Bright stars are fuzzy blobs

[jps]

My telescope is an F/3.75 or F/4 200mm newtonian(specs say an F/4 200mm with a 750mm focal length). It is an optex OP 600. My accessories are a TV 3x barlow, a 2x bintel barlow, a 25mm gso plossl, and a 9mm gso plossl, a celestron ultima 12.5mm on the way. Here's the story.

I am inexperienced. Everything is fine except for bright star images. Bright star images are atrocious. Some of the objects I have enjoyed with no obvious visual imperfectons are m80, m7, m6, the lagoon nebula, the eagle nebula, the trifid nebula, tuc 47, m24, the trapezium, the moon, jupiter and mars. When I point the telescope at bright stars such as Antares, Archernar or Rigel, the stars are a little blurry with the 9mm. Add a barlow and the image is so blurry you could never split a bright double requiring that much magnification. I have looked at bright stars at about 45 degrees above the horizon. Is this high enough? I've viewed the moon at 480x with both barlows and the 9mm(it was coming in and out a bit), so the lenses are fine.

I have addressed this on Cloudy Nights and have made inroads into my understing but still blurry bright stars so far. Even though I believe at the moment my collimation is imperfect, coma is only toward the very edge of view(remenber it's a F/3.75) and visually I find it not much of an issue compared to this blurry star business. I'm told it's unlikely that collimation would cause these blurry stars and only very limited coma. I'm also told that gross diagonal alignment might cause this problem. My mirror is not spherical. I intended to included a photo of the focusser view. The file is too large to include. A sketch was also too large. Any thoughts about whether I've got a fixable problem here and how to fix it?

Back to collimation. I've been told that the bintel laser collimator I bought is wrong for my telescope and that I need a sight tube/cheshire combination tool. If I was convinced this would fix my blurry star issue it would be purchsed immediately. I'm not convinced. I'm thinking it would only effect the edge of field coma. The other thing is I'm not sure if I can manage a star test. I can't find any diffraction rings. At what minimum power and brightnesss can this test be done? Any feedback greatly appreciated.

[ving]

its not seeing related is it my f6 reflector hass been experiencing the same of late due to poor seeing. I can split rigel ok but star "A" is a big fuzz ball. if you make an aperture mask it will cut down on the fuziness of these objects considerably. :)
works for me

[davidpretorius]

Three things: Seeing conditions, Collimation, Primary mirror Cooling

1. what is mars looking like at around the same latitude, can you see surface smudges on it or is it a red swirl. Moons, star constellations can hide the lousy turbulence and still give an alright view. If i see any distortions (heat waves) on the moon, then the seeing is not good and therefore surface detail on mars or tight structure on brighter stars will not be there. Sirius A is a classic. Last night after 8 hours of cooling due to constantly falling temps, Sirius A was very settled for the first time. My collimation was very good and the atmosphere was very calm at 3am. I was splitting many doubles easily with my 12mm.

2. *Collimation is different to coma. Coma will be more of an eyepiece issue . I have only just sorted out collimation after starting 5 months ago and stars are now pinpoint.

3. temp differences greater than 1 or 2 degrees from mirror to ambient will cause the same bad seeing conditionns as #1.

*Collimation is 3 steps for me:

1. secondary mirror.
2. primary mirror with a cheshire.
3. primary mirror with a star test.

http://www.schlatter.org/Dad/Astronomy/collimate.htm is great for one and 2.
http://legault.club.fr/collim.html is great for step 3.

I bought a $29 cheshire and whilst it is not the best, I have used it to get very good collimation. It can align the secondary reasonably well (a sight tube would be better) and then because i have centre spotted my primary mirror, it gets a fairly good line up for the primary. Star tests then finish this off

But don't try and star test without letting you scope cool. Try and let you scope cool for at least 1/2 hr after the temperature outside has stopped dropping otherwise the mirror won't keep up.

The main thing is to get a star of magnitude 0 or 1 at around 200x which you could do with your 9mm. defocus and focus

Then max up the mag on a -1 or -2 star.

Make sure they are nice and high 75 degrees or more and make sure the defocussed images are in the middle of the field of view.

I would suggest reading up the links above, think about buying a good sight tube and a cheshire. Get some prices and names of suppliers and come and share with us.

Come back and ask lots of questions.

I would say i am a fanatic and will try and get out there if there is even a hint of a view thru rain clouds. So if it has taken me 5 months, to get a handle on collimation is not a quick couple of tips. Unfortunately it is one of those constantly learning experience thingys!

[Stu]

It's hard to tell without looking at the image as fuzzy blobs could mean a lot of things. I think cooling and seeing will have effect, but you will only see it at high mag. Low mag should be OK, if this is the problem.

Bad collimation will make the image look skewed in one direction or the other, when defocusing you will notice it more.

Coma you will definitely have and I'm afraid to say that there is not really a cure is such a short focal length scope, unless you get youself a set of Panoptics. :prey: You will see the coma around the edge of field, maybe even all the way in, depending on the eyepieces you have.

If the bright stars look much bigger than the faint ones, well, umm...
Don't do what I did and go and look through a $15000 5" Takahashi APO. The brightest stars will look the same size as the faint ones and you will never be able to turn back and end up selling your newt like me and probably die a very poor old man with only one scope (But at least you will have lots of friends... Hi Mark!!!).

So do what Dave P. said. :)

[jps]

Thanks for the replies. Perhaps I haven't been letting my scope cool enough.



I thought letting it cool for 1-2 hours after dark was plenty. I hadn't really thought about when the temperature has stopped dropping. Perhaps the use of a thermometer to replace experience. Is there a typical time when this occurs. I guess weather varies too much.

Ving, this aperture mask you mention sounds very interesting. I have never heard of them before. Can you tell me more.

[davidpretorius]

http://www.iceinspace.com.au/forum/showthread.php?t=5541

we have an experiment going on as we speak re temperature.

not sure if this will work

http://www.iceinspace.com.au/index.php?id=63,187,0,0,1,0&hashID=e0af23cd79277d28690ba8bc9201 0a63

this is a hartmann mask to help you focus when you cant look thru the eyepiece (ie imaging).

basically you cut holes ie three circles in a piece of cardboard and then put that over the opening of your scope. you cut down the light, but you will still get an image!.

asimov, ving and others have some great piccies of them. different shapes have been cut

[RAJAH235]

jps, Just use a sheet of black cardstock from the newsagents. Cut to cover your tube + a bit to fold over the edge. Cut 2 or 3 holes in it, prob about 50 ~ 60 mm in diam. ( correct here if nec, guys). You can use the ap. mask to aid in focusing as well. When the objects come together, you're in focus....
They also cut down the brighter objects when trying to split doubles etc & as said, help when collimating.
I use 'blue tac' to hold in place & when storing away.
HTH. :D L.

[Robert_T]

Hi jps - thermal equilibrium and seeing as DP mentions may well both be contributing to your problem. However, both factors are more "take what you get factors" whereas for the third factor mentioned, collimation, you can actually do something about it and it's something you can easily rule in or out as an issue with a simple star test (you can get plenty of fancy collimation tools, but the best collimation tool is a defocussed star image). After an hour or with the scope outside start with an eyepiece that gives you say 200 times aim at a ~Mag 2-3 star and then defocus slowly. You should see the star dissolve into a series of concentric rings of varying brightness around a central point (the central rings themselves may be very faint from the effect of the secondary mirror obstruction). If the rings are clear and perfectly concentric at this power you're collimation probably isn't "way out" (to test and adjust critical collimation you'd need to up the power further) - if it isn't nicely concentric then your collimation is pretty off and this will really upset your views. The links DP provided should explain how to do the actual collimation if you find it lacking.


cheers,

[Blue Skies]

With such a short focal length collimation will be pretty crucial. For me, I know the collimation is quite good when I can see four minor diffraction spikes in between the four main spikes on a star image. The comment about the laser being wrong is right imo - lasers are great for big truss tube scopes where offest can actually be measured and you can see your secondary mirror in the open tube. But for a closed tube give me a sight tube and cheshire anyday.

The coma is more likely a combination of the very fast mirror and the eyepieces together and there isn't much to be done about that. (Its a funny thing, I've met people who can't stand any coma anywhere in the field of view, yet others are quite happy to have heaps around the edge as long as the centre of the field gives a sharp view). The eyepieces that come with the scope are not the best on the market - it is amazing what a good eyepiece can do to a mediocre telescope! This is not to say you should rush out and buy new ones, just dont expect too much from you have.

Local seeing conditions are probably also a factor, especially if you are viewing from the city and have lots of sources of heat around you.

If at all possible take a look through someone elses scope and compare.

[rmcpb]

With a focal length like that collimation will be an ongoing quest - perfection is needed and a combination of sight tube/cheshire and good laser collimater will be needed. Good luck with that!!

As for the temperature dropping just make a ply disk that fits over the back of your scope cut a hole in it that fits an 80mm computer fan and mount this, use foam weather strip (the stuff to go round doors) where the disk touches the scope to isolate vibrations and hold it on with velcro strips. This will help with initial cooling, will keep the mirror close to ambient as the temperature continues to drop throughout the night and will remove the boundary layer in front of your mirror. For better efficiency you can mount a baffle just in front of your mirror to force the air flow across the front of your mirror.

This will be a pretty touchy scope in regards to collimation and the effects of temperature.

Good luck.

[jps]

Thanks for all the replies.

The collimation sites mentioned by Davidpretorious were very good. Unfortunately I'm still a bit confused. The military man said the first thing you do is adjust the secondary alignment. The other site said whatever you do don't adjust the secondary alignment.

When I soon get my hands on a cheshire/sight tube collimator, before collimation should I adjust the secondary alignment?

I was thinking that if I take a photo of the focuser view with the cheshire/sight tube attached, you experienced guys could tell me if it needs adjusting. Would a photo without the cheshire/sight tube be of any use for this purpose. I'll include it as an attachment anyway, just in case.

Perhaps I should do what the military guy said and just jump straight into secondary alignment.

I'm definately going to make one of those masks. A fan sounds like a good idea too.

[rmcpb]

Bit hard to tell from your photo but it looks as if your primary is a bit out of alignment. Secondary alignment looks OK but like I said its hard to tell from a photo.

[davidpretorius]

Definately, you need to make sure your secondary is aligned properly.

From the photo, it looks fine, but remember the idea of a long site tube is to make sure that your eye is looking at the secondary from EXACTLY dead middle.

There is no point adjusting the secondary into a lovely circle only to find your eyes were looking at an angle, not from straight on!!!

When they say, don't adjust the secondary, it means you should not have to do it again for a long time unless you drop your scope or give it a really large bump.

The primary is the one that can move ie with transport, or even the perfectionists will star test all night as the mirror moves ever so slightly as you move the scope from star to star.

A site tube = yes and a cheshire = yes. Laser maybe. Star test = yes

Where abouts do you live, and do you have a rough idea of what the drops from and down to?? ie is it 16 degrees at 9pm when you take your scope out and then at 12am it is 2 degrees???

This will help determine if you need a fan.

[ving]

also when you barlow your 9mm your exit pupil is about 1.1 which is too small. really for crisp images your exit pupil shold be considerably higher. the 9mm by its self gives you an exit pupil of about 2.2 which is quite good in good seeing.
something else to consider. :)

[jps]

I think I now understand what I need to do as far as collimation goes. Very soon I'll get the right tools and jump into this.




I live in metro Adelaide. On the weekends I use my telescope about 120km away under darker skies. I really havn't watched the weather that much because I underestimated its importance. Two mins and maxs that I've noticed lately were 10 and 25, 13 and 24. I'm not sure when the mins occured but Ive normally packed up my telescope around 11 pm to 12 midnight which is probably too early as far as seeing is concerned. I'll have to start keeping an eye on the temp.



I think I understand what exit pupil means, but I didn't know it had this consequence. How can I get around this? I guess I buy a short focal length eyepiece with appropriate exit pupil. I don't think I will be addressing this until I have these other issues sorted.

[xrekcor]

Exit pupil use this formula

Focal length divided by Focal Ratio

So a 30mm ep in an f/6 scope will give you an exit pupil of 5mm

Or a 30mm ep in an f/10 scope will give you an exit pupil of 3mm

hope this helps

regards,CS

[xrekcor]

Actually when the seeing is pretty shot I find my hartmann mask helps
in focusing for visual too. Give it a try ;)

regards,CS

[jps]

Does this mean I can't get a decent exit pupil at high magnification with my F/3.75

[xrekcor]

Interesting, I know the formula for working it out, I know the optumim is
around 2mm, and when the exit pupil is larger than the iris of your pupil
you get an effect similar to sky glow. Correct me if I'm wrong, others
will be better at explaining it than me. But I think the effect could be
eye placement maybe more critical. But I'm not really sure.

John Bambury would be the person to ask. I think he was the one
who showed me the formula, and possibly a few other things in regards
to exit pupil and the effects. But unfortunately I forgotten...

Sorry I cant be of more help.

regards,CS