Is Upgrading from 90mm Refractor to 150mm reflector Worthwhile?

[Ean]

I'm a very casual observer and have a Skywatcher 90mm f10 refractor on an AZ3 ALT AZ mount, and would like a bigger telescope (but want to keep the 90mm refractor). I need to be able to carry the telescope and mount for 1/2 mile in one load, and I'm a pensioner. Using a rucksack for mount and accessories and carrying the telescope in my arms works for the refractor. The trip can't be done by car, and a wheelbarrow or dolly would be a liability. Observing from home is not an option. A 200mm would be ideal, and I like the Skywatcher f5 Explorer OTA, and its price, but at 8.8Kg is too heavy. The Skywatcher f5 Explorer 150mm OTA  is a lot better at 5Kg, about the maximum weight for me. I considered the Skywatcher Skymax 127mm Mak OTA, but the aperture is smaller than I would like. Where magnification is concerned, I prefer to err on the low side; even at the risk of missing out on the finest detail at times. It's more relaxing, the image is clearer and there's less shake.

I'm looking for a BIG difference, and as far as I can gather upgrading from my 90mm to a 150mm f5 Newtonian would be very broadly equivalent to upgrading from a 90mm refractor to a 115mm refractor, which to my mind is probably not worth the money or time. Having said that, excluding secondary mirror and spider loss, the 150mm Newtonian has about 2 1/2 times the light gathering capability as the 90mm refractor. I like using the AZ3 mount, and want to continue using it, and it's only about 3.7Kg weight. It's probably  a bit like using a Dob (too heavy for me) but with easy to use slow motion controls. The maximum rated total load for the AZ3 tripod is 10 to 12Kg.

My main interest is in the Moon and more accessible planets. Even the "easier" DSO's are a a bit of a struggle and are unimpressive with the 90mm, but  might try for the more accessible DSO's with a bigger telescope even though this won't produce miracles.

The bottom line is, is there much point for a casual observer in upgrading from a mid-priced 90mm refractor to an f5 150mm Newtonian (an f 6 or f7 would be preferable but more bulky and a bit more heavy)? The cost is about £225 for the 150mm OTA, a good 8 or 9mm eyepiece would cost about £50 and a ND Moon filter about £10, with about £12 delivery. This is driving me potty, please advise.

 

[Stu1smartcookie]

I have a WO ZS73 and i have just bought a 130PDS so in relation to your question its about the same difference in aperture . i like to think that both scopes compliment each other rather than directly compete . My zs73 will be used for observing and for photograhy , as will my 130. But the increased aperture will come in handy on the 130 . 

Stu

 

[Zermelo]

This seems to be your first post, so hello, and welcome to SGL.

I don't know how much you've looked around yet, but here are some links you might find relevant to your situation:

https://stargazerslounge.com/topic/196278-what-can-i-expect-to-see

https://stargazerslounge.com/topic/360627-refractor-vs-reflector

https://stargazerslounge.com/topic/357889-reflector-vs-refractors

https://stargazerslounge.com/topic/347526-my-4-refractor-is-seeing-deeper-than-my-51-reflector

 

Edited February 22, 2022 by Zermelo
typo

[mikeDnight]

I think you'll see things in a whole new light once you look through a 150mm reflector. Don't believe the old 3" refractor = 6" reflector nonsense in good old Patrick Moore books, it simply isn't true. There may have been a little truth in such a claim back when mirrors were made of speculum metal, or made by an inexperienced amateure over the kitchen sink, but not today. A 150mm reflector will have superior resolution and light grasp, and will be able to use higher powers. Don't worry about secondary obstruction, as it has very little effect to the light recieved and essentially no effect to the scopes resolving power. In the same way that a 4" refractor offers a genuine sweet spot in refractor apertures, so the 6" reflector is the often under rated workhorse of a serious observer. 

[vlaiv]

Hi and welcome to SGL

I'm a bit confused by your post - or rather some parts of it.

First - there is going to be quite noticeable difference between 90mm and 150mm - even if one is refractor and other is reflector. This holds true for both planetary views and DSOs.

Here is my first question to you - you say that you walk quite a distance, yet you mention that you mostly are interested in the Moon and planets.

You don't need dark skies to observe those. In fact - sometimes it is even better to have some light sources around you so you don't get dark adapted when observing moon and planets - as dark adaptation lets you see fainter stuff but reduces sharpness of your vision and color sensitivity - that helps to see detail in planets and the moon.

What is your budget?

From what I've gathered, maybe best scope type for you would be 6" SCT? It is still rather light weight - below 4Kg, and will sit nicely on that Alt az mount. Here is second thing that I don't fully understand - you mention that you prefer low power views.

There are couple of main drawbacks of SCT design (in my view) that limits its usefulness as all around scope - first is price. It is much more expensive than newtonian of same aperture (for some unknown reason), it has corrector plate that is often dew magnet (can be sorted with dew shield and dew heaters) and is F/10 system - which means that it is rather limited at wide field views.

When you say low power views - do you mean that you like to observe the moon at x100 instead of say x180, or you mean that you enjoy powers around x30-40?

With SCT 6" - you'll be limited to powers above x50-60 unless you go for very narrow field of view or accept serious vignetting (with 56mm plossl you would get 1500/56 = ~x27 but I seriously doubt that C6" will illuminate full 47mm field stop).

In any case - I'm not really sure that AZ3 will handle F/5-F/6 6" Newtonian. It is worth upgrade and certainly exceptional value for the money. If you want more compact scope (and money is not issue) - look at C6. Maksutov scopes will give you even higher magnification but are good planetary scopes. If you want 6" one - maybe check out 6" CC like this:

https://www.firstlightoptics.com/stellalyra-telescopes/stellalyra-6-f12-m-crf-classical-cassegrain-telescope-ota.html

It is a bit heavier at 6Kg, but does not have front corrector plate. It will properly illuminate something like 56mm 2" Plossl which will give you rather decent low power view at x32, and it will be excellent planetary scope.

Since you have 900mm refractor already - x32 is like using 28mm plossl on that scope (a bit less magnification than supplied 25mm EP but a bit more than say 32mm plossl).

Hope this helps.

[rl]

Welcome to the site.

Considering your requirement to carry the scope plus mount half a mile in one go I'd look very carefully indeed at the weight. 6" newts come in a variety of focal lengths from f/5 through f/8 to f/11.  The f/5 option made by Orion Optics might be your best bet on this score since the tube is thin aluminium. There is also the risk of knocking the collimation out of kilter carrying it in the dark, and you won't want the extra weight of collimation tools et cetera.  But there again what makes the OO scope light also makes it liable to getting dented easily, and they are a lot more expensive than the far eastern offerings. Carbon fibre tubes can be light but they also scratch very easily! You might want to invest on one of the specialist carry bags if you go down this route. 

Optically I'd put a good 6" Newt ahead of a 90mm Achro but I'm not sure I'd call the gain BIG...on that basis I'd probably stick.  The gain will be most apparent on DSOs which will also suit your efforts for access to a good site. But I'd agree with MikeDnight that the 6" Newt is much underrated and they can be good for a lifetime of casual observing (and not so casual). The fact that they are cheap (at least the Chinese ones are) means they tend to get put down unfairly. They can often be seen secondhand for less than #100...which might be a relatively risk-free option for trying one out. If it's not for you then just sell it on for what you paid out....

 

[Ean]

Thank you for being so helpful in your replies. I can't figure out how to reply to people individually, so am answering the four preceding posts in this single post. Thank you also for your kind welcome.

Stu- thanks, the complimenting of scopes is an interesting idea.

Zermelo - Thanks for the thought-provoking links. The one on what you see through 4 to 8" telescopes, with hints on observing, was particularly helpful. I have done web searches, including Youtube, and haven't found a clear article or video on my particular query. It would be easier if I was buying my first telescope.

Mike - Thank you for your clear response. Your kit is in a different league to mine!

rl - Thanks for your helpful comments; yes, I need to be very focussed on weight. I've looked into the need for collimation. Apparently it can be done simply with just an eyepiece holder with a pinhole in it, and acentre mark on the main mirror, but a light source is needed. This is a slight reservation with Newtonians, as I'm reluctant to make adjustments to an optical instrument in a dark field. I'll look into OO scopes.

Vlaiv - Thanks for your detailed reply. My budget is total £350 for a 6"/ 150mm reflector OTA and £650 for a super lightweight 8"/ 200mm reflector OTA (which doesn't seem to exist). What I mean by using low power is that my favourite magnification for my 90mm refractor is 120X . Using the theoretical maximum magnification with this telescope of 180x degrades the image. For me, 120x for the 90mm is a "sweet spot"; it's easy to use and seems to bring the best out of the telescope. For a 6" I would go up to 160x to 200x.  Much above this, seeing needs to be very good anyway. I don't plan to do imaging or specialised wide-field observation. The AZ3 tripod has a specified load of about 10 to12Kg. With the f5 150mm 6" Newtonian at 5Kg plus EQ mount counterweights weighing maybe 2 to 4Kg the AZ3 may not handle it, but I'm using the far lighter ALT AZ head. For me the total load is about 6Kg.

Edited February 22, 2022 by Ean
Added text.

[vlaiv]

4 minutes ago, Ean said:

My budget is total £350 for a 6"/ 150mm reflector OTA and £650 for a super lightweight 8"/ 200mm reflector OTA (which doesn't seem to exist). What I mean by using low power is that my favourite magnification for my 90mm refractor is 120X . Using the theoretical maximum magnification with this telescope of 180x degrades the image. For me, 120x for the 90mm is a "sweet spot"; it's easy to use and seems to bring the best out of the telescope. For a 6" I would go up to 160x to 200x.  Much above this, seeing needs to be very good anyway. I don't plan to do imaging or specialised wide-field observation.

In this case - I do think you should seriously consider 6" CC. It is a bit over the budget but it will be more compact and easier to carry (although it is a bit heavier).

It will also give you better views than say F/5 newtonian. F/6 and slower newtonians will probably be on par or even better, but fast F/5 newtonian is not the best choice for planetary views. Collimation needs to be spot on and faster scopes usually have larger central obstructions.

For comparison, TS scopes from FLO:

F/5 newtonian has 63mm central obstruction, weighs in at 5.9Kg and is 690mm long

F/6 newtonian has 45mm central obstruction, weighs again 5.9Kg and is 845mm long

6" CC has 58mm central obstruction, weighs 5.8Kg and is 440mm long

In order to reach say x180-x200 magnification with F/5 and F/6 newtonians you'll need short FL eyepieces - or use of barlow lens. Good short FL eyepieces and barlow add cost.

With 1800mm of FL - you can just use 10mm plossl or ortho (those still have some eye relief) and you are at x180 with excellent sharpness.

15 minutes ago, Ean said:

The AZ3 tripod has a specified load of about 10 to12Kg. With the f5 150mm 6" Newtonian at 5Kg plus EQ mount counterweights weighing maybe 2 to 4Kg the AZ3 may not handle it, but I'm using the far lighter ALT AZ head. For me the total load is about 6Kg.

This part I again don't get - why do you need EQ mount counterweights?

Not sure where you got that load for AZ mount - quick search gave me this:

"The Skywatcher AZ-3 is a practical altazimuth mount, easy-to-use and providing good stability.

It can serve as a base for terrestrial or astronomical observing and support telescopes or spotting scopes up to a maximum weight of 5kg (refractors of up to 120mm aperture and reflectors of up to 150mm diameter)."

and this:

I had AZ3 mount and I did not particularly like it, but if you are fine with it - then I think it will work for you (better with shorter scope than with longer one).

Again, I don't want to pile up expenses, but if you want stable light weight mount, maybe check out this:

https://www.firstlightoptics.com/portable-astronomy-mounts/scopetech-mount-zero.html

Coupled with decent sturdy photo tripod (I know, more expenses) - it will be lighter and better option than az3.

 

 

[nfotis]

Hello, and welcome to SGL.

For moon and planetary views, there's no need to venture outside the city (in fact, I use my Maksutovs on the rooftop for planetary viewing and imaging). For this kind of target, long focal distances of your tube are very helpful.

If you can, try to upgrade to a better mount, the AZ3 will be unpleasant at 5+ kg of loading.

For lunar/planetary viewing, I vote Maksutov scopes like the Skymax 150, because these offer refractor-like views (I own a Skymax 127 and a Skymax 180 myself).

N.F.

 

[Ean]

vlaiv -

Thanks for going into this in such detail. I think that different sources, and seemingly good ones, can give different specs for the same item. I have seen the AZ3 tripod offered  for sale with an EQ 2 (or 3) head fitted with  a 6"/150mm Newtonian. I think this is too much weight.  I don't need EQ counterweights; I was just saying they are part of the tripod load if used as an EQ, such as on the telescope and mount just described. Your image is exactly the same as my mount, and the specs you give agree with a check I made. Better to get real knowledge from a forum rather than some gibberish from some jounalist who may have no experience of astronomy.

I agree the mount will be better with the f5 version of the 6" Newt rather than the longer f8 version. Having said that, something like the 6" CC would be better. I have been advised a couple of times in this discussion that a f5 6" Newt may be difficult to use on the AZ3, and have decided not to try a Newt on this mount. The centre of gravity is near the mirror end and could cause instability in the mount, varying with elevation of the scope. With a physically short instrument such as a Cass, Mak or Mak-Cass 6", the C of G would be close to the centreline of the mount at all elevations, and there would be less than half the windage.

nfotis -

Thanks for your advice. I agree that for observing the Moon and Venus (phases only) that ambient light doesn't matter - it can help so the eye doesn't become too dark-adapted when viewing bright objects. As well as street lights, my house is built into a hill. There are 13 steep steps leading to the front door and the hill limits my view to between about WSW and NNW. Otherwise I can't get lower than about 50 to 75 degrees above the horizon. I'm taking your advice on not using a 6" Newt on the AZ3, you're not alone in thinking this. A Skymax 150 at 5.6Kg, although a bit heavier than the Newtonian at 5.0Kg, would be more stable on an AZ3.

 

CONCLUSION.

I'm grateful for the high quality advice received, and for what I've been taught. It seems that I have been saved from buying a telescope which would not work well with my AZ3 mount.

The best option would be for me to buy a physically short OTA such as a Cass, Mak or Mak-Cass 6",but these telescopes are beyond my price range. A search for used scopes from FLO, other dealers and ebay suggests that there are a lot of Newtonians on Dob or EQ mounts for sale, but almost no physically short  OTA's. A possibility would be to buy a used telescope with mount and use the AZ3. Anyway, this is now all on me. Spend the money, or not.

For the forseeable future, I plan to stick with my 90mm f10 refractor and AZ3 mount. Interestingly, this (and accessories purchased separately) suited me very well when I bought it, and still suits me very well.

[cajen2]

Sorry to stick an oar in, but there's another option that nobody's mentioned: a Skywatcher 150p Flextube dob. This closes up to be much smaller than a standard 150 and is also lighter. Mine is easy to carry in one hand. If you don't fancy carting the dob base around, it has a Vixen dovetail as standard so could be used on your AZ3 (though I haven't used that mount). Seems to me that would answer all your requirements.

[Peter_D]

Would a light Equatorial Mount not be a better option in the longer term? Something like a Eq3-2? If you are interested more in Lunar and Planetary, the next upgrade I would suggest would be a motor or motors to keep the planet in the fov? The next upgrade would be be the scope - maybe a Bresser 127/1900 Mak (think I'm correct in saying it operates at a full 127mm aperture?). 

 

Just speaking from experience but adding the relatively low cost Sky-watcher motors onto my EQ mount really helped my planetary observations. I had used a SW Pronto mount and I just found the constant moving of the axes really off-putting.

Edited February 23, 2022 by Peter_D

[Ean]

Cajen2 - Thanks for the suggestion. Some weight would be saved on the tube, but as it's at the top of the tube, the balance problem with nearly all the weight at one end would be more critical than with the full tube.

Peter_D - Thanks for the advice, but I like manual operation, and any EQ mount including counterweight (and motor (s) plus battery) would be heavier than the AZ3. Simplicity is my thing. A Bresser 127/900 weighing in at 3.4Kg would be light enough, or the cheaper Sky-Watcher Skymax 127 OTA, but I prefer the bigger jump to a 150mm scope to get a good upgrade.

Prior to my search for a new scope, I knew about the traditional ring mounts, but had never come across these dovetail mounts. They seem to be supplied with physically short scopes. For my AZ3 a mating part to receive the dovetail on the scope would have to be bought and fitted to my AZ3. Alternatively, I could remove the dovetail from the scope (if possible) and make an adaptor plate of some kind. If there are tapped holes in the telescope tube I could possibly bolt it straight to the mount without the dovetail.

[Catanonia]

I love this thread. Full of goodness and support and why I love this hobby.

[cajen2]

1 hour ago, Ean said:

Cajen2 - Thanks for the suggestion. Some weight would be saved on the tube, but as it's at the top of the tube, the balance problem with nearly all the weight at one end would be more critical than with the full tube. 

Sorry, I don't quite understand the problem. The scope is attached to the dob base using the dovetail, which is long enough for a large range of balance points. It's easy to achieve perfect balance that way. To attach to a mount and tripod, you just do the same thing.

I know there are several people using their 150 Flextube on such mounts very successfully.

[cajen2]

Additional: have a word with @Stu. I'm sure he mounts his 150 that way.

[Stu]

27 minutes ago, cajen2 said:

Additional: have a word with @Stu. I'm sure he mounts his 150 that way.

Yep, works very well. In fact, I’ve never used the dob base!

@Mark at Beaufort does a similar thing with his 130p very successfully.

[Mark at Beaufort]

This is my Heritage 130P on a camera tripod using a Pronto mount.

[Alkaid]

I can answer your question having owned both.

Moon:

The 90mm achromat I found to be sharper on the Moon than a 150mm F5.   Although the 150 F5 would naturally reveal more, this was offset by the view being slightly softer.  (Yes, it was collimated).  

However, I have also owned a 150mm F8.  This was a different league and was equally as sharp to the frac, plus much more detail.  It wins hands down and I regret selling it.

 

Planets:  

The 90mm is not wonderful, it just lacks the brightness at higher powers.  The 150 F5 is much brighter, and more detail can be teased out.    The 150 F8 is same brightness as F5, but sharper and reveals much more.

 

Deep Sky:

The 90mm is not wonderful, although I have attained many of the Messier objects form a dark site when camping.   The 150 F5 is much better due to light grasp.    The 150 F8 is roughly the same as the F5, with slightly tighter stars.

[Ean]

cajen2 - Thing is I've been advised in two posts that the AZ3 would not work well with the 150mm Newtonian because of its weight and because the centre of gravity moves a lot because most of the weight is at one end. Yet apparently people have done something like this with a flextube150mm, mount unspecified. The 130p is a lot lighter and would be OK with my mount, but of insufficient aperture to give a good performance boost over my 90mm refractor. Some years ago I found an excellent article on tuning up an AZ3 mount. The author actually fitted a rod to the top of the mount with a sliding weight to counteract C of G movement for his Newtonian. I need to be certain that any rig I put together will be lightweight, stable and provide a significant performance boost over my trusty 90mm refractor. For some reason, I'm being as fussy about the possible purchase of a new telesscope as I might when buying a used car!

My approach is to have the gear packed, scope pre-cooled, watch the weather carefully, then if clear at a moment's notice carry it to the site, set it up and go. Useful in our unstable climate.

Catanonia - Indeed it's wonderful to receive such generous help!

Mark - Thanks for the images.

Alkaid - Thank you for your clear, helpful and detailed comparisons.

I've never seen one of these dovetail mounts. Are threaded holes provided in telescope tubes to attach the dovetail, so it can be bolted on from outside the scope, essential with refractors and cats?

Another question, adjustable parts, movements or possible flexing in any mechanical system can lead to inaccuracies and play. With only two rods in the flextube, does this ever result in loss of collimation? Is there any significant advantage of the flextube over the full-tube version except that the flextube takes less space in storage?

 

Edited February 24, 2022 by Ean
Added text.

[cajen2]

As to your last question, all reflectors will need periodic collimation, particularly if they receive knocks or jars. I haven't noticed the Flextube needing more than usual. The rods lock into place solidly.

 

[F15Rules]

Hi,

Given everything you said in your first post, I'd suggest you get a 127mm Mak for portability and weight. This scope would be good on your stated favourite targets and observing power. (x120).

Then, I would sell your 90mm F10 refractor (as it is not a widefield scope, and neither is the Mak), and look to buy something like a an F5 Skywatcher 102mm Startravel achromat..lovely widefield views, not expensive (especially used).

Both would ride well on your current mount and are very portable, which is key to you.

If you had any budget left over, use it to get the best eyepieces you can afford.

The only danger of owning both the scopes might be that you could turn from a "casual" to a "serious" observer!😊

Dave

 

 

[Ean]

F15Rules -

Yours is the second post to suggest a127mm Mak. I'm getting lots of suggestionson what instrument to buy, including 150mm f5 Newtonians! On the basis of 30 to 35x diameter in inches of the main mirror, would such a telescope easily take a magnification of 150 to 170x? I prefer this criterion than the 2x diameter in mm one, which assumes really good optics and excellent seeing. I like 120x for my 90mm refractor because there's very little dimming for one thing. If I bought a Mak, I would be prepared to sacrifice better wide field performance, a little more light collection (but have lower CA  in keeping my 90mm) and not buying the 102mm achro. This would keep me close to budget.

Is it difficult to collimate Mak or Mak-Cass telescopes?

Stu -

I think the above post from F15Rules is in accord with what you said about telescopes complimenting each other in your first post!

Edited February 24, 2022 by Ean
Clarification.

[RT65CB-SWL]

Hi @Ean and welcome to SGL. 

1 hour ago, Ean said:

Is it difficult to collimate Mak or Mak-Cass telescopes?

I have a Celestron C6/SCT and a ‘re-modded’ Meade ETX105. Of the two, the C6 would be the easiest to collimate. I have had mine for a number of years and not done any collimation to it since I purchased it. It appears to be ok with a collimation cap and my two laser collimators.

[Zermelo]

As it happens I have an F/5 Newtonian, a Skymax 127 and a widefield refractor, so perhaps I should declare a bias there.

I wanted to cover all most of the bases with kit that I could use on a middling mount with a 5kg limit. A longer reflector would have been too unsteady. A larger Mak would have been too heavy.

Collimating a Mak is harder than a Newtonian, but they almost never need it. My Skymax doesn't gather so much light so it's not great on DSOs, but it punches above its weight in other respects. When conditions allow I've used it at 300x, and on really good evenings it has hit the 200/ Dawes limit on resolution. With your preference for solar system (bright objects, small details), it would be worth a look.