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

[Amaranthus (Barry)]

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??

[cometcatcher (Kevin)]

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/s...2&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 (Glen)]

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 (Barry)]

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 (Glen)]

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 (Barry)]

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 (Kevin)]

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 (Barry)]

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....-refractor.htm

[glend (Glen)]

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-Mess.../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 (Dunk)]

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

Sorry, couldn't help it

[Amaranthus (Barry)]

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

[Camelopardalis (Dunk)]

Awwww Barry missing out for sure

Keep the focal length as short as possible, although that's somewhat at odds with an achro
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 (Barry)]

In my ZS71 I get (theoretically) 6.3 degrees TFOV with my widest 2" eyepiece

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!

[cometcatcher (Kevin)]

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 (Barry)]

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 (Dunk)]

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

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

[MortonH]

A used one of these might be nice

http://telescopes.net/store/07338-de...telescope.html