spider in the way

[Rosie2]

I wonder if anyone can advise me . I am a beginner but have been reading as much as possible before deciding on another scope.I have ,which was given as a present a 76mm reflector. I dont think when i first got it and was looking at the moon the mounting for the secondary mirror was so visible it is such a nuisance as it is now .I can see a central blob and 3 I think there called diffraction spikes on any bright star or planet . I have been looking at Jupiter but no hope of seeing anything but the spider! I have seen an article on colimation but dont think it needs it as the spider is central with no bits missing . I have tried altering the focus and magnification. Sorry to drone on ! Thanks.

[gary1968]

Howdy and welcome to SGL. This sounds very strange, are you sure the you have the telescope focused correctly? The reason I ask is that you should not be able to see the secondary / spider at all. I can only see the secondary in my newt when the focusing is waaaaayyyyy off!

I cant offer much more advice on this but I'm sure there will be others along soon to offer their help...... :-)

Gary

[John]

The diffraction spikes on bright stars are normal for a reflector - they are caused by the spider vanes. Seeing the central part of the spider, ie: the bit that holds the small secondary mirror in place, means that the scope is not in proper focus as Gary says - it dissapears when sharp focus is reached. What eyepiece are you using with the scope ?

John

[Rosie2]

Hi John , I have 20mm 12.5mm and 4mm I have tried then all to see if this helps , Apart from turning the focus wheel is there another way? Thanks

[John]

Hi John , I have 20mm 12.5mm and 4mm I have tried then all to see if this helps , Apart from turning the focus wheel is there another way? Thanks

The only thing that I can think is that something is stopping the scope getting to sharp focus - check that the eyepieces are inserted fully into the focusser, ie: that there is no chrome barrel showing.

Have you tried looking at something like a distant tree ? - can you get that to focus sharply and is the spider still visable then ?.

I suppose a more radical alternative is that one of other of the mirrors have been moved so the scope won't focus - have you had it from new ?.

John

[Rosie2]

Yes John I will try tonight to see if the eyepieces are in properly It dosnt do it at all in daylight and focuses fine . I did get it from new.

[Rosie2]

Tried again tonight with 12.5mm making sure eye piece in all the way and then turned focus very slowly black blob still very visable !! ahhh! FREEZING !

[rigel1]

Hiya Rosie2.

There are probably a number of reasons why you are experiencing the problems you have, but for an initial start, try to stick with the 20mm e/p or the 12.5, but the 4mm would be of a high magnification and should only be used when the seeing is extremely good and you know what to expect from it.

Stick with the 20mm for now untill you have sorted out your problem.

All the best

Rigel1

[Milamber]

I think focus is *not* the issue here - if it were not in focus then there would be no diffraction spikes to speak of?

[SteelRat]

Hello Rosie I was using a very similar scope to yours (a Celestron 76x700) not many months back. Tell me, where did you purchase your scope from?

[ngc2403]

Hey,

I was wondering if the problem is with the eyepieces and not the focus?

to test look through the telescope and put your arm over the front, if you can see your arm then it is focus if not the it is a ep problem and i am not sure how or what it is but i know that telescopes with obseructions get it sometimes.

Does the black blob move the opposite way from ur motion when you move your eye over the ep?

ally

p.s. i did have a clearer post but windows Iexplorer decided to not remember it when i could not find the page %$&^%&%&^$&$^&

[Rosie2]

:scratch: Thanks to all . rigel1 Yes I will try the lower e/p

ally I did what you said and I can see my arm and my eye looking back ! all sounds a bit daft! What is " ur " motion . It is difficult to say what the black blob does ,it is just when I get the image of a bright star or planet which of course is unsteady the whole spider is visable over the object.

[scotastro]

Sorry if this seems a really daft question - Have you removed the whole of the cover on the front of the scope?

Many small scopes have a large cover on the front with a smaller one in the middle. Often beginners only take off the small one not realising the whole thing should be removed.

Difficut to sort problems like this sometimes any possiblility of posting a picture.

Mike

GAC

Galloway Astronomy Centre

www.gallowayastro.com

[rusirius]

Rosie, is the object you are looking at sharp, or is it blurred or distorted in any way?

I'm wondering whether the retaining clips or something on the primary mirror has been overtightened and the mirror is maybe pinched or distorted in some way?

What's the make and model of the scope?

[Rosie2]

No Mike as a complete newbie no question is daft to me I am just glad of all the knowledge I can get .When i first got the scope I took some time in lining up the view finder in daylight and then looked at the moon everything was fine no spider.It is a Bresser Venus 76mm reflector.I did take both covers off the front. Then was very excited to see Jupiter on 12.5 e/p it was a good image although the mount is a bit wobbley and not on level ground .My son came and looked but said what is that is it one of its moons but we soon realizes it was the spider with the 3 spikes. I cant send an image as no camera. Rusirius I could well have tightened something if this is possible from the outside, could you tell me more please. Cheers.

[rusirius]

Viewing Jupiter is great and even with your 3in refelctor you should be able to see the 4 Gallillean moons as 4 small points of light.

Unless the primary mirror cell was removed from the tube or adjusted in its mountings then its unlikley to be mirror pinch. I don't think you could have gotten to this from the outside but I stand to be corrected.

Most Newtonians have at least 3 small screws behind the primary mirror so that it can be adjusted. On most, the seconday mirror also has the similar arrangement. This is so that the mirrors can be optically aligned to each other and is called Collimation. I'm not sure whether your scope has these or not. Usually if the collimation is off, then the image is distorted, but you say that you did have a 'good image'. Can you think back to what may have changed or been done in between having a good image and the spider becomming appearent?

[philsail1]

Rosie2,

Can you try your scope in the daylight (even through a window on some distant object) - all your scope's controls will be easier to see and adjust, and you may see something that's obviously amiss?

(make sure you don't look at the Sun of course!)

With you saying you were seeing a "blob" I was going to question the eyepieces you were using, but they look OK. As with what eveyone else is saying, I would stick with the 20mm for now - until you sort out the problems you're experiecing.

Best wishes,

philsail1

[The Warthog]

Rosie, what you describe in your first post is exactly what you see when the scope is way out of focus. Has it been damaged in any way? If it used to work OK and now doesn't, something must be altered from what it was before.

Actually, as you say it focuses fine in daylight, I'm stumped. Try focusing on some distant object in daylight, then leave it alone until evening and point at Jupiter, or a bright star, without changing the focus.

Let us know...

[Rosie2]

Thanks again folks. I will try that in daylight with 20mm e/p . I am sure it has not had a knock or been abused in any way between behaving well and not. It has put me off reflectors a bit because from what I understand from this ,it is not abnormal seeing the spikes and they are not there on refractors?

[The Warthog]

Well, you see the spikes as little and rather pleasant points on finely focused stars, not with a blob in the middle of a fuzzy round disc. With some fast scopes, you may see a darkish patch in the centre of the FOV in daylight, when using a low power ep, but neither of those are what you are seeing. I'm sure if one of us could see your scope, we'd have the answer in a second.

[ringz]

Rosie,

Do you get the same effect if you switch eyes. I know this can be a bit difficult if you have one eye more dominant than the other.

Looking at stars through a telescope is one of the most testing things for eyes and can show up all sorts of problems, such as floaters and the like.

Looking at terrestrial subjects will mask these problems mostly.

--

Martyn.

[Rosie2]

Yes Warthog I think you should fly over and sort it out .LOL Thanks anyway.

Martyn, my son saw the same thing so its not the eyes. As soon as I can get out when it stops raining ,going to have a go in daylight and then at night again and report back.

Cheers.

[stefski]

Hi Rosie.

I own an SCT as you can see. I had exactly the same problem when using my 2" diagonal together with a focal reducer. I could focus in daylight on nearby objects, but at 'infinity' (ie stars), I always got it close but still saw the black circle of the central obstruction (ie what you are seeing).i was puzzled and so were the people at Celestron - as the SCT has a lot of back focus!!

I took the 'scope into the dealership for them to take a look this weekend just gone. We found that the secondary hadn't been put in correctly on the corrector plate. A quick fiddle and it was in place.

I took the 'scope home again. Spent an hour recollimating it and then tried the 2" diagonal and reducer.....it worked!

Good luck with it.

Stef

[Rosie2]

:cheers: Thanks a lot Stef. Its late but I'll think about my next step tomorrow .Cheers.

[The Warthog]

Well Stef, we're talking about a 76mm reflector, and diagonals aren't an issue. I think rosie should buy a 150mm Newt and end all her problems.