high magnification with a small refractor?

[sgazer]

hi,

A couple of nights ago it was clear, so I decided to try my C80-ED refractor on Saturn, at about 10pm it was just getting up to a decent height. I wasn't expecting much as the C80 focal length is only 600mm and I was using a 7mm ortho giving about 85x mag, not a lot!

However, Saturn was looking good, so I decided to do my usual trick of putting my 2" 2x barlow lens in the scope side of the diagonal, producing about 3x more mag, so approx 260x in total. I thought this would completely overpower the 80mm scope, however as I adjusted the focus, Saturn snapped into focus really nice and cleanly. I could see a little banding on the surface and I thijnk the shadow of the rings. It looked really good.

Now, conventional wisdom says that any magnification above twice the aperture (x160) is not worth it, however I was easliy getting 260x, even with a fairly cheap barlow and the standard sloppy R&P focuser. There wasn't a hint of chromatic abberation either.

Have others had experiences of a frac performing well at higher than recommended magnifications? I feel this little scope it a bit of a planet killer!

ps, I confirmed I was getting about 260x mag as I later tried the 7mm ortho in my C8 SCT and it gave a similar image size at 285x mag.

[Peter Drew]

Small, high quality refractors, on nights of good seeing, often perform well on bright objects such as the Moon and Saturn. Conventional wisdom normally suggests about 50x per inch of aperture is the average limit but 80x-100x per inch can be achieved with success on rare ocasions as the small aperture is less affected by seeing conditions. By contrast, an 8" would still probably work best on Saturn at 260x but would provide a brighter more detailed image. Try 80x per inch on an 8" and you will see that magnification is not everything.

[A McEwan]

I think those "conventional wisdom" formulae for maximum magnifications are a bit out of date now. I regularly use 180x to 240x in my C80ED, and did in my SW ED80 too. Experimentation should be encouraged!

[Astro_Baby]

My Nexstar 4 can go beyond its x2 per mm maximum but you need very cold weather and steady seeing.

I have heard the TAL 100 scopes can also exceed the standard limit but have yet to try it.

[bish]

Whatever works for you. Everybodies different - it's best to ignore the 'rules of thumb' and experiment. Some will say that magnifations of x150-x200 under normal conditions are best. It may be best for them but not necessarily for you.

[russ]

Whatever works for you. Everybodies different - it's best to ignore the 'rules of thumb' and experiment. Some will say that magnifations of x150-x200 under normal conditions are best. It may be best for them but not necessarily for you.

Exactly, the old 50x per inch rule is nothing more than a very vague and extremely rough guide to what will and what won't work. Experimentation is No1 with this hobby, it's what makes it fun.

Russ

[sgazer]

yes, it's definately worth experiementing. It was nice to see a short refractor working so well at high magnification.

The C8 SCT didn't snap into focus quite as easily as the refractor and although the C8 was reasonably well collimated, I feel the view was a bit clearer in the frac.

[IainR]

My Nexstar 4 can go beyond its x2 per mm maximum but you need very cold weather and steady seeing.

I have heard the TAL 100 scopes can also exceed the standard limit but have yet to try it.

Hello

I read your post and was wondering what sort of magnification you can reasonably use? I have a Nexstar 4 myself and have just bought a binoviewer with 5mm Parks Gold Series 7 lenses. I think I have probably been horribly overambitious given the telescope or maybe it is just that I am lousy at focussing (or both). What I need is for the moon to be out and about at a reasonable time.

Iain

[Capricorn]

You may simply have an ED refractor that by chance after assembly is set up to supply very good results. The two lens may be well matched and the EP may also be well matched and both to the other.

Also the conditions will have made subtle changes that could have improved it overall. As those lens cool they alter in radaii. Small but relevant.

All the components will be made via a production line, then assembled (randomly) and tested. As long as they pass they will go out, no-one will bother to check how well they pass. So one that scrapes through will get sold as will one that "magically" falls close to the optimum performance. The checks will probably be automated also. They were at the last place I worked as I developed some of the software for the testing. The final condition was "Did it pass this test" not "How well did it pass".

The problem is that we read of many instances where the person is saying I cannot get the magnification stated. This is about the only one I have seen saying I got better then expected.

I suspect that a good ED refractor will give good magnification, unfortunately a fair one will just give fair magnification. It is when people read that one did, so they then expect all to do so and it doesn't work like that.

Refractors started out as single lens, then achromatic doublets, then apo triplets and now I see ISTAR are advertising quadtruple Apo's. So yes I expect them to have improved. Not sure where to insert ED doublets as they are doublet and claim apo.

Just be pleased that yours does handle the magnification as I suspect that the next 5 people with the same scope will get no-where near.

[RichieP]

Last night I managed in excess of 400x on my Meade AR5 127mm achromat whilst viewing Mars. I did this by mistake (the Meade 5000 9mm and 5.5mm EPs are almost identical in the dark!) but I was stunned to see that the scope held at out this mag - I had never pushed it beyond 300x before.

Was exceptionally good seeing last night though.

Richard

[michael.h.f.wilkinson]

The limit given is based on the Abbe rules, and are based on diffraction limited optics. However, Abbe probably had very high visual acuity, as his rule only applies well to the sharper 5-10% of eyes in the world. For most people going beyond the limit will work. For me it does not, because of a high visual acuity (1.6) common to nearsighted people.

[jgs001]

I've done something similar and had amazing results at 240x with my C80ED, it was a mistake, I forgot about the change in focal length from my ST80 clone, soon after buying it, and used my 5mm Hyperion and 2x barlow ... I did once have a look at Saturn through my ST80 clone with a 5mm Ortho and a 5x Barlow... (400x) for the odd brief moment it looked fully 3d (which considering I was only using 1 eye is odd come to think of it...)... but the optics are really not up to it (nor was the seeing for that matter ...)

As for the sloppy R&P, £6 to £10 will really help on that... invest in some Teflon tape, stick it to the drawtube where the runners are. It packs out the space around the drawtube. I used to get serious star movement when focusing at 10x liveview, but last night, after this mod, it was solid as a rock. Packing out the drawtube, also slows down the movement of the focuser a bit, making it a little more precise.

[sgazer]

As for the sloppy R&P, £6 to £10 will really help on that... invest in some Teflon tape, stick it to the drawtube where the runners are. QUOTE]

thanks, I got some a while ago, but haven't got around to fitting it yet. It's on the list of things to do! Which bit of the focuser did you stick it to, the actual tube or the black plastic sliders?

[jgs001]

The draw tube... it's much easier... I turned the C80ED upside down (that's actually more important than it would first appear... removed the 4 screws holding the focus bar in place, lifted that assembly out of the way. Pull out the drawtube. There's a plastic piece that the screw on top tightens onto and that's only resting... that's also why putting it upside down works so well, as it keeps that in place. Then I affixed the teflon tape along the sides of the drawtube 120 degrees from the top (that's where the other two rests are). I used two strips stacked up to fill the gaps. Then slot the drawtube back in carefully, it's a bit of a squeeze with the tape fitted and it'd be a pain to strip it all off again.. attach the focus bar and assembly and away you go. I still have some coma around the corners, but it's all central now, and no eccentric shows up (that I can see). Unfortunately I can't think of a way of sorting out an FR/FF for a similar investment ....

[sgazer]

thanks John, job done, slop removed. It's much better now.

ps. I find the Celestron f6.3 reducer works well and sharpens up the edges. I get about 3deg FOV with a 300d. I reckon it drops it down to 400mm f5. Of course the FR is not cheap, but I use it on my C8 SCT too, so it made sense. For the frac though, it needs to be mounted on the camera ring adapter for it to still focus. Strangely though it still gives a similar reduction in focal length as it does on the SCT, despite it being significantly further away from the sensor when used with the SCT.

[jgs001]

Excellent. As for the FR... I was thinking the cost of the teflon tape I know dreaming... one day...

[sgazer]

Excellent. As for the FR... I was thinking the cost of the teflon tape I know dreaming... one day...

hmm, tricky one that!

[oceanheadted]

Guys,

where do you get the tape from/what sort /brand,

thanks

Alan

[jgs001]

Alan, I used oven liner, teflon tape, teflon tapes, uhmw tape, baking sheet, zone tape, oven mesh, baking sheets, over liners. It's not gonna be as good as a £300 focuser but it does a good job of removing play in the drawtube and tightening things up, which also slows down the rack a bit.