellar objects due to it's long focus and small central obstruction...but I digress.

 

Hi Lockie, yes good point but I still think a longer focus tube refractor and longer focal length eye piece make for better viewing. I'd like to see a theory and comparison between short OTA tube, short eyepiece Vs long OTA tube and long eyepiece. To see which puts up a better image. Ideally the glass should really be ED in both OTA, so who wants to make a 4" F15 refractor with ED glass ! Both the short and long tube at 4" diameter.

The longer eyepiece will of course make for more comfortable viewing. 

In an 60mm telescope with an F ratio of F7 you'd need an eyepiece of 2.52mm to equal 167X. With a 60mm F16.7 you need 6mm eyepiece to equal 167x. 

I still think what I mention about the light entering into the tube OTA is relevant. A long tube makes for a narrower light cone, and short tube a wider light cone as it leaves the OTA. I still think this will have an effect, no matter what eyepieces and glass is used in the OTA.

Dave

[Chris]

My 6mm Vixen SLV is both excellent optically and very comfortable, and they do SLV's right down to 2.5mm. I think the best compromise combo of long focal length and ED glass might be the ED100 at f/9, they are very very good. I've always thought the ED100 would be a nice scope to get 'Moonraker'd'   

[Stu]

21 minutes ago, Dave1 said:

I still think what I mention about the light entering into the tube OTA is relevant. A long tube makes for a narrower light cone, and short tube a wider light cone as it leaves the OTA. I still think this will have an effect, no matter what eyepieces and glass is used in the OTA.

I've posted a comparison about what eyepieces you use to get the same results in long and short focal length scopes. The net result is that so long as you have the correct eyepiece available there will be no difference in the field of view at the same mag.

To your quote above, it would make some difference to the edge performance of eyepieces (depending upon how good the eyepiece is) as a narrow light cone is easier for the eyepiece to handle. At high power I do not think there would be any particular effect other than the benefit of increased depth of focus for the longer focal ratio scope.

[Ricochet]

3 minutes ago, Dave1 said:

The longer eyepiece will of course make for more comfortable viewing. 

If you are using something like Plossl or other scaled design then yes, but with modern eyepiece design this is not going to be an issue. If you had an f5 achro plus 3x powermate next to an f15 achro of the same aperture then a single eyepiece would provide the same magnification with the same level of comfort in both scopes. Where the long local length refractor would win would be in terms of depth of focus, CA control and field curvature. However, if you are comparing a long achro to a shorter apo then the long achro wins on depth of focus and field curvature, but loses on CA control. However, a lot of apos have field flatteners available which would remove field curvature as an issue.

[Grotemobile]

I just thought you might want to see my Vixen 102mm F10 scope. About 30 years old . I

use a Apogee 5.5mm EP in it , which seems to give the best views. Still with the original

mount. Can be used manual or powered thanks to the clutches. Just changed the finder

scope to a 8x50  which  a lot better.

[Ben the Ignorant]

I've just finished assembling a modded 130mm f/7 newtonian. It still needs a few tweaks but it's usable; I tried it on the Moon in the night between Sunday and Monday, image purity was very pleasing despite rather strong turbulence. Went no higher than 225x but it's obvious it can take 260x, and maybe 300x.

This reminded me of a comment by a trusted italian reviewer: in order to avoid being the snake who bites its own tail with questions about chromatic aberration, and other constraints of achromats, it's better to stay clear of them, and either save for a true apo, or a pure reflector. Worth mentioning that even Schmidt-Cassegrain's are semi-apo's because of their giant lens (plate), same for Maks in a lesser way.

So the solution, or rather the change in compromises, is to get a newtonian with only two optical surfaces. My Schmidt-Cass has:

1) outside face of plate

2) Inside face of plate

3) first mirror

4) second mirror

5) diagonal mirror

That's five optical surfaces to bring aberrations and light loss. Refractors are twice less sensitive to surface defects in optics, thus a doublet (four surfaces) is on par with a newtonian in that regard. The compromise with apo's is they're necessarily not huge in diameter, and they cost a lot, but they don't compromise where it matters the most: the image.

I wish scope makers charged less for ED glass, I suspect their marketing dept inflates the price somewhat because they how much people crave for image purity. That would alleviate the dilemma between achro's and apo's.

[Dave1]

Owned a Newtonian, was not all that impressed. 150mm diameter F8, very susceptible to seeing conditions. I will take a doublet refractor over one every time.