Barry,
As a wide field junkie I think a bit over 2º is enough to take in. At a dark site the 150mm f8 achro's are fantastic at wide field , even though its only 2ishº.
So I use a 70mm f7 achro which gives 2.5º and thats pretty good enough for just about everything....Had a 102 f5 and the FC was pretty bad so I think get a 120mm f5, have a bit less TFOV but with less aberrations.
The view isn't good if it is full of seagulls half way out!
My 2 cents
Matt
Thanks Matthew, good advice. That, along with Kevin's good visual experience with the SW120 (and astounding results with some *very* patient post-processing astrophotography), has largely tipped me over to that choice. But I'll 'cool off' for a while and not 'pull the trigger' for a few days.
I must admit, getting 2.7 degree fields on the 120mm with my Panoptic does seem like plenty, and the 2.2 degree TFOV with my Nagler T5 16mm will also be nice. It's probably sufficient, as you note, little reason to push for 3 degrees.
Just one thing, you said:
Quote:
Had a 102 f5 and the FC was pretty bad
Could you expand on this a little? Is this field curvature problem a natural product of trying to get a smaller primary lens down to an F5, or an issue specific to the SW102?
Last edited by Amaranthus; 15-02-2014 at 10:18 PM.
Have to wonder how different f6 is from the a bresser 102/600 on Amazon UK...shipping works out about the same as VAT, so works out under 200 of the old notes.
Just had a look at a few objects. Sorry but my visual view is limited at this stage to just a few objects in the southern sky. Now there is also a full moon which may impact observations of CA greatly.
First off I looked at Miaplacidus in Carina. It's magnitude 1.65. Some violet fringing was visible around the star but not bright enough to bother me. With a Baader Semi-Apo filter I was unable to see any blue fringe. I would say anything fainter than mag 2 or 3 and CA is either not visible or reduced to a low level.
Next was Eta Carina. I couldn't see any CA of anything in the field. But the moon is so strong I could barely see any nebula either.
Acrux at the bottom or right at the moment of Crux. Acrux is magnitude 1.25, CA as a faint violet fringe around the star was noticeable but again didn't bother me. I had trouble splitting the main pair but the atmosphere is terrible tonight. Crux is low at the moment so low alt may play some role in that. I also had trouble splitting the pair in the 68mm F 8.8 scope. The Baader filter reduced the CA but I could still see a little if I looked hard.
Mimosa next. Also at Mag 1.25 and much the same story as Acrux. Some violet fringe but it didn't bother me.
Just down a bit was the jewel box cluster. No CA at all visible in the field. Looked quite pretty actually.
Gacrux on the left at magnitude 1.55 showed no CA that I could see though it was a strong red / orange colour but I think that's normal.
Had a look at the same things through the 68mm F8.8 scope. Same focal length etc. The 68mm F8.8 achro is about as perfect as a scope can be. I didn't see and CA or field curvature on anything. With the 120mm F5 scope there is some field curvature present and the edges of the field are distorted. I would say this was more annoying to me than the CA. But the little 68mm F8.8 scope was soooo dark compared the the 120mm! Give me aperture any day! For visual use anyway.
Well that's my visual report. Cloudy now so just got that done in time.
I can dig through my old pics for some raw pics through the 120mm if you want? I won't do anything to reduce the CA.
Thanks a LOT for that report Kevin. My targets are of course more in the Jewel Box, Eta Carina, etc. type category than mag 1 stars, so it sounds like the CA will be quite manageable for me on the visual tasks with the 120mm. And as noted, on the eyepiece end I have top quality glass, so I'm going to be getting the best out of the 'scope from that aspect.
I'd really like to see some example raw pics from through it, cheers!
So I dug up a couple of "Cloudy Nights" owner reviews of the generic Synta 120mm f/5 achromat (which is the basis for the Skywatcher and Saxon versions I was referring to above, and is also marketed under Orion, Astroview, etc.).
I think they cover the main bases pretty darned well. Overall conclusion seems quite favourable, with similar impressions to what Kevin have given for his model.
I know this is not going to be an Apo quality astrograph, but I suspect it will serve me very nicely... Almost crossed that 'decision threshold' now
Okay here they are warts and all! I've stretched some of them a bit so they are visible, but no colour modifications. I also shoot through an Orion field flattener. Basically it's mandatory on all refractors, APO, ED or achro so the field curvature we see visually is just about free of it on photos. All but 2 of these shots (Eta Carina and IC2944 are unfiltered). Maybe M46 had the Baader Sem-Apo on as well. It doesn't do much to CA photographically.
First is Mimosa and the Jewel box single frame. Photographically the CA is a bit too much to handle without a filter.
Corona Australis. This is actually a stack of frames which does not affect the CA. I also found the CA too much to handle unfiltered like this. The stars are a pretty blue / white normally.
M45 a single frame. This was a disaster of CA. All the stars in the field are way too bright to manage the CA.
Comet Lovejoy. The CA is manageable, especially once the stars are trailed. The SW 120 F5 is a great comet scope.
M46 single frame. Lots of CA, some heavy processing got rid of most of it (not shown).
47 Tuc. A field full of faint stars this one took minimal processing to make for a nice shot. Here it is unprocessed other than a bit of a stretch.
Eta Carina. This is through a CLS filter. The CLS greatly reduces CA as well as enhances nebula. There is some amp glow in the top left corner.
IC2944 Running chicken. A single frame. I thought the CA was pretty bad on this one in spite of the Baader Semi-Apo filter. Heavy camera amp glow in the upper left corner.
Nice work, and gives me a great feel for it! I really like 47 Tuc and Eta Carina (and look how much of it you got into frame!). For the brighter objects, can you control the CA (photographically) by taking shorter exposures?
I also noted in one of the reviews that the guy used an aperture stop, down to 80mm f/7.5, as a simple way of reducing CA when looking at bright objects. Have you tried that?
Nice work, and gives me a great feel for it! I really like 47 Tuc. For the brighter objects, can you control the CA (photographically) by taking shorter exposures?
No not really. A good photographic filter combination is a CLS stacked with a Semi-Apo filter and exposure longer. That's a pretty heavy CA killer.
As for stopping down, yes that reduces the CA but may as well use a smaller scope so I don't bother as I already have an ED80.
This is the prawn nebula single frame with a CLS / Semi-Apo combination stacked onto each other. There's not much CA left.
Yes, you have. Basically you have ignored how the aperture influences the "richness" of the field.
Many years ago there was an analysis which showed that a 6" (150mm aperture) f/5 scope is optimal in terms of its ability to pack the maximum number of stars into a given field of view - for a visual observer.
I won't go into the details here - too complicated - but I'll try to find a link for you explaining why this is the case. It is related to:
- the frequency distribution (by magnitude across the sky of stars and nebulae;
- the number of stars that can be seen as a function of aperture - this is not linear BTW;
- the sensitivity of the eye,
- the optical tradeoffs that occur regarding the actual field of view vs magnification and exit pupil.
Barry, where are you going to use to view from? Suburban or dark site?
The two things I find with RFT
1. Wide TFOV mean big exit pupils, great in the dark not so great in the suburban glow
2.My favourite hate aberration Field Curvature….
FC from the 102mm Objective is why I sold my 102 f5. The only eyepiece I used that was any good was a 32mm plossl, just had too big an exit pupil in a suburban setting.
Most of my viewing is in my backyard so I have gone for less wide with a smaller exit pupil of around 3mm. The scope that does that for me is a 70mm f7 achro with a 20mm 70º 1.25" eyepiece. My ES 24 68º vignettes on that scope a bit….not sure if its the 1.25'' diagonal or baffle placement.
The downside... 70mm losses quite a bit on the light gathering front.
I know it's not an f5, but a 150mm f8 achro in the dark is a fantastic RFT with the right 2" eyepiece, be that 68º 40/41 or 100º 20/21 Nice flat fields and more stars than it's possible to take in…for me at least. Never tried an f5 150 though.
It's a tough choice
Matt
The 150 f/5 has similar issues to the 102 f/5: field curvature and not good under light pollution. On the other hand the 150 f/8 is a very long tube that some observers don't find comfortable to use.
Overall I'd say the fast achromats are good wide field scopes when used from a fairly dark site; the aberrated outer field still gives context to the central area being observed.
I will assume with your budget your looking for an Achromatic refractor and you will be handling colour correction via a filter like Baader Fringe Killer or Semi-APO.
I recently went through this exercise IMO they do a poor to fair job as a lunar planetary scope.nd decided to buy a 102mm Bresser Messier f5.9, 600mm fl nice Bresser 26mm EP.
I have used my 11mm Nagler on it viewing Jupiter and its moons and the image is better than my 16" dob, richer with better contrast. It doesn't have the DSO reach of the dob but for planetary work its greatand wide ufield the brighter DSOs are good.
Hi Glenn,
If that is the case there is something horribly wrong with your 16" newtonian. It is either not cooled properly, not collimated properly, or has poor optics, or as I suspect, might be suffering from a combination of these things.
Comparing a 4"/f6 achromat to a well tuned 16" newtonian as a planetary telescope is like comparing a Cessna 172 to an Airbus A380 for use as a passenger aircraft. There may be ergonomic or logistic reasons why you would choose a 4"/f6 achromat over a 16" newtonian as a planetary scope, but quality of the planetary image isn't one of them.
These short tube achromats are designed for low to medium power wide field views of extended objects and they do an excellent job of that. Barry has stated that is the specific purpose he is considering one of these scopes for. IMO they do a poor to fair job as a lunar planetary scope. If they serve double duty as a grab and go scope you can use them for low to medium power lunar / planetary views in combination with a minus violet filter, but the quality of the views will not be as good as a well tuned medium aperture newtonian. These scopes also do a reasonable job of wide field imaging, of course an APO will do better.
Barry,
The larger scope will certainly give nicer rich field views due to the additional stars it will show in a given FOV. The downside is it is larger and heavier.
John I admit I probably did not explain it very well. There is absolutely nothing wrong with my 16" dob. The differences I saw with Jupiter for example may have as much to do with the Contrast Booster filter and atmospherics rather than optics. There was a recent post here on IIS that discussed why a refractor makes a better planetary scope and I may have paraphrased that incorrectly, I will see if I can find it again.
Next week I will have both the big dob and the refractor out at a dark site for a whole week and I will be closely assessing them side by side, and will report back.
My 16" dob beats every refractor I own hands down. But that's going a bit off topic.
Getting back to field curvature, my views through my SW120 F5 were somewhat ruined by this. For photography use I use an Orion field flattener for it which works well. My question is do they make field flatteners for visual use with fast refractors? The Orion flattener has a screw thread where a T-ring screws on, does anyone make adapters to fit an eyepiece to this?
I might MacGyver something up later to see if I can do this, though it will be straight through not with a diagonal.
My computer won't let me cut and paste it but google... Richest Field Telescopes by Mel Bartels
Lots of info there.
Kevin look at all the adaptors at Agena Astro and Teleskop-Express.
There is a field flattener for refractors for visual at TE but it is expensive. No idea if it works or not, sort of getting into the ED APO range of refractors by that stage.
Barry the other refractor that cures all the CA problems and gives 3.1º TFOV with a 40mm 68º eyepiece is the ED 100mm f9 from Saxon/Skywatcher. I have one and truly a nice scope.
Matt
Last edited by MattT; 16-02-2014 at 02:14 PM.
Reason: Correct TFOV
