90mm f6 refractor for observing planets is any good?

[aleixandrus]

Hi! I've just purchased a 90mm refractor, a TS-Optics CF-APO 90mm f6 FPL55 Triplet APO (540mm focal length). It will be used just for imaging (mono camera, eq6r mount) but I'm asking myself if I also could use this scope to show planets (Jupiter, Saturn) or the Moon to my friends. This would be a one/two times in a year 'event'. I live in a Bortle 4-5 village and I don't do visual. I know it would be a very sub-optimal experience at best case but it is what I have. I don't have any barlow or ocular, I'll need to purchase all accessories. So...

1. Should I forget this path or give it a try?
2. If yes, what diagonal/ocular/barlow would you purchase?
3. If no, I can purchase a SkyWatcher Mak 90 with diagonal and 10/25mm oculars for ~£130. I know this is a cheapo alternative but, is this a better approach?

Any advice is welcome

[Elp]

That's a nice scope and one I've been looking at on and off.

The response to the visual view will be down to the individual. I've had people look through my C6 at Jupiter at 1500mm FL and they said "it's a bit small isn't it?". I've looked at Saturn through my Z61 and was amazed I could see the planet and the ring crisply defined but the planet in realistic size was around 2-3mm, Jupiter is also not that big with this scope but the band details are sharp as anything.

So I'd suggest you try it, a Barlow will be useful. Having another scope for one thing I don't think is practical, it has to have multiple uses otherwise I personally cannot justify it as you might find you just wont use it enough.

In terms of diagonal any dielectric will do, you have to decide if you'd want the 2 inch capability in future for your eyepieces otherwise just get a 1.25 one. The WO ones are good, the one I have is an Altair model. If you can afford it get a TeleVue 2.5x Powermate, if not the Meade Telenegatives are good for a standard Barlow. Stick to around 2x, those 5x are tempting but you'll rarely get the seeing to support their use if your scope can even do the magnification, they also increase your focal length so the light transmission to the eyepiece is reduced.

[Chaz2b]

When I showed my mother Saturn for her first time, it was through a cheapish 60mm refractor, I Had no barlow lens either. It was small but you could make out that it was indeed Saturn from its rings. 
The refractor was an Achromatic on a eq1 tripod. I knew where to point it, got it in focus and let mother have a view. “My god” was her only remark, and kept viewing till it went sideways!

chaz

[rl]

It will be fine for showing people planets. On a good night you will see Cassini's division and the GRS, plus shadow transits on Jupiter (look them up first)

It's worth remembering that the maximum useful magnification in the UK is often about 200 times regardless of whether you're using a 4" or a 14" just down to conditions. Don't know about Spain.

I find it's worth managing expectations before taking people to the telescope. I hold a 5p/ 5 cent piece at arm's length and ask people to read the writing...it's useful in setting the context.

Edited February 18 by rl

[mikeDnight]

Beer & Madler once created a map of the Moon using a similar aperture refractor, though a long one, that was so detailed it brought selenography to a virtual stand still for decades. So a 90mm refractor is very capable for viewing the solar system as well as brighter deep sky.

[The60mmKid]

5 hours ago, aleixandrus said:

Hi! I've just purchased a 90mm refractor, a TS-Optics CF-APO 90mm f6 FPL55 Triplet APO (540mm focal length). It will be used just for imaging (mono camera, eq6r mount) but I'm asking myself if I also could use this scope to show planets (Jupiter, Saturn) or the Moon to my friends. This would be a one/two times in a year 'event'. I live in a Bortle 4-5 village and I don't do visual. I know it would be a very sub-optimal experience at best case but it is what I have. I don't have any barlow or ocular, I'll need to purchase all accessories. So...

1. Should I forget this path or give it a try?
2. If yes, what diagonal/ocular/barlow would you purchase?
3. If no, I can purchase a SkyWatcher Mak 90 with diagonal and 10/25mm oculars for ~£130. I know this is a cheapo alternative but, is this a better approach?

Any advice is welcome

That telescope will provide thoroughly enjoyable planetary and lunar views. There's no need to think that it will be suboptimal or that a 90mm Mak will do better (which it won't). A 90mm refractor can be a wonderful all-rounder for visual astronomy.

[Captain Scarlet]

I’ll echo what @The60mmKid said. I have a Kowa 88mm spotting scope, which has a pure fluorite front element and is as “apo” as they get. Its focal length is 510mm so f/5.9. It has an adapter that accepts 1.25” eyepieces. I lent it to a neighbour for the Jupiter/Saturn conjunction a couple of years ago and we were utterly gobsmacked by the view.

As for comparison to a 90 Mak. The Mak 90 has a ~30% central obstruction. Suiter in his famous book says that to a very close approximation an obstructed scope will deliver the equivalent unobstructed contrast of ‘aperture minus CO’, ie in this case 90 - 27 = 63. Ie effectively a 63mm scope.

So yes, definitely go with the TS, and a reasonable quality 10 (54x) and a 4-5mm (108-145x) eyepiece will give all the magnification range you need. In fact get the svbony 3-8 zoom, very highly rated and inexpensive.

my tuppence worth 

Magnus 

Edited February 19 by Captain Scarlet

[Louis D]

I have a TS-Optics 90mm f6.6 FPL-53 Triplet APO that I use visually.  It's fine on planets and DSOs, but the limited aperture compared to an 8" Dob is really noticeable in resolution and light gathering.  It shows the bands and moons of Jupiter, the rings, Cassini division, and brighter moons of Saturn, the phases of Venus, and the largest features of Mars.  The moon shows a wealth of details.  However, don't expect to see a wealth of details in Jupiter's bands like barges and whorls or fine details on Mars.  Lucky imaging with it might reveal some, but the human eye needs more aperture to pick up such details visually.

Here's a comparison image of the moon I took through various sub-6" scopes to give you some idea of visible image scale and details:

Ignore the peachy color in some images.  That was introduced by the eyepiece (12.5mm APM Hi-FW) that I was using for afocal projection into my smartphone camera.

As for a diagonal, I like the various dielectrics made by GSO and sold under such many brands including Revelation in the UK.

[badhex]

Last year I was trying to chase down Enceladus with a 73mm F5.9 FPL53 doublet, 7.7mm eyepiece, and 2x Powermate under similar LP levels. I didn't catch Enceladus itself but I did get all of the larger moons with no problem, as well as Neptune. Saturn and Jupiter are small with this setup but extremely crisp, with plenty of larger detail visible. 

As you can see from an example of a smaller scope, yours will be more than capable and also get better image scale and resolution. 

You could consider an Svbony 3-8mm zoom if you're only doing planetary observation. They are well regarded and relatively inexpensive if you find the right place to purchase from. 

Edited February 19 by badhex

[JTEC]

I have a 90mm f6 Stellamira (by Long Perng) that I bought as a grab ‘n’ go option and to travel with and like it a lot. It gives very nice views of Moon and planets and, given suitable skies, many of the brighter DSOs.  Incidentally, I like it a lot better than the Starfield 102 that receives frequent praise on this forum and that I tried before settling on the Stellamira 90.  The 90 is a triplet, so one might expect better colour correction and, imo, it has. If you have the neck for it, viewing straight through gives optimal image quality, I found, but it’s pretty tough with objects high in the sky and I for one can’t last that way for long - interesting to try though.  Advice I had from TEC scopes was to use mirror diagonals with (their) refractors to avoid unwanted colour, so I do, but even with a relatively short focal ratio like the ones we’re discussing, I doubt you’d see much difference between mirror and prism at the powers most likely to be used. And prisms do seem to have countervailing advantages, like relative freedom for scatter.  Bottom line, I really like my 90mm f6 - it’s good quality, versatile and convenient.

[Stu1smartcookie]

My advice to the op is "go for it" I, like @Elp  have had great views of planets in small scopes , small but sharp. I used an evostar 72 ed . More importantly , do what you want in this hobby , nobody should judge you . 

[Deadlake]

2 hours ago, JTEC said:

The 90 is a triplet, so one might expect better colour correction and, imo, it has

How long does the 90 mm take to cool down?

[Louis D]

My 90mm triplet takes at most 30 minutes to reach equilibrium with a modest temperature differential.  Until then, it appears to have pinched optics.  My 72ED FPL-51 doublet has no such cool down issues.  The issue with the triplet may come down to an under-engineered lens cell.  I know AP and other premium APO makers spend a lot of effort on cell stability across temperature changes for imaging reasons.

[JTEC]

Hi @Deadlake. About the same as @Louis D says.  Perhaps a bit less. But, by then, it’ll have settled down and be as good as it can be. I don’t think I see any pinching; what I’m aware of is that, until the scope is settled, colour correction is not optimal and it progressively improves until it reaches its best point. To be honest though, I don’t tend to scrutinise these aspects. I’ve been looking through scopes a long time and am happy to rely on an overall impression of image quality. I noticed the colour correction aspect when I hastily banged in a high power eyepiece shortly after taking the scope out. It bothered me initially but I watched as the scope gradually settled down and, as it did so, the colour ‘error’ gradually went away.

[Louis D]

By pinched optics, I mean that bright stars have spikes emanating radially in oddball orientations from the center of each of them.  I was panic stricken the first time I looked through mine because of it.  However, I noticed it getting better over time, so I went back inside to let it acclimate for 20 or 30 minutes.

I haven't noticed color correction changing during acclimatization, but it would be hard to discern among the bright spikes with my scope.

Edited February 21 by Louis D

[900SL]

I have the TS CF 90 f6, it's a great scope for imaging DSO. I've used it down to -25C no issues, takes 45 minutes or so to acclimatise.

I use it with a  Baader hyperion zoom and a ASI 178mm camera for lunar. It's a bit short for Jupiter and smaller galaxies, but stunning on the moon, if the seeing is good.

 

 

Edited February 20 by 900SL

[Louis D]

I forgot to mention that the 90mm APO shows exceptionally nice solar images with an affordable 1.25" Herschel wedge.

[JTEC]

I speculated that the gradual coming together of colour correction could be explained by progressive, differential cooling/equilibration within the bodies of the lens elements and/or of the 3 elements of the objective in relation to each other - but that could well be complete nonsense 🤣

Edited February 20 by JTEC

[Louis D]

Because of this thread, I went out tonight with my 90mm APO and looked the moon, Jupiter, Collinder 70, and the Orion Nebula.  It was basically color free compared to my 6" f/5.9 achromat, no surprise.  It showed a wealth of detail on the moon with excellent contrast.  It showed the main two bands of Jupiter quite clearly along with obvious size differences among its moons.  However, I could not discern any detail in the main bands.  Collinder 70 was rendered beautifully and in context at low power with pinpoint stars.  The Orion Nebula looked great from low power right up to the highest powers.  The Trapezium was always obvious as multi-member.  However, I could not get even a hint of the E and F components tonight.  I loved how light and compact it is compared to my 6" achro.  It makes for a terrific grab and go scope.

Edited February 21 by Louis D

[aleixandrus]

Sorry for the late reply, busy week 😅 Wow, thank you so much for all reply and comments, I appreciate it so much. Very valuable info there. Some thoughts:

• I've never done visual except for some very-very casual observations using a very old 1000/100 eq1 manual newton and a 12" dobson from a friend, time ago. I know that the dobson is in another league, I get my 'wow' moment with Saturn using it and I do know the 90 APO will no provide anything similar. But it is fine. In addition and to be fair, I don't know any of those features of the target objects: bands, transits, divisions, etc. In the past I've just enjoy the object itself so my expectations are quite low, probably more in the "I want bigger" than "I want details". Of course I'm probably wrong thinking this way.
• Never try sun observation, it's not high on my priority list but I'll take a note on this for the future.
• Based on your comments, I'll discard the Mak 90 for know, give first the APO 90 a try.
• I don't have the scope yet so I have time to rob borrow some oculars from friends. I'll try to get something such 4-6-10mm ocular plus x2 barlow just to test magnification in the range of x90 to x180. Scope specs mention "up to x300", this may be just a marketing scam but I'll try to push the "aperture x 2" magnification limit if I find the right oculars. Here in north of Spain, close to the Ocean Atlantic, I think seeing is similar to UK (clouds and rain most of the year) so I don't probably could go that far with magnification very often. Time will tell.
• That Svbony 3-8mm zoom seems quite appealing at least for casual use. The Baader Hyperion zoom is simple out of my budged for now. I'll probably need to open a thread in the future before purchase anything as I said, never did visual and I'll probably screw it up. Can't figure out yet how the take into consideration things such as 'exit pupil', 'field of view' or 'eye relief'.
• Do I need a diagonal (or prism?) or it will simply prevent me from doing the kamasutra with my scope? The focuser as almost 150mm of travel so I should do focus without extenders. Isn't it?

As I said, thank you so much for all your insights.

[Louis D]

5 hours ago, aleixandrus said:

Sorry for the late reply, busy week 😅 Wow, thank you so much for all reply and comments, I appreciate it so much. Very valuable info there. Some thoughts:

• I've never done visual except for some very-very casual observations using a very old 1000/100 eq1 manual newton and a 12" dobson from a friend, time ago. I know that the dobson is in another league, I get my 'wow' moment with Saturn using it and I do know the 90 APO will no provide anything similar.
  • Saturn's ring will wow you even in a smaller scope.  Keep in mind, they're closing down to a thin line over the next year.
• But it is fine. In addition and to be fair, I don't know any of those features of the target objects: bands, transits, divisions, etc. In the past I've just enjoy the object itself so my expectations are quite low, probably more in the "I want bigger" than "I want details". Of course I'm probably wrong thinking this way.
  • You can certainly go bigger, but aperture still limits resolution.
• Never try sun observation, it's not high on my priority list but I'll take a note on this for the future.
  • Can be quite rewarding with just a Baader Solar Film front filter.
• Based on your comments, I'll discard the Mak 90 for know, give first the APO 90 a try.
  • I found 90mm Maks dim for their aperture compared to refractors.  I do have a 127 Mak and find it quite handy for camping trips at dark sites due to its compactness.
• I don't have the scope yet so I have time to rob borrow some oculars from friends. I'll try to get something such 4-6-10mm ocular plus x2 barlow just to test magnification in the range of x90 to x180. Scope specs mention "up to x300", this may be just a marketing scam but I'll try to push the "aperture x 2" magnification limit if I find the right oculars. Here in north of Spain, close to the Ocean Atlantic, I think seeing is similar to UK (clouds and rain most of the year) so I don't probably could go that far with magnification very often. Time will tell.
  • Bummer about your weather.  I've always pictured Spain being semi-arid like parts of western Texas.  Apparently, that's mainly in the Tabernas Desert.
• That Svbony 3-8mm zoom seems quite appealing at least for casual use. The Baader Hyperion zoom is simple out of my budged for now. I'll probably need to open a thread in the future before purchase anything as I said, never did visual and I'll probably screw it up. Can't figure out yet how the take into consideration things such as 'exit pupil', 'field of view' or 'eye relief'.
  • I was actually using my Svbony 3-8mm zoom last night to speed up checking high power viewing.  It is excellent from 5mm to 8mm and very good from 3mm (really 3.5mm) to 5mm.
  • Exit pupil is simply the eyepiece focal length divided by the scope's f-ratio.  It tells you how big the exit beam of light is coming out of the eyepiece.  I was struggling at tiny exit pupils with the zoom set at 3.5mm to 4mm last night due to the wealth of dense floaters I have in my observing eye.  It made using those powers very frustrating.  At larger exit pupils, floaters don't tend to bother much at all.
  • The zoom's eye relief was tight enough that I had to tilt my head around while wearing eyeglasses to see the outer parts of the field of view.  This isn't a big deal when observing planets, but it is a distraction with the moon, though.  At the highest powers, I could remove my eyeglasses due to the tiny exit pupil negating the effect of my eye's astigmatism.
• Do I need a diagonal (or prism?) or it will simply prevent me from doing the kamasutra with my scope? The focuser as almost 150mm of travel so I should do focus without extenders. Isn't it?
  • Exactly.  Apparently, Japanese observers tend to prefer straight through viewing.  If you can recline in a beach chair, you might be able to make it work while remaining comfortable.

As I said, thank you so much for all your insights.

See inline comments in yellow.

[aleixandrus]

Well, @Louis D, Spain is very different from north to south. Here in the north, in my town, there is an average of 190 rainy days yearly. Add a few tens of "normal" cloudy days and... yes, astro is hard here. In the south you can find a lot of places with less than 25 rainy days yearly. Quite a difference! I've been born in the wrong place 😅 Well, not really, I wouldn't change my land for the south, it is too brown, almost a desert and summers are getting more and more horrible. It is ok for a holidays trip but nothing else (to me). Tabernas Desert, in Almeria, is probably one of the more arid lands in Spain, it is not fair to compare to!

About your comments, thanks a lot. I'm "in a rush" to observe Saturn before it turns sideways. I'll take note of the exit pupil and eye relief explanations. I started wearing glasses two years ago (myopia, 2 diopters) and I've never done visual wearing them. As I'm not very familiar with visual observation in general, I suppose I need to try first and check if I feel comfortable.

[Louis D]

20 minutes ago, aleixandrus said:

I started wearing glasses two years ago (myopia, 2 diopters) and I've never done visual wearing them. As I'm not very familiar with visual observation in general, I suppose I need to try first and check if I feel comfortable.

Check your cylinder (CYL) diopter amount for your observing eye in your eyeglass prescription.  It is a measure of how much astigmatism you have in each eye.  I have around 2.0 diopters in each eye, so I need to wear eyeglasses all the way down to fairly small exit pupils to avoid seeing spiky stars and blurred fine details.

Tele Vue has this chart for their DIOPTRX™ line:

It's fairly accurate for most observers.  For instance, I can get by without correction below 1mm exit pupil.  It still improves the sharpness a little bit by putting eyeglasses back on at sub-1mm exit pupils, but not tremendously as at larger exit pupils.

[aleixandrus]

I'll check that info, kind of new to me. Thank you!

PS: I've received the 90 APO yesterday but, as expected, rain is announced for the next two weeks. I think I have some time to get the oculars for the first test 😅

[ScouseSpaceCadet]

On 22/02/2024 at 19:27, Louis D said:

Check your cylinder (CYL) diopter amount for your observing eye in your eyeglass prescription.  It is a measure of how much astigmatism you have in each eye.  I have around 2.0 diopters in each eye, so I need to wear eyeglasses all the way down to fairly small exit pupils to avoid seeing spiky stars and blurred fine details.

Tele Vue has this chart for their DIOPTRX™ line:

It's fairly accurate for most observers.  For instance, I can get by without correction below 1mm exit pupil.  It still improves the sharpness a little bit by putting eyeglasses back on at sub-1mm exit pupils, but not tremendously as at larger exit pupils.

Also new to me. Thanks Louis.

SGL just keeps on giving! 😀👍