Refractor vs SCT

[damnut]

H There,

There will always be the debate between sct & refractors. I have both and another on the way! My main scope is a 10" sct, heavy and when you set up its fo the evening not for 20 mins! So I also have a short tube 120mm and an alt az mount for hols a 102mm mak because I liked it and a 127 mm refractor on delivery.

So which is best, the one you use for the moment, but I guess like me (grey hair) kit accumulates along the way...

Good luck with your choice and for opening pandoras box!!

Best Regards

Damian

[MjrTom]

Thanks again for your input guys, much appreciated

However I spoke too soon and missed out on that second hand Dob so it's back to the drawing board again for the moment at least...

Still fancy a 8 inch SCT but really could do with checking one out first before parting with such a wad of cash....

[MjrTom]

Hello again,

Thought it might be time for a quick update.

Last nights sky was pretty clear and being a wednesday was the Liverpool astronomy society meeting I thought I would go along and see who was about.

Not a massive turn out but what it did allow me to do was check out a few newtonians and a Meade SCT telescopes on some familiar objects.

Firstly the 16 inch Dob (Meade Lightbridge), very nice views and very bright. first thing to note was being an f/4 the coma was very noticable and only really the centre 10% of the field was pin sharp. the views of M13 were spectacular. Detail resolved right down to the core and even the Propellar was visible.

Next I looked at an 8 inch SW Newtonian mounted on an EQ mount.

Also a very nice scope. The difference between the 16 inch and the 8 inch is huge with respect to brightness however optically a touch slower it still showed an irritating amount of Coma. The views were very nice, pretty contrasty and sharp (in the centre at least) The ring nebula looked very nice even with a lot of LP from the moon.

Lastly I got a look through a 8 inch SCT (Meade LS 8" ACF)

Wow very nice! very similar in performance to the 8 inch newt but without all the coma. The contrast was slightly lower but absolute sharpness did not seem to be much different. The stars were sharp from the centre to very close to the edge.

The ring nebula looked very similar to when viewed through the 8" Newtonian. not really any more detail could be seen and would call it a draw at this stage.

Well to summarise, I want a 16inch Dob lol. Well not really as they are massive

So realistically it looks like the SCT has won me over.

I was really impressed with the physical size of the SCT and the views it gave.

I suspect that my mind is now made up and I shall start looking at the C8 SCT's hopefully not a great deal different from the Meade LS ACF I looked through yesterday.

On a side note since conditions were pretty good I checked out the moon last night with my Equinox 80. My intention was to see how higher magnification I could pull from this scope. I paired the scope with the trusty 3-6mm Nagler Zoom and pushed the magnification upto 167x which I have used many atime before. Realising that conditions were pretty good I added the 2.5x powermate to the EP and pushed the magnification higher. It only started to fall to pieces at approx 250x amgnification which is pretty impressive I would say from a 3 inch scope.

Thanks to all who suggested I should push it further!

Anyway Thanks!

[Moonshane]

good stuff Mark

check out my 16" dob build thread. I bought a used older version paracorr with a tunable top and used it for the first time last night. my scope is f4 and there was virtually no coma visible even in my 26mm Nagler. they really do work superbly.

[MjrTom]

Thanks Shane, a Paracorr would be needed if I go the newtonian route, I cant abide stars not being round.

The question is do I go for the fastish 8 inch newt with a Paracorr or do I go for a SCT on its own. Both cost approximately the same. (£800)

[Moonshane]

for me it depends what you want to observe as the SCT will presumably have a much narrower field - that said, most things will still fit in said field.

[MjrTom]

Thanks again Shane.

Yeah difficult choice.... The maximum FOV the SCT can muster is just over 1 degree of the sky with a 2 inch EP. This is bassed upon a 2000mm focal length.

Vs about 2 degrees on an 8 inch f/5 Newt.

[ollypenrice]

It's a funny thing but our 20 inch F4.1 Dob has remarkably little coma, something that has surprized me and a number of others who've used it. The default EP is now a 26 Nagler and while I don't say there is no coma I would say that you do have to look for it.

I does have some astigmatism due to the secondary, a new one being in the pipeline. The mirror, apart from its freedom from coma, is of OK quality but not in any way exceptional.

Olly

[MjrTom]

Thanks for your input Olly

So not all newtonians are cut from the same cloth then

Some of the society do think the Meade LB 16 inch is not as good as it is made out to be... both optically and definitely its build quality.

[Moonshane]

I think there's a lot of talk about the meade primary mirrors being stuck down with double sided foam tape and many owners remove this with dental floss and refit with carpet tabs. this might be the source of some issues I suspect.

my cell was made so the mirror sits under its own weight and is free to move, held in position effectively by two teflon strips 10mm wide each at 45 degrees from the bottom centre. the mirror clips are a good 6mm above the mirror too. this way the mirror can flex freely when cooling but is pretty much immovable when collimated.

[MjrTom]

Thanks Shane,

I know the Society Committee had a load of hassle with it when it arrived and had to do a lot of modding before it was fit for use....

Maybe using this scope as a reference is going to skew my results a bit if it is atypical.

[ollypenrice]

In my Dob the mirror cell is pure John Dobson. It lies on a sling passed under it and sits on an 18 point flotation system. Nothing holds it in at all, though there are three stops, not in contact, which would stop it falling forward if ever it tried to do so. I have never touched the sling in five years. So no pressure of any kind is applied to the mirror (other than the gravity which pulls it onto the sling.)

Ralf Ottow found that the secondary was too rigidly bonded to its PVC support so he and I sawed through the silicone and redid it in a much more mnimalist and sparing way. 75% of the astigmatism disappeared at this point.

Olly

[MjrTom]

thanks again Olly,

After following Shane's 16 inch Dob thread I have been sucking up all the knowledge on mirrors and mirror supports should I end up with a newtonian down the road.

So not only the quality of the optics are important but just as much as the physical engineering of the optical train.

I hate to buy something to find it does not meet expectations. I tend to only buy things once... buy quality, well with exception to the refractor which its focuser sucks. Well sometime you learn the hard way lol

[michael.h.f.wilkinson]

Thanks again Shane.

Yeah difficult choice.... The maximum FOV the SCT can muster is just over 1 degree of the sky with a 2 inch EP. This is bassed upon a 2000mm focal length.

Vs about 2 degrees on an 8 inch f/5 Newt.

I get 1.38 deg with a 40mm Paragon with my SCT (which is indeed not a wide field master), which is big enough almost al objects. Otherwise I have an 80mm APO for wide-field observing. The F/5 Newtonian can theoretically double the SCT FOV, but at an exit pupil of 8mm for the Paragon. Moving to an exit pupil of 5, the 26 Nagler would yield 2.13 degrees (which doubles the area seen).

The SCT actually has fewer hassles with collimation than the newt, and is much lighter. It also beats the APM 80mm triplet APO by miles on planets.

[Mr Spock]

Same here, 1.3° with my C9.25, a 42mm 72° LVW. I use the C9.25 for planets and deep sky, and an 80ED for widefield - 5° with the 42mm LVW

The widefield 'superstar' is the 80ED with a 22mm T4 Nagler - 3° and a lovely rich field full of star colours.

[michael.h.f.wilkinson]

Same here, 1.3° with my C9.25, a 42mm 72° LVW. I use the C9.25 for planets and deep sky, and an 80ED for widefield - 5° with the 42mm LVW

The widefield 'superstar' is the 80ED with a 22mm T4 Nagler - 3° and a lovely rich field full of star colours.

Same here, I get 3.76 deg with the 22mm Nagler, and 5.6 with the 40mm (as finder really, the exit pupil is a bit big). The 80 mm is the best widefield scope I have, and bagged me such gems as the North America Nebula and Pelican, and the Veil

[Nexus 6]

Michael is spot on with regard collimation of an sct, my 925 is still holding up well 2 years later, I have moved it around a lot and wondered how it may perform as time went by, point proven when I was using it last Saturday/Sunday morning, one of the best nights I have had with it , I tried a few double stars with the Moon being so prominent, Albireo was a stunning sight with both stars holding steady and showing wonderful colour!

Am still to collimate it!

[MjrTom]

Thanks Michael and Allan

That is very promising info thanks.

In all likelihood I shall have both the SCT/Newt along side the current refractor on a dual head Alt/Az mount.

[Nexus 6]

Thanks Michael and Allan

That is very promising info thanks.

In all likelihood I shall have both the SCT/Newt along side the current refractor on a dual head Alt/Az mount.

why stop there? surely you need a dob too

must admit I am hoping to get hold of a large one myself, although I have re evaluated what size that may be as 16" and upwards do appear to be a bit more than one person can handle

[michael.h.f.wilkinson]

why stop there? surely you need a dob too

must admit I am hoping to get hold of a large one myself, although I have re evaluated what size that may be as 16" and upwards do appear to be a bit more than one person can handle

A well, they are still compact compared to the APM 530mm APO refractor (that's aperture not focal length )

[great_bear]

Personally I think planets aren't really interesting until you get 7" or more in aperture *and* a tracking mount. I've not seen tiny wispy cloud detail on Jupiter on smaller scopes. Maybe others have? Also, whilst I know skilled dob owners are proud of their tracking skills, but I feel that you really need to relax and really "probe" with your eyes to see details - and I struggle to do that if I'm manually moving the scope. With tracking you can just concentrate on probing for detail and waiting for those few fleeting moments of perfect seeing.

Also, I'm very happy with my Mak 180 - but I do appreciate having wider scopes to hand as well...

[michael.h.f.wilkinson]

Personally I think planets aren't really interesting until you get 7" or more in aperture *and* a tracking mount.

I know skilled dob owners are proud of their tracking skills, but I feel that you really need to relax and really "probe" with your eyes to see details - and I struggle to do that if I'm manually moving the scope.

With tracking you can just concentrate on probing for detail and waiting for those few fleeting moments of perfect seeing.

Also, I'm very happy with my Mak 180 - but I do appreciate having wider scopes to hand as well...

Very true. My RA motor control broke down recently. That's OK when the 80mm is mounted on the Great Polaris mount, but problematic when viewing planetaries with the C8. With planets, and even higher magnification it gets worse. I fixed the control now, hence the clouds and rain

[Moonshane]

Personally I think planets aren't really interesting until you get 7" or more in aperture *and* a tracking mount. I've not seen tiny wispy cloud detail on Jupiter on smaller scopes. Maybe others have? Also, whilst I know skilled dob owners are proud of their tracking skills, but I feel that you really need to relax and really "probe" with your eyes to see details - and I struggle to do that if I'm manually moving the scope. With tracking you can just concentrate on probing for detail and waiting for those few fleeting moments of perfect seeing.

Also, I'm very happy with my Mak 180 - but I do appreciate having wider scopes to hand as well...

for higher powers I totally agree that tracking is a real boon. I am likely to make an equatorial platform for my big dob for this reason in due course.

[MjrTom]

Thanks guys!

Yes that was one thing that was troubling me. I really am not a fan of equatorial mounts and have always preferred the ease of an Alt/Az mount.

The focal length of the SCT is slightly off putting but the way I looked at it is that its the magnification that makes the difference. In the UK I have very rarely been able to use more than 250x magnification due to seeing conditions.

How I achieve this magnification could either be a 1000mm scope with 4mm EP (newt) or a 2000mm scope with an 8mm EP. (the SCT)

So either way the Newt or the SCT still shall perform the same in this respect.

Both will have the same FOV and the object would move at the same rate across that FOV.

I have never minded hand tracking my current setup at 167x on an alt/az especially with very wide angle eyepieces.

I guess its also not much different than using this magnification on a big dob as well I guess the majority are not driven

If you are interested I have picked up this mount which is what I have in mind for this prospective OTA.

GiRo III - Alt/Az mount for up to 18kgs - Twin view for 2 instr.

With a photo of the current setup for scale.

[Moonshane]

I was tracking the moon easily at 300x last night with my 16" dob and with my Nagler zoom (50 degree field). this sort of tracking is made much easier when you are seated to observe.

for manual tracking I tend to use the drift...nudge....drift......nudge technique.