Double image problem with a weird phenomenon

[Planetarian]

Hi, I've got a 76/700 newtonian telescope. The problem is that I see double images. When I try to focus, the two images get closer and closer to each other, and when they are very close, each of them are sharp, but when focusing further, they become one, but the image is blurry. There is a strange thing tho, that when seeing two very close sharp images for example of Saturn, then if I cover half of the front opening of the telescope by my hand, one of the images disappear, and I'm left with the other, sharp image. Same problem with the stars, when they seem sharp, all of them are doubled. I collimated the telescope with a pinhole cap (I'm gonna use a laser collimator soon to double check it). I'm using good quality coated plossl eyepieces with different focus lengths. All of them act the same way, so I'm assuming the problem is with the telescope optics. The primary mirror is not chipped or anything, the secondary seems to be silvered on it's surface so that seems fine too.

So can this be an issue still with collimation?

[Geoff Barnes]

Welcome to the forum @Planetarian.

That's a curious thing, I've never heard of such a problem before, hopefully one of our more experienced members can help you out.  

[Ruud]

What does the front of the telescope look like? Is there a plastic cap with two holes in it? If so, remove the entire cap and try again.

It's a wild guess, but some larger telescopes have a dust cap like this.

 

[Planetarian]

The cap has one hole in the middle with a smaller cap. I normally remove the entire cap when using it. 

[Geoff Barnes]

Might seem a daft question but you're not indoors viewing through the glass of a window are you?

Just a process of eliminating possible causes.

[Planetarian]

No, I was viewing outside.  

[Waldemar]

maybe there is a micro or hair-crack in the main mirror?  I think that is the only reason for such an effect...

Edited September 14, 2020 by Waldemar

[Planetarian]

Well, I've inspected the primary mirror and can't see any signs of damage. I'm gonna wait for the laser collimator that I ordered to see if that helps.

[Waldemar]

Micro cracks are hard to find... you need to do a real close up check.

[Planetarian]

All right, I'm gonna try to remove the mirror later and see from close.

[Pixies]

Try taking our your eyepiece and have a look at the secondary mirror. It's that cracked?

[Planetarian]

I took out both the primary and secondary mirrors. The primary is in perfect condition. The secondary doesn't seem cracked either although when shining light on it from the side, I can see a very thin half circle half way to the middle of the surface, but this could be a little scratch on the surface only. Otherwise when looking at the mirrored image, can't see any problem. As my laser collimator I ordered, hasn't arrived yet, I tried to do it with a star. I got the concentric circles when defocusing, but one side of the circles were stretched (oval), then I realised that probably the secondary mirror needs to be rotated a little. By the end the stars looked much better, didn't see two of them this time but they were always a little stretched to a certain direction depending on how I moved the secondary mirror. Couldn't test the double image problem on planets as they have gone out of sight by the time I finished messing with the mirror. Anyway I don't have practice yet with alignment using a star so I'm gonna wait for the collimator. 

[Geoff Barnes]

21 minutes ago, Planetarian said:

By the end the stars looked much better, didn't see two of them this time but they were always a little stretched to a certain direction depending on how I moved the secondary mirror.

I think this is showing that it could well have been a collimation problem, though still curious as to why you were getting a double image.

[Planetarian]

Yes, very weird, but anyway, after a proper collimation I'm gonna post an update here. Hopefully with good news. Thanks for the replies so far. 
Also wanted to ask if it's worth going with reflectors in the future if I decide to invest in a more serious telescope, or might be better off with refractors?

[Geoff Barnes]

54 minutes ago, Planetarian said:

Also wanted to ask if it's worth going with reflectors in the future if I decide to invest in a more serious telescope, or might be better off with refractors?

Ooh, that's an issue that creates huge and passionate discussions on any astro forums!

It really boils down to personal preference in the end.

I suppose in an ideal world we would all have at least one refractor and one reflector and use whichever one gave the best results on any given target on any given night  😁

[Planetarian]

Haha, good answer. I know it's a 'dangerous' question, but if I asked in a separate topic, after a big argument the result would be probably 50 - 50 for each which is useless to me. Just wanted to know your opinions. Or pros and cons. Or what instrument do you have?

[Planetarian]

Oh I've just found the info under your comments. I'm too new here yet. Anyway I'm still curious at which occasions or for which targets do you use refractor and when to use reflector.

[Geoff Barnes]

I only ever use the 12 inch Dob now for all visual targets. It is streets ahead of the 4 inch achromat in every respect.

I have never looked through a quality ED refractor so I have no reference point to compare, but I can't believe the views would be much better if at all than my Dob, for example, I'm out tonight in great seeing conditions and viewing intricate details on Jupiters cloud bands and watching Europa transiting across the northern equatorial zone. Also seeing Saturns C ring and Enceladus moon easily at 275x with a Baader Morpheus 6.5mm EP. 😀

Edited September 15, 2020 by Geoff Barnes

[Planetarian]

That's a nice large dobsonian. Is it stable with those extending bars? Do you also do astrophotography?

[Geoff Barnes]

Yes it is rock steady, the bars are very strong and heavy duty. I'm purely a visual observer.

[Planetarian]

Update: So I've done a little experiment (still without the laser collimator). I aligned the mirrors using a pinhole cap, and the situation is this: the image is not sharp, I mean everything has a shadow. For example a distant telephone wire looks double, or as it has a shadow very close to it. This happens with any eyepiece. When I cover half of the front opening with a piece of paper, the image becomes crisp and perfectly sharp even with the highest magnification that I can achieve (350x). Of course the image is dimmer due to the paper I covered half the aperture with. Then I removed the paper to open up the whole front again to see if adjusting the primary mirror does anything to the image. It did nothing. The shadow of the objects still remained, no matter how I moved the primary or even the secondary mirror.

My conclusion is that the primary mirror must be really bad quality or the secondary mirror is not at the right distance from it. Anyway I decided to return my scope to the seller I bought it from. Now I'm thinking about getting a SkyWatcher Heritage 130p Flextube Parabolic Dobsonian as a starter telescope. I like the portability of it. Or something similar.

[Ricochet]

56 minutes ago, Planetarian said:

Anyway I decided to return my scope to the seller I bought it from. Now I'm thinking about getting a SkyWatcher Heritage 130p Flextube Parabolic Dobsonian as a starter telescope. I like the portability of it. Or something similar.

Great idea. The heritage 130p would be a massive improvement even if your telescope had been in perfect condition. Sending the current one back is the right call, it sounds like there must be something seriously wrong with it. 

[Planetarian]

Update 2: I've got my heritage 130p and I haven't regretted buying it. There's no double image, it's brighter then the other telescope and gives a great image. 

It was the first time i could see Jupiter's bands, but wasn't too visible, probably because the planet is too close to the horizon nowadays and can't get a great viewing condition. 

 

[Planetarian]

First Moon sighting with the Heritage 130p and a phone camera.

[Geoff Barnes]

2 hours ago, Planetarian said:

First Moon sighting with the Heritage 130p and a phone camera.

Excellent! Nothing wrong with that image at all for a phone camera!