Refractor observation

[Galaxyfaraway]

While waiting for the moon eclipse, some thoughts and questions I have been pondering about today:

Yesterday I set up my WO GT81 refractor and Celestron EdgeHD 8" SCT side by side to check what I can or can't see through either of them.

I expected to be amazed with the SCT but instead noticed something curious: even though the FOV was obviously greater at first with the GT81 refractor, when I zoomed in with the eyepiece to bring it roughly to the same FOV as the EdgeHD, I felt the same stars appeared clearer with the GT81 (I could also see more of them). Is that supposed to be the case? I can't quite get my head around this and how the the maths & physics of it works to understand what's going on. (I now regret not paying enough attention in physics classes).

So the GT81 has a focal ratio of f5.9 and a focal length of 478mm while the EdgHD 8" has f10 and a focal length of 2032mm. Big difference and in theory, I should be able to see much more with the EdgeHD, no?

The eyepiece for the EdgeHD I have is 40mm (resulting in a magnification of 50x if I got it right?) and the eyepiece for the GT81 is a zoom eyepiece of 6.5mm to 19.5mm. (To get to roughly the same 50x magnification with the refractor I would need to zoom to about 9.5mm with the eyepiece?).

In any case, I am not quite certain how the eyepiece affects the focal ratio. Obviously the higher the magnification the more light is 'lost', resulting in less clarity/detail in the image. I wonder at what point does the EdgeHD 'overtake' the GT81 in terms of photons acquired? (As in, at which magnification or focal length does the focal ratio on the GT81 becomes <f10?? Would it be at the focal length of 810mm? i.e. f10 * 81mm. So to get to 2032mm on the GT81, the focal ratio wold need to drop to around f25?. So it really can;t be that the objects looked clearer at f25 on the refractor compared to f10 on the SCT, right?)

Presumably there are also other factors at play since one refracts light and the other reflects it? (Does it mean there is further loss of photons due to the SCT being a reflector of light? And that the f10 on the refractor with the same focal length would still be better/clearer?)

I guess I am trying to work out and understand the 'usefulness' of the SCT versus my refractor if the image isn't that much better on a f10 SCT. 

In the absence of eyepieces (when it comes to imaging): is having a faster refractor with a shorter focal length still preferable than a slower SCT? The image presumably can still be cropped so the FOV is not so relevant (or only relevant to an extent). But then the sensor size probably also plays a role in the calculation?

In any case it would be good to put some numbers on it or perhaps just to explain where the usefulness of a 'slow' SCT lies. (I feel guiltier for spending so much on it, if it might not be of much use after all!)

Apologies for the ramblings.

gfa

[Thales]

Hi, I'm sitting here waiting too - saw your post. Very interesting.  There will be better answers I'm sure, but for my tuppence worth I'd think that the difference is mainly due to the slower and faster f ratios?? f/10 will darken the fov while f/5 will brighten.  The same effect can be seen in a camera with full aperture focussing lens in filmn slr cameras.  ie you focus at f2 to get a bright composing image and then click a button to stop down to the f number you want to see what's in focus - but it will always darken the fov...

[rwilkey]

Hi there, the GT81 is a rich field scope, so you should expect better detail and sharper stars in my opinion.  Just because a scope (the SCT) has a longer focal length does not make it better at starfields or even DSO's.  The SCT's and similar excel at planetary observation, though in my view the refractor you have will beat it on every occasion.

[Galaxyfaraway]

"Just because a scope (the SCT) has a longer focal length does not make it better at starfields or even DSO's."

 

Yes I gathered that. I gather it's all about the focal ratio (and hence how much light can be gathered by the OTA). However for the GT81 to achieve a similar focal length as the EdgeHD, the focal ratio would need to up to f25 (if I got this right - I am not certain that I have), compared with f10 for the EdgeHD. I am not certain why the GT81 should still be beating the EdgeHD because the numbers certainly imply that it shouldn't.

Is there a back of the envelope formula to figure out at which point a SCT or a Newtonian would be yielding better results than a refractor by any chance? Or is this too subjective?

Thanks, gfa

 

 

[Galaxyfaraway]

I just found this:

Under average seeing conditions, a useful rule of thumb in astronomy is that a good quality 3" to 4" refractor can often outperform an average quality 6" to 8" reflector or catadioptric telescope for seeing details on the Moon and planets, splitting binary stars, and resolving globular clusters. 

https://www.astronomics.com/why-buy-a-refractor-telescope_t.aspx

It seems to bear out what I found (and what you were saying). Why then do people still purchase SCT's or Newtonians (the latter are much heavier and suffer from all sorts of problems)? My 3.2" refractor (WO GT81) was cheaper than the 8" SCT (EdgeHD). Are there circumstances where the EdgeHD would beat the refractor?

It seems a bit of a con (for people who don't quite understand about optics, myself included).

[MattJenko]

Can we rule out any mis-collimation of the SCT as a cause?

[Thales]

 

 

 However for the GT81 to achieve a similar focal length as the EdgeHD, the focal ratio would need to up to f25 (if I got this right - I am not certain that I have), compared with f10 for the EdgeHD. I am not certain why the GT81 should still be beating the EdgeHD because the numbers certainly imply that it shouldn't.

Hi again, my reading is that for star fields and larger dsos it is the *shorter*, not the higher f ratio of the 81 that's advantageous -ie the f/5.9 would give you brighter views and better fov than the longer focal length sct.  However, dependning on the optics, I don't think you can 'push' magnification of the 81mm lens as much as the 8" on moon and planetary objects (this is what I've been told, so was planning to go for 9.25 or 11 sct myself if wanting to outclass my current 5" refractor) . So I'd expect your sct to hold its magnification better across the fov (again, just repeating what people have told me). As to Newtonians, I would get one because you do find large apertures and low focal ratios in those - I've seen f/4.5 on 8" and 17" scopes!  So I'd expect lots of light capture and bright images - but I don't think that can be achived with the sct design.  I hope others enter this thread as I'd like to find out if I am correct too!!

[Galaxyfaraway]

"Can we rule out any mis-collimation of the SCT as a cause?"

It's completely new. There shouldn't be any. But I will check, thanks, gfa

[Davey-T]

Decent refractors used to be horrendously expensive even for a 3" one which may have led to the rise of the SCT packing a long focal length into a manageable size and being mass produced by Meade and Celestron, a bit of competition helping to keep the prices down.

Nowadays unless you're a dedicated planetary person with a 12" - 14" SCT then a refractor is probably preferable, I don't see any point in buying a 6" SCT but that's just my opinion, I have a 10" SCT and it's useful for imaging small galaxies and nebula but useless for imaging stuff like NGC7000 or M31, a 2000mm  f/10 refractor is quite long so an SCT may be more user friendly.

So as usual you pays your money and takes your pick but really nice to have the best of both worlds, I think around 5 scopes covers most things then there's Solar 

Dave

[John]

I just found this:

Under average seeing conditions, a useful rule of thumb in astronomy is that a good quality 3" to 4" refractor can often outperform an average quality 6" to 8" reflector or catadioptric telescope for seeing details on the Moon and planets, splitting binary stars, and resolving globular clusters. 

https://www.astronomics.com/why-buy-a-refractor-telescope_t.aspx

It seems to bear out what I found (and what you were saying). Why then do people still purchase SCT's or Newtonians (the latter are much heavier and suffer from all sorts of problems)? My 3.2" refractor (WO GT81) was cheaper than the 8" SCT (EdgeHD). Are there circumstances where the EdgeHD would beat the refractor?

It seems a bit of a con (for people who don't quite understand about optics, myself included).

My 12" F/5.3 dob outperforms my ED120 refractor under all but the poorest conditions. On the planets the refractor gets closer than you might expect given the large aperture difference and on double stars the tighter star images of the refractor are more aesthetically pleasing. On deep sky objects there is no contest at all of course - 4.7" just can't get close to 12" of light gathering power.

My 12" dob takes around the same amount of time / hassle to set up as the refractor does (not much for either as they are both alt-azimuth mounted).

I had a 4" ed refractor and an 8" SCT at the same time for a while and found a similar situation to the above. The 8" SCT (a Celestron) actually gave me some of the best views of Saturn I've ever had with any scope on a couple of memorable occasions. SCT's do need to be in good collimation and well cooled to give their best though. I do wonder if some that are in regular use are not collimated as well as they could be ?

[Davey-T]

As you say GFA a 4" refractor is probably a better choice than an 8" SCT / Newtonian but just see how much a 6" apo or triplet refractor is so after about 6" reflectors rule the bigger the better 

Dave

[Galaxyfaraway]

Interesting. Many thanks for the replies. I will have to do some more testing.

thanks! gfa

[Stu]

Hi Gfa,

I have a similar combination of scopes ie a Tak FC-100 and an EdgeHD 8". I have tested them side by side, including a lengthy session under dark skies in Devon recently.

To me they are both excellent scopes, but do different things well. The refractor just has beautifully resolved stars and lovely contrast. Last night on the DC it was just incredible, really lovely. It is quick and easy to set up and does not need much cooling and no collimation. It is lighter and more convenient than the SCT and will take high power (well over x200), whilst also allowing fields up to around 3.5 degrees.

The SCT is a very different beast. It is heavier, needing more counterweights and more complex in terms of its cooling and collimation requirements. It is also a dew magnet so needs a dew strip and dew shield to keep it clear. It also suffers from mirror flop so I have had to fit a second focuser on the visual back to allow me to lock the mirror.

So, why bother?

From a visual observing perspective, compact aperture is what I would say. I go away camping every year and have never managed to fit anything bigger than a 4" refractor into the car until I got the SCT. Being able to easily get a decent sized scope under dark skies is great. This really comes into play for DSO observing where the greater light grasp does make a significant difference. I saw some incredible sights down in Devon and the frac just would not have gone anywhere near as deep. The narrow field of view is actually not so much of an issue on most DSOs, you can get around 1.2 degrees with a decent wide field eyepiece which covers all but the largest objects.

I'm unsure about the reason for your results, the SCT should show significantly more and fainter stars than the refractor at the same fov, assuming they are there to be seen. A good trial would be to point them both at a faint DSO, perhaps M1 and see how they compare.

The other benefit of aperture is resolution. Compare the view of M13 in both scopes, and you will see far greater numbers of stars resolved in the 8", so it is not just a brightness thing.

The frac is the best option for deep sky AstroPhotography and wide field observing. It is faster, has a shorter focal length and will be much easier to guide. The SCT will blow it out of the water for planetary imaging though in pure resolution terms.

I think you have got a little muddled about focal ratios and lengths. The focal ratio of the scope essentially remains constant regardless of the eyepiece you are using. This is not totally true for the SCT but for the purposes of this discussion let's say it is. So the SCT is fundamentally a slow scope and is easy on eyepieces, but slow to accumulate data for AP. Conversely the GT81 is fairly fast, and will be a little more demanding in eyepieces but not too bad. Barlows and focal reducers are the main accessories which extend or reduce the focal length of a scope, and thus slow it down or speed it up.

Finally (!) star shapes. As I mentioned, the frac will probably give you better star shapes under most circumstances. SCTs just always have a slight 'fuzziness' to them which is not as crisp. I find my refractors more capable of splitting double stars than the SCT, but certainly practising collimation and ensuring proper cooling will go along way to maximising the image quality, and don't forget those brightness and resolution benefits to balance up the argument.

That's all, sorry for the long post!!

Cheers,

Stu

[Moonshane]

For visual observing, focal ratio has no effect on image brightness or photons gathered assuming the same magnification. This and resolution/ image scale are due to aperture of the scope.

[Peter Drew]

Try pointing the GT81 and the 8" SCT at M13 and see if you can spot the difference. 

[Galaxyfaraway]

Many thanks Stu for a fantastically informative response.

Still so much to learn.

Next stop: collimation & mirror flops...  

(Btw does anybody know how I can point both scopes on the same target? I have a side-by-side set up on a Azeq6 mount: like this one: http://www.optcorp.com/media/catalog/product/cache/1/image/400x400/9df78eab33525d08d6e5fb8d27136e95/1/7/17761-L.JPGbut have not found a way to align them.)

Thanks !!

gfa

[Stu]

You are welcome

Mirror flop may or may not be an issue for you whilst observing, but if you choose to image with it it almost certainly will be. SCTs focus by moving the primary mirror, and when you change direction of focusing they can shift slightly, causing the image to shift and making it harder to focus.

When imaging, when the scope goes past the meridian then again the mirror can flop slightly, causing issues.

Spend some time with the scope first to see how much of a problem it is for you before looking at solutions. Don't forget to unlock the mirror locks before changing focus, that can make mirror flop worse!

Here is my solution which is what many people end up doing. There are plenty of options out there all the way from Revelation up to FeatherTouch so it doesn't have to cost a fortune!

Regarding side by side mounting, I assume you are using the mount in EQ mode? For visual it may be worth using it in AltAz mode; I have the same mount and have never used it in EQ! It is easier in terms of alignment, and the eyepiece position remains easier to access regardless of where the scope is pointed. The other benefit is that you can mount the second scope on the end of the counterweight shaft and have both scopes side by side. This method does have adjustment in altitude, but not azimuth. I use a Skywatcher guide scope mount with a dovetail bolted to it to allow me to properly align the two, use this for the lighter scope.

This is the guide scope mount I use, needs a vixen dovetail attached to mount it on the AZEQ6.

http://www.firstlightoptics.com/guide-cameras/skywatcher-guidescope-mount.html

Hope that helps, let me know if you have any questions.

Cheers,

Stu

[glowjet]

Great set up Stu, like

[Galaxyfaraway]

Amazing stuff. Wonderful set up. Thanks very much. Do you only do visual? (if you haven't yet used the mount in EQ mode).

What is your collimation product? I can't quite make it out. I guess it's made by Baader from the pics.

I don't yet know what the symptoms are of a badly collimated SCT but will read up on it.

I see you are using telrads on both scopes. I only bought one red dot finder (from William Optics) which completely transformed the search in the night sky for me. I presume in order to have both scopes set up and fully functional I will need individual reddot finders (or telrads) on each scope otherwise it will be too much hassle to keep swapping them between the two (and presumably having to adjust them each time as well).

I haven't yet tried the AltAz mode on my mount. I can manage to mount both of the scopes side by side (with a side by side rail posted above) but will give the AltAZ mode a go once I get the guide scope mount to make this set up useful.

Thanks again and fantastic set up!

gfa

[Stu]

Amazing stuff. Wonderful set up. Thanks very much. Do you only do visual? (if you haven't yet used the mount in EQ mode).

What is your collimation product? I can't quite make it out. I guess it's made by Baader from the pics.

I don't yet know what the symptoms are of a badly collimated SCT but will read up on it.

I see you are using telrads on both scopes. I only bought one red dot finder (from William Optics) which completely transformed the search in the night sky for me. I presume in order to have both scopes set up and fully functional I will need individual reddot finders (or telrads) on each scope otherwise it will be too much hassle to keep swapping them between the two (and presumably having to adjust them each time as well).

I haven't yet tried the AltAz mode on my mount. I can manage to mount both of the scopes side by side (with a side by side rail posted above) but will give the AltAZ mode a go once I get the guide scope mount to make this set up useful.

Thanks again and fantastic set up!

gfa

Thanks gfa

Yes, I only do visual, other than the odd dodgy iPhone shot

Not sure what you mean by collimating product? The first three images are a Baader SteelTrack focuser, with a SteelDrive motorized unit fitted so I don't have to touch the scope when fine focusing. It is not a collimating product.

This thread may be interesting to you. Post 43 has a link to information about collimating SCTs properly.

http://stargazerslounge.com/index.php?/topic/248523-Living-life-on-the-Edge%2E%2E%2E%2E%2E%2E

In terms of finders, I actually have a Rigel on the SCT and a TelRad on the Vixen. Both work very well. If your scopes are aligned properly on the mount then you only need one finder. You could just have the RDF on the refractor, then use a widefield eyepiece to find the object in the frac, it will then be in the SCT field of view. If using them separately then yes, having a finder for each is easier in terms of not having to realign them when switching between scopes.

As I said, the AZEQ6 can align vertically so it may be worth having a go to see how close they are in azimuth. The guide scope mount certainly makes it easier to do and possible to achieve very accurate alignment. I definitely think that the AltAz mode is the way to go for visual.

Stu