SCT & Refractor - Star Quality

[Pig]

I would appreciate some help & input to determine if my SCT needs collimating. When checking for concentricity via the doughnut shape all seems ok but its not perfectly aligned.

When I view the Moon at high & low power with my 6" SCT I can achieve very good detailed views of the lunar surface. However, when viewing stars they always appear a little fuzzy. Could slight misalignment cause such a difference between these two types of targets ?

I have also been wondering if this effect is purely down to the fact that stars are pinpoint through the Esprit and I have got used to this kind of sharpness.

I really don't fancy messing about with collimation of the SCT if there is no need, on the plus it does have Bobs  Knobs.

Thank you in advance

Shaun

[Stu]

Hi Shaun,

I suspect this is as much, if not totally related to cooling and the fact that it's an SCT rather than collimation.

They are however very sensitive to miscollimation and planetary contrast falls off pretty quickly if they are out.

John posted a great link on SCT collimation on my thread about my edge. I still need to have a proper go at this but think it is worthwhile.

In my experience though, stars just aren't as sharp in an SCT as they are in a good frac.

[Chris]

I agree with Stu, standard SCT's do show 'bigger' stars and coma even when well collimated. The Edge HD I owned for a year was much better I found with tighter stars and a flat field, but still not as pinipoint as a frac. 

I think going from viewing through a triplet to an SCT would exagorate things purely by the comparison. 

If a de-focused star in the centre of the FOV is concentric then I'm sure things are fine Shaun

[alan potts]

I am going with the flow, My SC is not as good as my refractors for pinpoint stars and I bet everyone else will say much the same.

It is a fact though that even the slightest amount out looking at the de-focused star will give poor results, try a defocused star with about X300 and view it that will give you a clearer picture, if you know what I mean.

Alan

[Chris]

If things are a touch out Shaun, just make very small tweaks to the Bob's Knobs. 

This should help:

[Owmuchonomy]

My experience is the same as Stu.  The SCT of long focal length, mine is 2300mm, and prolonged cooling period never resolves stars as well as my 4" frac.  I assume the SCT 'pulls in' the seeing too; a bit like looking down a warm road tunnel in summer.

[DRT]

My experience is the same as Stu.  The SCT of long focal length, mine is 2300mm, and prolonged cooling period never resolves stars as well as my 4" frac.  I assume the SCT 'pulls in' the seeing too; a bit like looking down a warm road tunnel in summer.

+1

Perhaps I need to trade in my 11" 2800mm focal length SCT for a refractor with the same vital statistics?

[cfpendock]

I don't fully understand the reason for this, but I, too, have found it difficult to achieve tight stars on my SCT.  I always assumed that this was because of the longer focal length - I use 2800mm, compared to the refractor operating at typically 530mm or (reduced) 385mm focal length.  I presume that the Airy disk is affected by focal length?

Chris

[Chris]

I bet there are a number of factors leading to large stars in SCT's:

-Long focal length hence high mag

-Poor focus due to mirror flop

-Central obstruction

-Aberrations such as coma and field curvature

-Tube currents due to long cool down time

Several of these things are addressed in the Edge varient which is probably why they are sharper (well at least to my eyes).

[Tinker1947]

+1

Perhaps I need to trade in my 11" 2800mm focal length SCT for a refractor with the same vital statistics?

Well i wasn't very happy with the view so it had to go...11" SCT.....back to a reflector.....

[Pig]

Thank you very much for your views and feedback

I did suspect that the type of scope was the main contributor and I have been spoiled by grace of the Esprit. However, the collimation is a little worse than I originally thought so I will give it a little tweak as per the clip above.

I have left the scopes outside for quite sometime tonight so hopefully this should minimise any cooling issues.

Stu,

I don't suppose you have the link you mentioned?

Once again thanks to you all, I will give it a shot tonight and let you know how it goes

[Stu]

Here you go Shaun

http://www.sctscopes.net/SCT_Tips/Maintenance/Collimation/collimation.html

It's post #43 in this thread, or there abouts.

http://stargazerslounge.com/index.php?/topic/248523-Living-life-on-the-Edge%2E%2E%2E%2E%2E%2E/page__view__findpost__p__2704847

[Pig]

Thank you young Stu

[Stu]

Thank you young Stu

If only!

[Pig]

Its getting quite clear here tonight so I will be able to give it a go soon or 1:00 am    Some lovely dark blue skies atm

[John]

I've owned a few SCT's but I've never managed to get refractor-type star images from them, even when well collimated.

Mak-cassegrains and mak-newtonians are the closest non-refractor type scopes that give refractor-ish star images from the scopes I've owned.

Having the SCT well collimated is still a good plan though - the contrast and resolution on planets falls off quite quickly when they are a bit out.

[Pig]

Thank you John, after a quick check the SCT is out so it will be a worthwhile exercise  especially id it helps out on Lunar and planetary.

[Pig]

Some good news  I have just completed collimation and it has made a difference the stars are much better, not in the same league of the frac but much better.

I found it a bit fiddly at first and at one point wished I hadn't touched it   But once I familiarised myself with how each screw worked it was fairly straightforward

[Peter Drew]

One thing that does not help the star images from a SCT is the passage of light three times through the tube before reaching the eyepiece.

[Mark in Macc]

If you observe at the same magnification, how do they compare? Apples and oranges?

[Stu]

If you observe at the same magnification, how do they compare? Apples and oranges?

I would say yes to that Mark. Beautiful tight diffraction patterns in the frac, slightly fuzzy stars in the SCT. I still have it to learn on mine though, so will give it more time. I bought it for 'portable aperture', not pin point stars though so it's not a problem for me.

[Pig]

Mark,

I would say the refractor would deliver more pinpoint stars.

In my 2 scopes it would mean a 10mm eyepiece in the frac and a 37.5mm in the SCT to achieve the same magnification, thus quite a big difference. I think the focal ratio would be in favour of the F5  with the right eyepiece.

[Live_Steam_Mad]

With your 6" SCT, what eyepieces have you got? Can you get to around 300x magnification? If so, that is a 0.5mm Exit Pupil, and any Astigmatism you have from your eyes is going to be contibuting virtually zero effect on the image. Now examine the in-focus Airy disc at x300. Adjust the collimation whilst AT focus and get the 1st diffraction ring to be continuous around the Airy disc. If you are way off collimation at 300x, then first try at 100x, get the secondary shadow centred in the outside of focus diffraction disc (make sure to always finish focusser movements turning the focusser knob Counter Clock Wise), then try at 200x, then 300x, then at 300x at focus.

Once you have a 1st ring which is continuous and there is no tendancy for the light to go into any particular part of the circumference of the 1st ring at 300x, then examine the Airy Disc, it should be a hard round sharp disc, with one moderate brightness ring and the 2nd one much fainter and the 3rd one rather dim (do all this on Polaris if you have an Alt-Az mount, or on a star 45 degrees above the SW horizon if you have an EQ mount / wedge. If you see the Airy disc and it looks as I describe it, try looking at Epsilon Lyrae, you are looking for 4 hard round balls of light each surrounded by a single ring. If so, you have very good optics, and you are seeing the optimum sharp stars and no improvement can be ever made short of refiguring the optics.

If you see an oval pattern when defocussed a little (like if you pull the eyepiece out about 4.4mm for F10) at 300x and it swaps over orientation from intra to extra-focal, then you have Astigmatism in your optics. If the orientation of the oval follows your head as you rotate your head then the Astig. is from your eyes (you might see this at very low powers, it depends on the level of Astig you have in your eyes).

Make sure to let the SCT cool for about an hour before tests, passive cooling should suffice with a 6" in these warm temperatures. My C11 needs my home made Lymax style fan and cheap Ebay medical gel ice pack for an hour before I can use it at high mag., but has very good optics. I cannot see sharp stars at exit pupils over 1mm because of the severe Astigmatism in my own eyes, so I only ever see crosses on stars at say x48 with nearly 6mm exit pupil, and aberrated spikey stars between x48 and about x215. At 700x in the C11 I see sharp Airy discs surrounded by a couple of rings, and Epsilon Lyrae looks very sharp at 350x.

Best Regards,

Alistair G.

[timwetherell]

Mak-cassegrains and mak-newtonians are the closest non-refractor type scopes that give refractor-ish star images from the scopes I've owned.

That would be my experience too. I once tested a 4" mak-cas against a 4" apo and to be brutally honest, the mak was more "refractor like" than the refractor. SCTs do have the aperture advantage though, the smaller ones having the same aperture as the biggest maks.

[Peter Drew]

Apart from other considerations, the refractor will always have the advantage of its small size, and I class 6" as small. Apertures in excess of this will always be more subject to the interference of seeing conditions. I have seen textbook images on the test bench using 8"-12" SCT's only to find that they give mushy stars in real outside use. 4" SCT's and small Maks, if collimated properly, usually give refractor type star images. My 8.5" refractor seldom does.