Quadruplet vs. Doublet (1 Viewer)

Musoman · Mar 4, 2011

Was just looking around the Astro-Tech site and found 2 reasonably priced scopes in shorter FL , like Fernandos' TS 804. We know Fernandos TS has a bit of curvature at the edges, as stated by him because of the short focal length, but apparently the 65ED is almost free of curvature according to reviews, perhaps because a flattener is already mounted inside ?

The 72ED also gets great reviews but probably suffers more from curvature than the 65ED

Astro-Tech AT65EDQ 65mm f/6.5 ED quadruplet spec

Length: 13"
Weight: 4.75 lbs.
Focal Length: 420mm
Focal Ratio: f/6.5
Aperture: 2.6"
MSRP: $549.00

Fully multicoated four-lens/two-group dual ED element refractor optics

Amazing flat field astrophotographic capabilities

Dual-speed 2” rack-and-pinion focuser with 10:1 ratio fine focusing

Non-marring 1.25” and 2” compression ring accessory holders

Rotating camera angle adjuster

Split mounting rings with Vixen-style dovetail

Sliding lens shade with lock knob

It uses an FPL-53 element in its 65mm f/6.5 apochromatic triplet objective lens, and has a separate field-flattening ED singlet lens at the rear of the optical tube.

-------------------------------------------------------

Astro-Tech 72ED 72mm f/6 ED doublet spec

Length: 12"
Weight: 5 lbs.
Focal Length: 430mm
Focal Ratio: f/6
Aperture: 2.83"
MSRP: $379.00

72mm f/6 air-spaced ED doublet optics

Premium ED (Extra-low Dispersion) glass from Ohara, Japan’s best known specialty optical glass manufacturer

Fully broadband multicoated, with blackened lens edges

Dual-speed rotating 2” Crayford focuser with 10:1 ratio micro- focusing

Non-marring 1.25” and 2” compression ring accessory holders

Self-storing retractable lens shade and 3 internal light baffles for better contrast

Combined dovetail/tripod bracket for use on either a photo tripod or a Vixen-style equatorial or altazimuth mount

Pic 1 and 2 AT 65ED Pic 3 AT 72ED

FernandoBatista · Mar 7, 2011

The 72ED is the roughly the same as my ED70 (420 F/6), so curvature would a be a bigger problem than the 804 and CA is not as good as the ED80 either, it would still be capable of good photos but not what we are used too from the ED80.

The 65 is a nice small scope, free from CA and with a completely flat field (due to the flattener) but IMO too small for this, 70mm aperture is as small as I'd be willing to go.
BTW that scope is also sold by TS:
http://www.teleskop-express.de/shop...Quadruplet-Astrograph---44mm-field-diam-.html

Musoman · Mar 7, 2011

Thanks Fernando

I was thinking that the extra quality of the quadruplet might be good enough to take a 2x TC without much less in IQ, giving 840mm, or at least a 1.5x TC giving 630mm, which is still a very good reach

Paul Corfield · Mar 7, 2011

I reckon you could do alright with that 65mm scope. There's a guy on flickr who uses the small Televue 60mm f6 and gets great results on birds.

Anything listed as an astrograph should be optimized for photography and have a flat field of view.

Paul.

Paul Corfield · Mar 7, 2011

The guy I was thinking of who uses the small Televue TV60 can be see here. http://www.flickr.com/photos/markkilner/sets/72157594344569266/

Paul.

FernandoBatista · Mar 7, 2011

Well, I also think you could do alright with a 65, I have a friend who made great shots out of a 70-300 VR, so with a 420mm scope you can only get an easier job, specially since it would probably takes TC superbly.
But for me I still prefer to keep it above 70mm at least, 80mm being preferable. Specially since it's not exactly a cheap scope too, with more than half a stop smaller aperture and about the same weight as the TL804.
Ok, the field maybe completely flat, but that's not all that important for birds... it is important, but I wouldn't trade that for aperture

Musoman · Mar 7, 2011

Paul Corfield said:
The guy I was thinking of who uses the small Televue TV60 can be see here. http://www.flickr.com/photos/markkilner/sets/72157594344569266/

Paul.

Nothing wrong with those photos - though i guess he would have to be nearer the subjects, compared to most Scopes you guys use ( and me ) unless he uses a TC as well.

Fernando, your TL804 is only 480mm, so are you also saying that this is not your preferred focal length ? Is that 60mm difference bewteen your TL804 and the AT 65 a big deal ? You had good success ( leaving aside curvature ) with mounting a TC on your TL804, so i was thinking the same with the AT 65.

The AT 65 would be a respectable 630mm with 1.5x TC, with quality optics already built in, plus flat field element already built in and matched for this particluar scope.

The aperture is perhaps a bit narrow for me at f6.5 @ 420mm, before even fitting a 1.5x TC. I wonder if this could be the biggest problem

FernandoBatista · Mar 7, 2011

My prefered FL would be something like 1000mm or so... if I didn't have to carry it |

|
Actually I'm thinking for sometime to get a bigger scope. The ED80 was a nice do it all scope, but now I want two, a shorter, lighter, take anywhere scope (the TL804 is doing that perfectly) and a longer beast, something along the lines of a 700mm F/7 or even 800mm F/7.

When I'm mentioning aperture on the 65 is not so much the F ratio but the diameter itself. For the same focal length (using TCs) a 80mm scope is about 2/3 of a stop faster than a 65mm, that's what I don't want to lose.
I could go as little as the 70mm on the ED70 I have but that's my limit. And the ED70 would have been my choice for the smaller scope if it wasn't for the CA and curvature. If that 65 was a 70 with all the other specs I'd be all over it.

As to mount a TC on the 804, curvature is actually much better than the naked scope, it's nearly as good as the ED80 alone.

Musoman · Mar 7, 2011

yes i can see your point about aperture ( diameter ) of 65mm losing out a bit to 80mm. Also the f6.5 at only 420mm before TC is a bit of a worry. With 1.5x TC adding one stop .. about f9.5 or f10 at 630mm would you say ? This is not good

A few posts ago, you mentioned that you might try a better quality Field Flatener on your TL804...did you try this, or did you find that using a TC on your scope helped with curvature so that a new Field Flatener was no longer necessary?

FernandoBatista · Mar 7, 2011

F/9.75 @ 630mm, if we consider the ED80 have nearly the same FL at F7.5, with much less money, superb quality and flat enough field, it doesn't look so good. Ok the ED70 is roughly the same specs but it only costs 270€, and weighted less than 2Kg with the nikon mount.

Eventually I'll try other flattener, but I'm not in a real hurry. First because I don't really notice the curvature that much in the photos with the naked scope, there's probably some where the curvature will hurt the image, but I didn't made one yet. Second I tend to use the 1.4x TC most of the times and with it the curvature is not so noticeable, so it doesn't bother that much any more. That said, it's the first thing I look for when I review the images on the computer so for that reason alone I'll probably be pursuing another flattener option, if nothing else for peace of mind

But it will wait, right now what I really want and need is auto aperture, F/6 makes for a very shallow DOF, in a full frame shot of any small bird F/6 isn't enough to get even a birds head completely focused. It's really shallow. And to have the dandelion chip working perfectly I need the lens fully opened when I'm focusing, so the iris must work automatically.
Also this will fix most images where curvature will be an issue, by increasing the DOF, I can have the complete frame within DOF. By what I've tried that will happen at about F/8.5.

Also, I'd be trying some other options as TCs, most likely negative elements like Paul is using, maybe I can find one that corrects the curvature too, it wouldn't be surprising since that's part of their job in the objectives.

Fowl Mouth · Mar 9, 2011

Pete, there isn't much I can add to what Fernando has already said. 65mm is starting to get a bit small for a general purpose lens. The huge advantage of the 65Q having a flat field is for star points. Really, if you haven't done astrophotography (I've only started) you'll be surprised to see how a lens you thought was flat in daytime use, really isn't. Pinpoint light sources, in this case stars, are the most demanding test for lens correction at edge of field. Stars will make your best lenses weep, showing FC, CA, or coma, often all at the same time. OK, I digress.

I think the extra 7mm of aperture at these smaller sizes might be more important for daytime photography than a crisper EOF. Besides, I think you are using a crop sensor (Pentax 1.5x, right) which makes the edges less pronounced. If I had a budget for the 65Q, and I was only using this for birds, I would actually prefer a scope like the 80/600 ED as sold by Orion Telescopes, et al. That is an amazing scope for the money on axis, and gets you a better FL for birds right out of the box. But since you didn't ask for that advice, you can sweep my unsolicited tip under the carpet. :-O

If you have some time to wait, I can do some test shots with my 72ED on both my 1D (1.3x) and my 20D (1.6x), where your Pentax should settle somewhere in the middle for comparison's sake. I have a high enough resolution "home made" ISO12233 chart. I can give examples of the scope shot virgin, with field flattener, with 2xTC, and with both flattener and 2xTC. Let me know if this interests you Pete, and I'll work on it this week.

Musoman · Mar 9, 2011

Thanks Jason - yes i take onboard what Fernando said.

I have had an 80ED too, and still have my 120 non ED, but i'm going to go for lighter weight / shorter scope sometime in the future.

This seems to mean smaller aperture, obviously. I'm trying to figure out what i can get away with, and still get a decent scope

I would be real interested in your comparisons if you get some time to sort it, i know you are busy from our PM's

PS yes, Pentax K200D 1.5x crop

Fowl Mouth · Mar 14, 2011

OK, Pete. I finally had a couple hours to setup the AT72ED today.

Here is how I ran the resolution test.

I have a jpeg file that I downloaded from the internet (FOUND HERE). I cropped the chart to a 2/3 format, printed it using high quality setting on my Canon inkjet printer across 9 pages, and "puzzle-pieced" together on a flat sheet of cardboard. I mount the chart as built on my inside house wall and mount my camera/lens atop a heavy-duty video tripod with geared column and leveled heavy-duty video head.

I tested the AT72ED in each of the four following setups:
•Mounted to Canon 1D
•Mounted to Canon 1D + Astro-Tech Field Flattener
•Mounted to Canon 20D
•Mounted to Canon 20D + Astro-Tech Field Flattener

In each of the setups I took shots at approx 1/10 mm intervals, from 2mm front focus to 2mm back focus. The sharpest result for center focus and the sharpest result for corner focus were each used for sake of this comparison.

For sake of interest, I added the results of my Sigma 150-500 OS lens at 403mm shot both ƒ6.3 and ƒ8.0 using the 1D (separate test done a couple weeks ago). All samples are best of multiple shots, using mirror lock-up and release cable.

...and the Bigmos

I don't know if this will be of any help to you. If you have any questions don't hesitate to ask and I'll do my best to answer.

Jason

Musoman · Mar 14, 2011

That is brilliant Jason - many thanks for those tests. Those flatteners really do work dont they. There's such a difference with them fitted, and so plainly obvious in your test shots. I wonder if Fernando could find himself a better quality flattener for his TL804. I think he will try one in the near future, maybe not right now though.

Were the 72ED shots taken with a 2xTC as well as the flattener ?

By the way, i also found the 150-500 to be sharpest at f8 when i owned one. As proven by your 2 Sigma shots

Nice work pal :t:

Paul Corfield · Mar 14, 2011

To my eye there's the obvious improvement in the corners but maybe some loss in the center sharpness and a loss of contrast. Could just be the the more contrasty one looks sharper. I'd probably try baffling the front of the flattener to improve the contrast.

Paul.

Musoman · Mar 14, 2011

Baffle everything , even the wife

Paul Corfield · Mar 15, 2011

The trouble with most astro gear is they always paint it shiny black. I know in the dead of night when it's pitch black there's no stray light to bounce off that shiny paint but in the day it's a real pain. They just don't consider us daytime users. :-C

Paul.

Fowl Mouth · Mar 15, 2011

Musoman said:
That is brilliant Jason - many thanks for those tests. Those flatteners really do work dont they. There's such a difference with them fitted, and so plainly obvious in your test shots. I wonder if Fernando could find himself a better quality flattener for his TL804. I think he will try one in the near future, maybe not right now though.

Were the 72ED shots taken with a 2xTC as well as the flattener ?

By the way, i also found the 150-500 to be sharpest at f8 when i owned one. As proven by your 2 Sigma shots

Nice work pal :t:

Hey Pete, you are welcome. I agree that the field flattener makes a marked improvement with the corners regarding curvature.

No, I didn't do any tests with the 2x TC. I ran into an "ERR 01" issue with the 1D trying to use the TC with the scope. It's a known issue with certain lenses and early 1 series bodies, but I had never tried this combo so it was a small hiccup. I plan on doing the test again with the TC, maybe I have to tape the pins on the TC to render it invisible to the 1D. I can at least try it with the 20D. I'll hopefully get a chance to do that soon.

Fowl Mouth · Mar 15, 2011

Paul Corfield said:
The trouble with most astro gear is they always paint it shiny black. I know in the dead of night when it's pitch black there's no stray light to bounce off that shiny paint but in the day it's a real pain. They just don't consider us daytime users. :-C

Paul.

Right on, Paul. You and I have both had a hunch about my previous photos with this combo. The baffling isn't good enough for daytime use, and this particular test has made it painfully obvious. I've still got about a square foot of that flocking just waiting for a non-lazy day... :king:

Musoman · Mar 15, 2011

Look forward to seeing what you can do with a 2xTC on your 20D Jason - thanks for your comparisons

I also need to strip my OT and flock the insides, and the hood and ext tubes. Its something i did on the 80ED when i had it, but have not got round to that on this 120 ST

Quadruplet vs. Doublet (1 Viewer)

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