Jump to content

Stargazers Lounge Uses Cookies

Like most websites, SGL uses cookies in order to deliver a secure, personalised service, to provide social media functions and to analyse our traffic. Continued use of SGL indicates your acceptance of our cookie policy.

sgl_imaging_challenge_banner_lunar_landings.thumb.jpg.b50378d0845690d8a03305a49923eb40.jpg

spaceboy

Is a 4" Apochromatic ever going to beat a 8" reflector?

Recommended Posts

Hi SGL Members

What's the thoughts on the above?? I know on the moon there's no beating the contrast & sharpness of the Refractor but on the more distant planets how would a 4" fair against the light grabbing power of an 8" reflector?

Thanks

SPACEBOY

Share this post


Link to post
Share on other sites

If the reflector has rotten optics or the coatings are shot or the collimation is miles out, the little frac might do as well. But really it shouldn't be a contest.

Normally a 8" scope will show objects about 1.5 mags fainter than a 4" scope ... irrespective of the type of either.

Share this post


Link to post
Share on other sites

Last weekend at SGL6 I was using a 6" f5 reflector which was well collimated and I was using my Ethos EPs. Even though I have a nice 4" APO frac (used this the previous night at SGL6) I was seeing fainter DSOs with the 6".

Mark

Share this post


Link to post
Share on other sites

I'm assuming you mean for visual use. It depends whether you are talking mass market or serious-money 4"refractors on the one hand, and what quality and design of newtonian on the other. An APM TMB 4" triplet for instance will leave any five or six inch reflecting telescope, of whatever description, stone dead on any target, and will naturally have better contrast and fewer diffraction effects on planets even against an optimal 8", though it won't out-resolve it. But I scarcely need to add that such performance doesn't come cheap. A refractor will always seem to offer a better image because its contrast is better, but that isn't the whole story. None the less serious planetary enthusiasts will pay huge sums for big high performance refractors.

The other point to bear in mind when comparing telescopes of dissimilar sizes is that poorer seeing frequently favours smaller apertures.

Share this post


Link to post
Share on other sites

Thanks guys.

It's how they would fair against planets was my main concern. I know the 8" due to aperture should be the winner regards DSO's but as Jupiter & Saturn are some way away I was wondering dose the light grabbing power of the 8" also win here also or dose the contrast of a budget Apochromatic 4" compensate for this and in the end give better planetary detail ?

Top quality Triplets are never going to be in my budget Neil :D

Edited by spaceboy

Share this post


Link to post
Share on other sites

Top quality Triplets are never going to be in my budget Neil :D

Then look at reflecting telescopes specifically adapted to planetary use. Specifically look at a long-focus newtonian (an 8" f/8 instrument would still be quite manageable if mounted as a dobsonian) with a small central obstruction and a high accuracy (1/8 wave or better) mirror. The longer the newtonian, and the smaller the obstruction, the better the contrast. Less coma is also a result of longer focal ratio.

Another thought : a 7" mak-cass? Not by any means a flexible instrument because of its long focal length, but if your commitment is to planetary observation, then this could be the design of choice.

Share this post


Link to post
Share on other sites

Despite being a refractor buff I will concede that a well collimated 8 inch, ideally a fairly slow one, should win. For me, with refractors, it ain't what you see, its the way that you see it. The night sky, to me, is always at its most captivating in a fine apo. But that's not what you asked!

Olly

Share this post


Link to post
Share on other sites
The longer the newtonian, and the smaller the obstruction, the better the contrast.

So it's the unobstructed view that gives the good contrast in refractors and not the design in it's self?

Share this post


Link to post
Share on other sites

I have yet to come across an instance where my 4" refractor has come even close to outperforming my 8" reflector!

Share this post


Link to post
Share on other sites
a 7" mak-cass? Not by any means a flexible instrument because of its long focal length, but if your commitment is to planetary observation, then this could be the design of choice.

Only if it's very vell ventilated ... the Meade LX90 7" Mak (discontinued) was great when it was at ambient but took all winter & half of spring to get near ambient ....

Share this post


Link to post
Share on other sites
I have yet to come across an instance where my 4" refractor has come even close to outperforming my 8" reflector!

I doubt this not at all, but for the record our 10 inch SCT hasn't a cat in hell's chance on the planets against our TEC140. Trust me, I' a doc... refractor nut!!!

Olly

Share this post


Link to post
Share on other sites

Not that I know much but it also depends on viewing style - some prefer to use a higher magnification, in which case the greater light gathering of the reflector would be essential.

My experience of small scopes is that the small exit pupil makes eye floaters very visible, and a major irritant for me, anyway - I understand these would be less apparent in a larger scope as the exit pupil would be larger for the same magnification.

Share this post


Link to post
Share on other sites
Not that I know much but it also depends on viewing style - some prefer to use a higher magnification, in which case the greater light gathering of the reflector would be essential.

My experience of small scopes is that the small exit pupil makes eye floaters very visible, and a major irritant for me, anyway - I understand these would be less apparent in a larger scope as the exit pupil would be larger for the same magnification.

Good point Ags and one I had over looked once again. :D

Share this post


Link to post
Share on other sites

I have never used an APO but have used a 120mm f8.3 achromat. Compared with my 6" f11 dob, there was no comparison. The contrast in the newt was massively better, the sky blacker, no CA/colour and the image sharper even at higher magnification than the 120mm. We really should get together sometime and you'll see what I mean. Even with the small(ish) secondary at 25mm, there's not much if any edge darkening either. It's OK on DSOs too but this is not where it excels.

I would guess that a good refractor will give nice star images, a little tighter , but that's about it for me - only based on my experience.

I confess though I keep seeing these little f10 102mm/80mm Vixens and feeling very tempted. It's a problem I know.

Do you fancy the Peak District star party? I'll be there and will hopefully be able to fit in both scopes then.

Share this post


Link to post
Share on other sites

Would love to have a look through your OO's Shane. Other than Galloway I'd not given any other star parties a thought. My dark site competes magnitude wise with most star party venues so it's the extra expenses that are somewhat off putting. Being short of cash is the reason behind the question. Since getting the Evo 120 I now enjoy the moon due to the added contrast but due to the false colour the planets suffer. In comparison my Explorer 200p gives superb views of Saturn but I am wondering if more detail through contrast (using a refractor) can be achieved. To do this I'm thinking I would need a false colour free Apochromatic refractor but due to cost would be no bigger than 4". At almost half the light grabbing power of the 8" reflector on an object that is so much further away would it really show any benefit? I am skint so sticking with what I have seems the better option as from what I have read other than a top brand triplet I'm not going to get any significant improvement from a C100ED. I'm thinking the EVO 120 for the moon (no real distracting false colour) and some open clusters and the 200p for planets & DSO's. Dose this sound like a plan??

Share this post


Link to post
Share on other sites

no worries mate. if you change your mind re the peak star party, let me know!

why not buy a 6" f11 apochromatic newtonian from OO? :) I know they made a special order recently for another member. might be cheaper than a 4" apo and I bet will beat it.

Share this post


Link to post
Share on other sites

...if you fancy the ATM route, you could try a Chiefspiegler, this gives an unobstructed path, open parabolic mirror, folding mirror to bring the eyepiece to a pratical height and two lens to correct the chromatic and astigmatic error to diffraction limited spec. Its one nice design for planetary viewing - and will outperform an apo.

Share this post


Link to post
Share on other sites
for the record our 10 inch SCT hasn't a cat in hell's chance on the planets against our TEC140.

Olly

I had much the same experience, Olly.

Years ago I owned a Televue Genesis and a C8. The Genesis routinely provided a more attractive planetary image than the C8. I've no doubt that used for imaging, and with the appropriate contrast enhancement in processing, more data could be teased from a C8 image, but for visual use the 4" refractor won hands down.

It's just a pity that I have had to lead a life of crime to feed the refractor habit ...... :)

Share this post


Link to post
Share on other sites
our 10 inch SCT hasn't a cat in hell's chance on the planets against our TEC140.

Olly

was the cat reference intentional? very clever :)

Share this post


Link to post
Share on other sites

I've just reminded myself of the original question which used the term "beat". I think the response depends on the definition of that term to be honest. Perhaps we should have been pendantic and asked the OP to define "beat" before launching into the discussion !.

If it's nice images at short notice, catching the gaps in the clouds, then my 4" refractor "beats" my 10" newtonian every time :)

Share this post


Link to post
Share on other sites
I've just reminded myself of the original question which used the term "beat". I think the response depends on the definition of that term to be honest. Perhaps we should have been pendantic and asked the OP to define "beat" before launching into the discussion !.

If it's nice images at short notice, catching the gaps in the clouds, then my 4" refractor "beats" my 10" newtonian every time :p

some people! :p

good point John. another perfect example of 'both' being best if you can have them. :) and more than one scope being a good thing if possible.

Share this post


Link to post
Share on other sites

I agree regards actual observing time the closed tube design of the refractor should beat the reflector but it's the new found contrast from my Evo 120 that has me asking the question. On the moon the Evo 120 view wise for me beats the 200p in both sharpness and contrast but due to the CA loses out on the planets. What I wanted to know would an Apochromatic refractor with out CA offer any significant contrast and sharpness over the 200p on the more distant planets or is aperture more important to tease out detail ? I am more than happy with the 200p on the planets but it is poor on the moon. The idea was if a C100ED was going to do as well on the planets as the 200p then I can observe the planets and the moon with good success with just the one scope. As it stands at the moment the Evo is great on the moon and only OK on planets where as the 200p is great on the planets and some what washed out on the moon. (Just for the record they are both good on DSO's) I just find that when the moon is out in force and coupled with LP there's not really much else to observe other than the moon and brighter objects like planets. In my current set up I would have to have 2 scopes set up to do this. If there is another alternative like contrast boosting filters for the 200p or CA removal filters for the refractor then this would also be an option.

Share this post


Link to post
Share on other sites

I use a correcting lens called a Chromacor in my 6" F/8 Meade achromatic refractor. The Chromacor is designed to i) remove most (in practice around 80%) of the chromatic aberration and ii) correct the spherical aberration found in most chinese achromat refractors (under correction in the case of my Chromacor).

Unfortunately not on the market these days and rarely found, the Chromacor's really do work and I can summarise the benefits by saying i) my 6" F/8 shows very little CA around bright stars and virtually none on the lunar limb at focus, and ii) magnifications of 300x plus are readily usable now wheras without the Chromacor little or no improvement in resolution was seen at powers over 200x.

I wrote a review of this device a while back (then being used with a 6" Konus F/8):

http://stargazerslounge.com/attachments/equipment-reviews/39531d1279755811-chromacor-chromatic-abberation-corrector-ccreview.pdf

Share this post


Link to post
Share on other sites

After the mention of one of these in your http://stargazerslounge.com/members-equipment-gallery/133805-my-big-frac-waiting-saturn.html I'd already searched for one but found the prices in old adverts to be out of my budget so if one were to come up for sale now I'd hazard a guess they are going to be even more expensive. I'd only got the funds for a cheap C100ED or maybe budget filters. I was hoping to leave some back up funds in the kitty to cover any eventualities so in truth was in hope that the majority was going to say a 4" Apo refractor will not beat a 8" reflector on planets and finally put my mind to rest. Please forgive my way of putting questions across but having asked the refractor vs reflector question (basically what this is) a couple of times before and notice threads can get a little side tracked to what people own and prefer.

I feel the question has kind of been answered and the easiest/cheapest option is to work with what I have. Your option would be perfect John if only a little difficult and pricey to obtain but I'm guessing they didn't take off as there was some thing sort of similar or more compatible out there?? Whats peoples view on the Semi-apo filter? I know Moonshane recently applauded the moon/ skyglow filter on the moon which is basically the same set up as a Semi-apo but the SA has the bonus of fringe cutting in refractors??? Could this filter be the best of both worlds offering contrast for the 200p and CA reduction in the Evo ??

Edited by spaceboy

Share this post


Link to post
Share on other sites

Join the conversation

You can post now and register later. If you have an account, sign in now to post with your account.

Guest
Reply to this topic...

×   Pasted as rich text.   Paste as plain text instead

  Only 75 emoji are allowed.

×   Your link has been automatically embedded.   Display as a link instead

×   Your previous content has been restored.   Clear editor

×   You cannot paste images directly. Upload or insert images from URL.


  • Recently Browsing   0 members

    No registered users viewing this page.

×
×
  • Create New...

Important Information

By using this site, you agree to our Terms of Use.