Refractor Questions

[Sunshine]

After having some time with my refractor and, being able to more closely make observations, I have come across some phenomenon which I would like to better understand. First off, CA, it happens here but not there, mild here more severe there, for example, when looking at the moon, I notice barely any CA, I must really pay attention to see the thinnest most subtle yellowish line, not even sure if this is CA. On the other end of the spectrum, bright stars and, Venus, show clearly visible blue/red CA, not so much to ruin the experience but, unmistakable. It also seems to vary in intensity, assuming I am looking at the same star, depending on the day and elevation. Second phenomenon is focus, or maybe not focus at all but my own eyes, this one baffles me a bit, I find it easier to bring stars to a fine focus when at high power than when using low powers. Example, looking at the trapezium with my 9mm Morpheus at 88x (or double that when barlowed) I can bring stars to fine focus, even those delicate ones at the very threshold of vision appear as absolute beautiful pinpoints. By comparison, when using my 14 or 18mm eyepieces at 57 and 44X, bringing stars to points is more difficult, even when I think I am focussed, I get these small spikes which move depending on the orientation of my eye with respect to the glass, slight ocular shift will make spikes rotate around the star. Is this is a result of my eye, the eyepiece, both?.

there is one more conundrum to this, the smaller the star, even at lower powers, the less these spikes become an issue and the more they look like they are proper points without any spikes. That being said, if I were to summarize it, imagine looking at a cluster with stars of drastically different magnitude and, apparent sizes.  With my 9mm Morpheus, they all look like points, well focussed, now swap it for the 18mm AMP, suddenly, the smallest of those stars can be brought to a pinpoint focus but, the larger and brighter stars have those small spikes ( or do they all at low power but my eye cannot resolve the spikes on the smallest stars?) who knows.

 

Edited March 29, 2020 by Sunshine

[cotterless45]

Loving to use refractors , I'll give it a go. Firstly ca, don't bother looking at it , ignore it . You can fit a semi apo filter or fringe killer filter into your diagonal that'll fair control it , not eliminate it , it'll remove some of the light coming through . For me , ca may be a big issue on bright binary stars at high magnification.

Focus becomes harder with old age , if you're a certain age you might find this. Dual speed focusers are a great help and long refractors give you a greater range to snap into focus , more than short fast refractors. 

You also have to train your eye to relax and get accustomed to observing . Don't squint or use one eye . Use an eyepatch or a handy hand to cover the non seeing eye. Else you'll be straining your face muscles and will get tired pdq. Sitting comfortably helps , adjust chair to relax at the scope. Don't be climbing up step ladders in the dark or squatting down to get a view. Adjust your tripod first , avoid full leg length . A pillar extension on the mount will aid getting the right height.

Spikes and wobbles : you're not actually seeing marble stars. What you're seeing is a diffraction ring which is how your Optics collect light to focus.If you go out of focus either side of focus you'll see great Airy discs that'll give you an indication on how good your Optics are , https://en.m.wikipedia.org/wiki/Airy_disk

Spikes and wobbling are caused by the atmosphere. Low down you'll be looking through 90 miles of it . in addition any thermal differences produce air movements , often the air settles after midnight. Don't think there's anything wrong with your eyes or scope , it'll be the atmosphere. This can be bad on a hot day followed by a cold night, or over buildings where heat is rising .

Collimation: yep , unless factory set , they can be out . Use a Cheshire to check. That applies to diagonals as well. It's not critical unless you are using high magnification on planetary features or tight binaries. Not a big issue unless you drop the scope. 

I hope others add to this and get your queries answered . Anyhow refractors are proper telescopes ! Enjoy the purity of views ,

Clear skies ,Nick.

[johninderby]

Would help to know which refractor are you using.  

Edited March 29, 2020 by johninderby

[Sunshine]

31 minutes ago, johninderby said:

Would help to know which refractor are you using.  

Orion Eon 115mm triplet, 805mm FL, William optics dielectric diagonal, when needed I use a televue powermate 2.5 barlow (which is a great little toy). when observing the trapezium at 220 power, on a decent night, I can clearly make out E&F stars as the most delicate pinpoints, I guess I shouldn't worry too much.

Edited March 29, 2020 by Sunshine

[johninderby]

I’m wondering if the eyepices used might have something to do with it and of course seeing conditions. But with your scope wouldn’t expect it to be a real problem. 

 

[Second Time Around]

Do you have astigmatism in your observing eye?  If you wear glasses do you use them to observe?  Doing so would correct the astigmatism and may solve the problem. 

I'd add that astigmatism shows up more with lower powers.

Edited March 29, 2020 by Second Time Around

[Stu]

First off a question, is this something you see with Newtonians or just a refractor issue?

Second up, if you are seeing E and F clearly there is not a lot wrong with the scope I would suggest.

My thoughts are that refractors tend to show a cleaner star image because they have no secondary obstruction and have no spider which causes diffraction spikes. This means any other issues show up more clearly, and I suspect the fact that this occurs at low powers only indicates the problem is perhaps with your eyes. Low powers mean larger exits pupils and this brings astigmatism and perhaps other defects in your lenses into play, causing the spikes.

Coming to your first point last, CA. A triplet refractor should not show any CA really. The most likely explanation is that it is atmospheric CA caused by the air column, and this gets worse the lower the altitude of the object. Venus shows this quite frequently, the earlier and higher you catch it the better it is though. This is also related to, and made worse by poor seeing so I think the fact that you say it varies depending on the day and the elevation of the object indicates that it is atmospheric.

[CraigT82]

Yes as Stu says the blue/red fringing you're seeing sounds like atmospheric dispersion. Nothing wrong with the scope there. 

[F15Rules]

 A few questions..

-I see you are in Canada. Which region are you in? What are your local atmospherics like?

-You are using a Triplet refractor. Can you have the collimation/spacing checked? (I wouldn't advise you to try this yourself). Pinched optics or misaligned lenses could affect the images but that fact you get great views of some objects makes the optics being the problem seem less likely.

- Your age and eyes. I'm 64 and my right eye (my main observing eye since I was a teenager) has been steadily deteriorating in sharpness for some years. But my left is still pretty good. I only really noticed this after an eyetest 2 years ago. I have retrained my left eye up to be my main observing eye, it to took a while but it is much easier now. 

I also use binoviewers more now, and that helps on some objects, especially planets and moon.. I find they make viewing more relaxed, and a relaxed eye sees better.

For single eye viewing I use a black eye patch more now.

It's worth having an eyetest (when you can, assuming you are in virus lockdown as we are in the UK).

-your eyepieces (certainly the Morpheus's) are great, so unlikely to be the problem.

My money would be on your eyes, or atmospherics, or a combination of these. Some great tips from Old Nick  (Cotterless) in the first reply

Good luck, don't worry, and I hope you get to the bottom of this😊👍.

Dave

Edited March 29, 2020 by F15Rules

[domstar]

From the Cambridge Double Star Atlas- 'The exit pupil is inversely proportional to the apparent diameter of the Airy disk-as the exit pupil becomes smaller, the Airy disk will appear larger.' It goes on to say that for most observers exit pupils of 1mm or less is when the Airy disk becomes visible. I must say, this works for me. At lager exit pupils my stars are like you describe but at smaller exit pupils I can see a perfect dot on brighter stars (the Airy disk). Fainter stars are always points but are too faint to show that Airy disk dot.

[cotterless45]

''Tis thé joy of refractors , getting marble like stars.

For those with large apertures, closing down using a simple aperture mask can provide superb views . Don't leave it in place though like I did the other night !😳

Keep safe , old Nick.

[Space Hopper]

4 hours ago, Sunshine said:

Orion Eon 115mm triplet, 805mm FL, William optics dielectric diagonal, when needed I use a televue powermate 2.5 barlow (which is a great little toy). when observing the trapezium at 220 power, on a decent night, I can clearly make out E&F stars as the most delicate pinpoints, I guess I shouldn't worry too much.

If you can see Trap 'E' and 'F' you are in the right ballpark !

They can be very elusive from UK sky's.

[Timebandit]

 

 

I have a SW 120ed refractor , don't see any diff spikes on that with stars, pin point stars that's one of the benefits a refractor should have over a reflector. Now the reflector I have, will show a slight diff spike, central obstruction, which is expected from the reflector. The CA on the moon is not really noticeable on my refractor when looking at the moon, handles CA very well.

It could be a number of factors, scope, eyepiece, atmosphere, your eye's. Personally I would get another observer by the side of you one night ,get them to confirm if they can see they same view as you through your scope , and start ruling out your eye's, atmosphere, eyepieces, scope.

A good well set up refractor should have clean ,pin point stars. 

 

 

 

[Sunshine]

Thanks everyone! great info here, I decided I will not preoccupy myself with these perceived issues till I’ve had a much longer time to enjoy the scope. I’ve had instances where it floored me with breathtaking lunar views and a stunning trapezium. This thread will be invaluable for reference once I have given the scope a fair chance in a myriad of conditions over time. We all tend to be quick to judge our scopes and, point out what we fear may be flaws, I feel i am guilty of this before having allowed for enough time and trials.

Edited March 29, 2020 by Sunshine

[mikeDnight]

What appears as CA can sometimes be caused by the eyepiece and not the scope, also, on bright objects like Venus or Sirius, atmospheric refraction can be an issue due to their brilliance and usually low angle. From personal experience with a 127mm version of your refractor, I can honestly say it was a true apo triplet with no visual CA and was a pretty impressive scope.

[Sunshine]

2 hours ago, mikeDnight said:

What appears as CA can sometimes be caused by the eyepiece and not the scope, also, on bright objects like Venus or Sirius, atmospheric refraction can be an issue due to their brilliance and usually low angle. From personal experience with a 127mm version of your refractor, I can honestly say it was a true apo triplet with no visual CA and was a pretty impressive scope.

Thanks, i am sure you are perfectly correct and, i am pretty sure that my 3 nights of observing under fairly good conditions is by no means a platform from which to judge my scope just yet. Sometimes i get carried

away by something i observe, sound an alarm bell only to realize i was paranoid.

Edited March 29, 2020 by Sunshine

[Stu]

5 minutes ago, Sunshine said:

Thanks, i am sure you are perfectly correct and, i am pretty sure that my 3 nights of observing under fairly good conditions is by no means a platform from which to judge my scope just yet. Sometimes i get carried

away by something i observe, sound an alarm bell only to realize i was paranoid.

It is very easy to observe on a poor night and suddenly believe your kit is rubbish! Amazing how poor seeing can make even the best scope appear like it is hopeless, and conversely how even modest scopes can perform quite well under excellent seeing or darkness conditions.

[Ricochet]

On 29/03/2020 at 05:02, Sunshine said:

I get these small spikes which move depending on the orientation of my eye

This is astigmatism in your eye, which becomes more noticeable at larger exit pupils (so longer ep focal length, lower power). If you want to remove these spikes you will have to wear your glasses while using eyepieces that result in larger exit pupils. 

[mikeDnight]

If spikes continue to occur, you could try rotating the eyepiece.  If the spikes rotate with the eyepiece, its probably oil on the eye lens, which can be cleaned off using lens cleaner and a lens cloth from an opticians.

[Louis D]

I would hope you've tried observing with eyeglasses that correct your astigmatism to see if that improves your low power views.  If you don't have eyeglasses to correct your astigmatism, you need to get a pair.  What you've described is exactly what astigmatism of the eye looks like.

[Sunshine]

How about contact lenses? I was wondering about this, so many use eyeglasses but, for us amateur astronomers glaring into eyepieces, wouldn't contact lenses be far more practical?. My astigmatism has never been an issue in daily life, I see very short  streaks coming off lights and things, nothing that has ever alarmed me. While looking into the issue for the sake of observing only, I do notice they do make contacts for astigmatism which seem to be disposable one day use type of thing. I will visit my eye doctor, if these contacts are an option then maybe I would only apply them on night when I plan to observe.

Edited April 3, 2020 by Sunshine

[Louis D]

37 minutes ago, Sunshine said:

How about contact lenses? I was wondering about this, so many use eyeglasses but, for us amateur astronomers glaring into eyepieces, wouldn't contact lenses be far more practical?. My astigmatism has never been an issue in daily life, I see very short  streaks coming off lights and things, nothing that has ever alarmed me. While looking into the issue for the sake of observing only, I do notice they do make contacts for astigmatism which seem to be disposable one day use type of thing. I will visit my eye doctor, if these contacts are an option then maybe I would only apply them on night when I plan to observe.

Toric lenses to correct astigmatism depend on gravity to pull them into the proper orientation (the bottom edge is weighted).  Guess what, if you don't look straight ahead into an eyepiece, the lens will tend to start rotating out of the proper orientation.  Some people report having had good luck with them, but many more report having had enough issues with them that they quit using them for astronomy.  If you've got them anyway for daily wear, it's certainly worth a try.