120 mm or 102 mm f/5 refractor for rich field visual?

[Amaranthus]

After much internal ruminations, I need some feedback :)

I'm looking to get a low cost (but acceptable quality), portable refractor for primarily rich/wide field visual observations (open clusters, large nebulae, comets, sky scanning etc.).

I'd like it to be something that will slot in between my Celestron Nexstar 8SE (which I use as a workhorse for planetary and smaller DSO visuals and short-exposure astrophotography and imaging) and my 15 x 70 binoculars, which are great for really wide-field stuff and grab'n'go.

I'm looking at the Skywatcher/Saxon alt/az models, either the 120 or 102 f/5 achros, but am open to other suggestions (though I want to keep in under $1000 budget, preferably well under -- if I want to get serious about a refractor in the future then I'll get a proper APO when the time is good and proper :) ).

My indecision is largely over the aperature vs focal length trade-off. For instance, take the a scenario using the following 3 EPs that I own: TV 24 mm Panoptic, 16 mm Nagler and 11 mm Nagler.

For the 120 mm (600 mm FL), I would get the following TFOVs:
24 Pan = 2°35' at 25 x magnification (yields a nice 4.8mm exit pupil)
16 Nag = 2°7' at 38 x (3.2 ep)
11 Nag = 1°25' at 55 x (2.2 ep)

For the 102 mm (510 mm FL), it would be:
24 Pan = 3°2' at 21 x magnification
16 Nag = 2°29' at 32 x
11 Nag = 1°40' at 46 x

So obviously the 102mm gives the wider field (but still well below my binoculars at 4°24', so it 'fits' nicely (my 8SE with the Panoptic and a 0.63 FR gives 1°13'). The 102mm is cheaper (costs $150 less than the 120), and probably is more steady on the AZ3 mount.

However, the light gathering capacity of the 120mm is much better (an alluring 38% more than the 102!), and it can resolve to 58 arcseconds vs 73 for the 102 (I minor consideration here, I know).

All in all, it's a tough trade-off. Have I missed any other factors? What do do?? :eyepop::lol::help:

[cometcatcher]

Comparing the 120mm F5 to the 102mm F5, both will give about the same exit pupil / brightness per eyepiece. The CA will be a little worse on the 120mm (see this post by Don http://www.iceinspace.com.au/forum/showpost.php?p=1056242&postcount=16 ) The field of view will be more narrow on the 120mm but the limiting magnitude will be fainter.

Alternatively what about a fast Newt? No CA, just coma and collimation to worry about. Those 200mm F4 astrographs are cheap. Maybe on an AZ4?

[glend]

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 and decided to buy a 102mm Bresser Messier f5.9, 600mm fl achromatic. This scope is not available in Australia but is available directly from Bresser in europe. I bought mine (OTA only because I have a iOptron goto mount that will handle this light OTA) from Amazon UK and the price was very good and it was delivered via UPS in less than one week.
Amazon also has the 127mm version on sale as well.

In my testing it has proved to be a very nice scope with some CA as you would expect with a fast achromatic but it is pretty much corrected with my Baader Contrast Booster filter. Its a great visual scope. The tube is well made, aluminium, with a rack and pinion focuser, long dew shield, well blackened and baffled interally. It comes with a nice Bresser 26mm EP. As you might expect there are some things that will need upgrading, like the diagonal. I replaced the original with a GSO 2" diaelectric, but that's all I have had to do.

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 field the brighter DSOs are good.

[Amaranthus]

A 200 mm F4 (800 mm FL) is getting toward a little too narrow a TFOV compared to what I was seeking, and may be less portable. I think a refractor fits my requirements better overall, but at least it's another idea...

Yes, they're both f/5 so yield identical exit pupils. From what we discussed in the other thread, the 120mm that you are using sounds like a great little unit for visual work, so it's still probably just top in terms of my rank right now.

As for diagonals, I'll typically use my Televue Everbright on it, so I've got the quality eyepiece/diag situation well covered. I have a LPR UHC and an OIII filter already - how would they go in mitigating the CA, do you think?

[glend]

Not with those filters, you will need a good CA filter to get good corection but honestly the Bressier is acceptable to me, visually, as is. I have also used it with my newt solar filter on the front and the sun contrast is amazing around the sunspots - better than my 130mm newt for sure.

[Amaranthus]

Thanks Kevin for that CA chart from Don Pensack - very useful! It seems from that chart that the 102 would be in a preferable CA zone, but from what you said in the other thread the CA for visual DSO work on your 120 is okay?

I am definitely NOT looking to go high power planetary on this scope - the max I might ever push it is my 9mm Nagler at 67x for the 120 mm (57x for the 102).

[cometcatcher]

I'll take a peek tonight with my 13mm Nagler type 6 and tell you at what magnitude the CA becomes unnoticeable. My eyes might be a bit different to yours but should be similar I think.

[Amaranthus]

Thanks Kevin, that'd be useful - I have the T6 11mm and the T5 16 mm, so it should give a decent comparison to those.

[MortonH]

I've considered a larger achro for the same purposes that you suggest. I didn't do it in the end but I had done enough research to decide I would want something slower than f/5. The Bresser scope mentioned above is around f/6, while Explore Scientific do achros at f/6.5.

You can get the HiOptic 127mm f/6.5 for $720:

http://www.telescopes-astronomy.com.au/hioptic-pricelist-telescope-astronomy-refractor.htm

[glend]

The Bresser I was talking about is the AR102S (for short 600mm fl), there is a longer one too I believe. Here is the link (which I hope is working):

http://www.amazon.co.uk/Bresser-Messier-Optical-Assembly-Telescope/dp/B00EPC5HV6

Note it is OTA only, but they do have them with mounts too I believe - but of course shipping a mount is more expensive as it would be two boxes. Interestlingly it came in a Meade Europe box out of Rotterdam. I understand Meade had bought Bresser at one point but the Bresser family may have bought the brand back recently.

[Camelopardalis]

Just think of the FOV you could get with a nice 2" diagonal, Barry :lol:

Sorry, couldn't help it ;)

[Amaranthus]

2" diag AND some new eyepieces that together will cost a lot more than the refractor I'm aiming at. Thanks Dunk :P Nah, I've vowed to stick with the nice, light-and-easy 1.25 inchers for my C8 :)

[Camelopardalis]

Awwww Barry :lol: missing out for sure :thumbsup:

Keep the focal length as short as possible, although that's somewhat at odds with an achro :P
In my ZS71 I get (theoretically) 6.3 degrees TFOV with my widest 2" eyepiece ;)

[clive milne]

Build a pair of 8" f5 binocular newtonians.... they will have similar field of view of a small refractor but the light grasp of a 12 inch telescope (9x brighter than a 100mm refractor) use the money saved on good eyepieces.

[Amaranthus]

True, but you're only talking a magnification of 10x (assuming a 41mm Panoptic), and not great contrast.

As noted above, I'm really focusing on the trade-off between aperture, focal length (for good TFOV) and CA. The ZS71 ticks the boxes well on the latter two criteria, but only has the same aperture as my binoculars. I'm looking for something a bit more in between. I'm thinking more and more that the 102x510 is the right compromise for me, but I'm not yet convinced! It's a really tough choice between this and the 120x600...

Someone, pleeeaaasseee, give me something definitive to decide on! :D

[cometcatcher]

How much money do you want to spend Barry? That will give us a better idea of what would be in your price range.

[MortonH]

Sounds like you need to find a used f/5 model to try it out. If it's not for you at least you should be able to make your money back.

Agree that you probably need something in the 100-120mm range to differentiate from your current gear. Some of the Orion/Skywatcher models have sold on here very cheaply.

[Amaranthus]

Not looking to spend big at this stage (I may invest in a quality triplet in a few years time), and for now I'm focusing on rich field visual (not planetary or high mag), as detailed in the OP.

Hence the decision to go for an achro and basically isolating the choice to between the SW120 and SW102. I can get the former telescope for $575, the latter for $425 (both mounted on an AZ3), so the argument to go for something different at >$600 would have to be pretty persuasive.

I'm basically seeking feedback on the pros/cons of a 120x600 vs 102x510 achromat...

[Camelopardalis]

You've already got a C8, so the stuff requiring larger aperture and higher magnification is covered.

So keep the focal length to a minimum and keep it portable. I'd go with the 100 and see how you get on. If you find it's insufficient, whip out the C8.

Just be prepared...at f/5 I'd expect the CA to be pretty bad, but for big faint stuff it goes against the price requirements. It'd be an interesting experiment :D

I hear an NP101 is a really nice scope ... :lol:

[MortonH]

A used one of these might be nice

http://telescopes.net/store/07338-deluxe-100mm-f-6-0-refractor-telescope.html

[MattT]

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

[nebulosity.]

What about 100mm binoculars? I have the 25x100mm Celestron Skymasters and love them, with dark skies the views are just 2.5 degrees of :eyepop:

They have a bit of CA but it doesn't worry me, viewing with two eyes is just superb :thumbsup:

Cheers
jo

[Amaranthus]

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:


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?

[Camelopardalis]

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.

[The Mekon]

F5 way to short to give nice flat field at over 2 degrees fov. I would not recommend the 120 at all.

[cometcatcher]

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.

[Camelopardalis]

Is the FC so bad it can't be mid focused out? Meaning, focus on a star somewhere between centre and edge and let your eye adapt.

[Amaranthus]

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!

[Amaranthus]

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.).

http://www.cloudynights.com/item.php?item_id=2168
http://www.cloudynights.com/item.php?item_id=646
http://www.cloudynights.com/item.php?item_id=1602
http://www.cloudynights.com/item.php?item_id=1257

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 ;)

[cometcatcher]

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.

Edit, the order is messed up sorry.

1. Mimosa and jewel box.
2. M46.
3. M45
4. IC2944.
5. Eta Carina.
6 Corona Australis.
7. Comet Lovejoy.
8. 47 Tuc.

[Amaranthus]

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?

[cometcatcher]

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.

[cometcatcher]

War and peace nebula single frame with a CLS / Semi-Apo stack.

I need to revisit these things again in winter with my Nikon full spectrum camera. The Pentax isn't very Ha sensitive.

[Wavytone]

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.

[MattT]

Must have been cloudy every where last night!

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 :confused2:
Matt

[casstony]

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.

[ausastronomer]

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.

Cheers
John B

[glend]

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.

[cometcatcher]

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.

[MattT]

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

[cometcatcher]

Thanks Matt, I'll have to check them out. In the meantime....

Take one back side lens cap for the T-ring, add one GSO 2" to 1.25 inch adapter, duct take them together. I ran out of black duct tape so had to use grey. :rolleyes: Bayonet fitting to T-ring. Slide Expensive TV eyepiece into assembly and hope that it doesn't fail. When has duct tape ever failed? Don't answer that. :P All ready to observe! Cat accessory not included. ;)

[cometcatcher]

Well I just did some preliminary testing on trees about 150m away. Comparing views through the field flattener to straight through... the Orion photo FF actually seems to work. The leaves were in focus from edge to edge with the FF but definitely had field curvature without it. Of course I will have to wait until tonight to do some real star tests. But this problem seems solvable, which is what I wanted to know.

[Amaranthus]

I'm currently observing mostly from my backyard in Bortle 6 suburban skies (north Adelaide), but will be moving in October to a new place in Tasmania, south of Hobart, which is a lovely dark Bortle 2 location. Can't wait!

Overall, I've found the feedback/discussion on this thread extremely useful -- thanks. I'm hardly a newbie, but I've never looked seriously before at that 'transition zone' between narrow-field large aperture Newts/SCTs and the ultra-wide-field but very low mag binoculars. So I've really been working to fill my gap in knowledge regarding the rich-field refractors.

The point about a sweet spot in terms of the aperture vs FOV trade-off in picking up the best that rich-field viewing has to offer was very important and something I'd not really computed in my mind until now, as was the issues of field curvature (I'd mostly been thinking about CA, but for what I want to use the scope for [visual, large DSOs], I'm pretty assured that this will be manageble.

This feedback has helped a lot. I've decided to go with the SW120 f/5. It's hardly a large expense, and if I get the taste for wide-field refractor work, I will look seriously at upgrading to a quality APO in the future. For now, a simple, low-cost achro with a decent aperture will meet my 'in between' needs nicely, I think. My SCT will remain my workhorse, but this will be well used too.

I'll be sure to report back on this thread with some real-world feedback once I've got it up and running and tested for a while. (I'd like to take it up to some dark skies within the next month or so, for a proper stretch of its legs!).

Thanks all!

[Amaranthus]

Kevin, out of interest, where did you get the Orion FF, and was sort of $$? (I may start dabbling -- strictly for personal edification at first! -- in some astrophotography using the SW120, hence the general interest in this...)

[cometcatcher]

Barry, got it at Bintel for $149. http://www.bintel.com.au/Astrophotography/Reducers--Correctors--Flatteners/Orion-Field--br-Flattener/679/productview.aspx

[cometcatcher]

Just tried the field flattener visual and it works. Stars were great right across. Straight through not good on the neck though. I wonder why these aren't more common? Can't they be built into a diagonal or something?

[Stardrifter_WA]

Hi Glen,

That is not correct Glen, Bresser are available in Australia from Extravision in Queensland, as they are the Australian agents for Bresser.

http://www.extravision.com.au/catalog/product_info.php?cPath=33&products_id=2008

Cheers Pete

[Larryp]

Andrews Communications in Sydney are also Bresser dealers:)

[Stardrifter_WA]

Hi Laurie,

They probably get them from Extravision anyway, as I know they are the Australian distrubutor.

Cheers Pete

[Larryp]

Quite possibly. Peter:)

[glend]

What I said was that model Bresser Messier AR102S f5.9 600mm fl OTA is not sold, or stocked in Australia. I know there are Bresser agents here but none of them carry this scope. It is also unavailable in the US. Only Bresser in europe offers this model. I don't know why that is so. The longer AR102 f9 1000mm fl scope is what is sold here.

If you don't believe me check the Andrews website or ring the other stockist.. I only mentioned that scope to try and give the guy an option. I have given plenty of my business to both Andrews and Extravision. I'll give up helping people out and leave it to the experts.

[Larryp]

Hi Glen
That scope is available in USA from Telescopes.net, which is part of Woodland Hills Camera and Telescopes.
It is f4.5 according to their website, and seems to be recommended for low power wide field work, which is what the OP wants.
Cheers:)

[Stardrifter_WA]

Hi Glen,

Sorry Glen, from reading the website at Extravision website it states:

" The OTA has a free aperture of 102mm with a focal length of 1000mm and therefore a focal ratio of F9.8"

"The Bresser Messier AR-102 on EXOS-2 offers the ambitious amateur astronomers a good look with a very stable universal mount."

It appears that it is the same model, just with a mount. I would hope that the OTA is also available, but that I simply don't know. However, having dealt with Extravision for years, I have always found them accommodating.


Cheers Peter

[Larryp]

Peter, Glen is correct-the AR102S is a very different scope from the AR102:)

[Amaranthus]

Well, I got a V-style dovetail mounting bracket from Bintel, and with that was able to successfully mount my SW120 onto my 8SE GOTO alt-az tripod. See attached picture.

It works very well indeed, and with the dovetail clamp, it takes all of 30 seconds to switch around the OTAs! (SCT or refractor). The only disadvantage is that because of the length of the focusser + diagonal, the SW120 scope will hit the mount after about 75 degrees altitude (c.f. ~85% for my SCT; I set my slew limits to avoid that). So zenith viewing is out, but I can live with that...

The AZ3 mount that came with my SW120 has not gone to waste -- it provides a MUCH more stable mounting for my 15x70 binoculars, with fine controls to boot.

Tomorrow night I'm taking the new gear up to Stockport Observatory to give it a first 'dark site' run in. Can't wait.

[cometcatcher]

I see you got a nice black and white one. Mine's blue.

I got hold a second hand Meade 102mm f7.8 achro to play with. Should be interesting to see how it goes.

[Crazydog]

Looks great....they are nice looking scopes, at least. I'll be interested to hear your review, too, Barry. I'm still umming and arrhing over the 102 vs 120 at this stage. It's really going to be a financial decision in the end, I think, even tho neither of them are really expensive in the big scheme of things.

[MattT]

Looks good Barry! Shame about the viewing angles.
Both Baader and Blue Fireball Technologies sell a female to female T thread adaptor that would fit the Orion FF, so an eyepiece adaptor, also Blue Fireball, with T threads on one end and compression spring on the other would mean eyepieces/diagonals will fit without duct tape :lol:

Agena Astro in the US have all the bits needed. I'm going to try an Orion FF on my f7 frac soon.
Like to know how the 120 is going Barry.
Matt

[Amaranthus]

Okay, so an update on how the SW120 f/5 has performed.

Eyepieces I used for testing were:
TV Panoptic 24 mm (magnification = 25x, TFOV = 2°35')
TV Nagler T5 16 mm (38x, 2°7')
TV Nagler T6 9 mm (67x, 1°11')
TV Nagler T6 11 mm with 2.5 x TV PowerMate (136x, 34')

I used a TV Everbright diagonal, plus UHC-LPR, OIII, Lunar, and Wratten filters.

First light was under bright Adelaide suburban skies, mounted on the AZ3 that came with the instrument. The mount was okay (the altitude movement is rather stiff -- it is controlled by a friction bolt), and the fine adjustment controls were useful. The finder is a red-dot sight, and was functional.

Bright objects (Sirius, Jupiter) showed obvious bluish CA, but it was quite tolerable and essentially invisible on any object below about 1st magnitude.

However, what was striking was the 'washed out' look I was getting under the wider views -- the sky brightness (a Bortle 6 site) dominated the view to the extent that any significant 'rich field' benefit was lost. With the LPR and OIII filters in, nebular DSOs were significantly improved, and indeed the LPR helped with the stellar DSOs too by darkening the background sufficiently without losing too much light - though it would be a case of diminishing benefit for the fainter (mag 9 and below surface brightness).

My overall impression was that it was 'okay' for wide-field viewing from my backyard, and I'll continue to take it out maybe once a week, but not great.

Then on Saturday I had a chance to take it to Stockport Observatory site in the north of Adelaide (in Bortle 2 skies). Well, my impression turned to :eyepop: It was then mounted on my Celestron 8SE single-fork-arm GOTO mount. Simply superb -- what a difference. Rich fields in the Pan 24 and Nagler 16 were stunning, and when I did the Nagler 9mm x OIII combo, I got my best EVER look at the Rosette Nebula - the gases just hung over it beautifully like a grey shroud.

I ended up getting through about 25 DSOs over about 4 hours of viewing (selected for their wide FOV characteristics, ranging from M45, to Eta Carina, Tarantuna, 47 Tuc, Southern Pleiades, etc.). For these objects, there was essentially no hint of CA (maybe a touch on the M45 stars, but hey, it just made the cluster look cool ;) ) and I really didn't notice any significant field curvature -- certainly nothing that bothered me visually.

The galactic fields around Centaurus, Carina, Vela etc. were magnificent, just gradually wandering over them with the Pan 24.

At the end of my view, for a giggle, I chucked in the Nagler 11 mm with a 2.5 PowerMate and a Neutral Density filter, and looked at Jupiter. Well, it looked pretty darned good -- very little sense of CA with the filter in, and the great red spot and major cloud banks were clear. I was surprised -- I certainly didn't buy this scope for planetary observation, but it did remarkably well.

In short, I was in love with the instrument at the dark site.

So, my conclusion is that it will be a regular traveller with me to Stockport, and when I myself move permanently to a Bortle 2 site in Tassie later this year, well, it's going to get a LOT of use. Overall, very happy.

[MattT]

Thanks Barry, a nice wrap up of the 120. The use of high quality eyepieces does make a difference in these scopes, specially the Pano 24.
I have seen in my 70mm f7 the same thing, FC with cheapish eyepieces and nearly zero with good ones, in my case the ES 24mm 68º is pinpoint to the edge where as the 20mm SWA 70º shows quite a bit of FC. There goes the Orion field flattener idea out the window....:cool:
Matt

[Camelopardalis]

Sounds good Barry, now you just need a nice 2" eyepiece to widen the view even more :lol:

Btw I can't see it on their site any more :confused2:

[Amaranthus]

The field flattener? It's still there: http://www.bintel.com.au/Astrophotography/Reducers--Correctors--Flatteners/Orion-Field--br-Flattener/679/productview.aspx

Re: 2" eyepiece -- given the washed out look of my Pan 24 under urban skies, I'll wait until my move before I even consider this.

[Camelopardalis]

No, the 120 f/5 scope :confused2: I must be blind :help:

Yeah I get what you're saying about urban skies, where I used to live about 60-70x was about as low as I could stomach the orange glow :sadeyes: but on trips out to darker skies here it's :love:

Btw, the 2" barrel isn't just to facilitate longer focal lengths, but also to enable wider apparent field of view of mid focal lengths..even some shorter ones have 2" barrels by default (ES100 for example) for stability. I just love the immersive wide views from 82 and 100 degree designs in my scopes, but appreciate it's a question of taste. My favourite to date is the 17mm Ethos which is just spectacular in all of my scopes. But I'm just :screwy: as you can tell :lol:

[Amaranthus]

The 120 f/5 scope is here: http://www.bintel.com.au/Telescopes/Refractor/SkyWatcher-SW120/1796/productview.aspx

...and various other places around the net.

I'm getting 82 degrees in my 16 mm and down, and 68 at 24 mm, so it's fine for now. Yes, I will go down the 2" route, but it's not a priority. I seem to have it the sweet spot, IMHO, in terms of useful FOV where I live right now.

[cometcatcher]

Great review Barry and very glad you like the scope for visual use.

[Crazydog]

That was a good review and pushed me over the edge....I ordered a sw120 today. I won't have the nice eyepieces, which I know will make a difference, but I'll have the dark skies, and I think I'll be impressed with anything compared to my old Tasco 60mm!

[Amaranthus]

I think that you won't regret that extra aperture, Lea. And you can always start a premium EP collection in the future - no rush!

[raymo]

I find the tripod is stable, and the slow motions work o.k. but one huge
drawback is that even with my little SW 80 on it, if I add my 1100D I have to hang a kilo of weight from the lens hood to stop the targetfrom moving. If I tighten the large nut enough to stop this happening,
I can't move the platform by hand when the slow mo adjustment runs
out of thread. Being as I almost exclusively do imaging, I now never use the tripod.
raymo

[clive milne]

It's a shame that your comment didn't get any traction Matt, it was probably the best piece of advice given in this thread. iirc) the article Mel wrote on this subject was derived from a fairly lengthy discussion amongst members on the old ATM list. I think it was around 1999 or 2000 and should still be in the archives if that helps.

As you intimated earlier, there are legitimate reasons why a 6" f5 Newtonian is close to the optimum rich field instrument. There is one configuration that is demonstrably better though. That being a 6"f5 Newtonian binocular with coma correctors.

best,
c

[Amaranthus]

I took notice at the time Clive - I agree it was an excellent summary!

[Camelopardalis]

That sounds interesting...anyone got/making one?

[MattT]

Read your way through this...http://www.bbastrodesigns.com/tm.html

Recently I bought a 102 f5 thanks Zane :thumbsup: and put an ES 30mm 82º eyepiece on it and :eyepop: A revelation compared to any other eyepiece I tried in my first 4" f5. A tiny bit of FC and a TFOV of 5º... very nice. Perhaps a Nagler 31mm might be better, but my wallet says no way!
Matt

[clive milne]

Mark Sutching had an 8" binocular (I don't think it is still around) I had look through it probably 20 years ago. I remember two things clearly, it showed objects as well as the 12" nearby, and the aesthetic appeal of the views were second to no other telescope on the field irrespective of aperture or configuration (including a 7"AP). I have been sold on the binocular Newtonian principal ever since. I built one for myself (all be it with larger aperture) Dave Moorehouse saw some pictures of my bino's and built a pair of 16's, 3RF have a pair of 12's, and there is at least another pair of 14's over on the east coast.

In my experience, if you are a purist and would like to see the universe rendered in the very best way possible, binocular Newtonians are a very cost effective way to do it.