Secondary mirror shows on image in eyepiece

[MalusDB]

Hey everyone, my first post here, so apologies in advance for my complete lack of knowledge! And probably for also asking a question that has been asked before

I just got a Skywatcher 76 dob, and have been trying to use it tonight/this morning. I calibrated it as per intructions (aim at something terrestrial about 500m away) and every thing seemed fine, during the day i was practicing focusing in on random objects, and felt I had the hang of it.

So tonight, during a break in the clouds I managed to find what I believe to be Jupiter, but i noted a small black dot in the centre of the image. When I increased the mag, I found out it was actually the secondary mirrors image superimposed on the image of Jupiter(?). I'm wonder what, if anything, I have done wrong, or just some insight into what this is all about. Feeling pretty deflated, despite the fact the Telescope works, this has really put a dampner on my experience.

 

Thanks in advance for any help

[Mak the Night]

Which eyepiece were you using, the 10mm or the 25mm? I have one of those little Dobsonians and I can often see the secondary with a low magnification.

Edited April 4, 2016 by Mak the Night

[MalusDB]

5 minutes ago, Mak the Night said:

Which eyepiece were you using, the 10mm or the 25mm? I have one of those little Dobsonians and I can often see the secondary with a low magnification.

I have a 10mm and 25mm wideview, the stock ones that came with it. I actually seem to notice it at higher magnification, but I can see it using both. Hoping it wasn't damaged in transit, although if it was clear enough to get a look at the moon maybe I could figure out more clearly what is going on. I took a look down the focuser with the eyepiece removed to make sure it's collimated right, and it seems fine, but I noted there's only the one screw attached to the secondary anyway, and no spider, so changing anything there would be beyond me for sure. Is it considered a normal occurence to see the secondary like this? Once I brought Jupiter up to the point it near enough filled the eyepiece the secondary was covering at least 2/3rds of the planet.

 

EDIT: Thanks for taking the time to reply to me btw, appreciate it

Edited April 4, 2016 by MalusDB
because thanks

[Dave In Vermont]

First off - Greetings and welcome to SGL - it's nice that you've found us! Secondly - no need to apologize! We love helping folks find answers to their astro-related questions. It's how we all learn. As someone else coined: The only stupid questions are the one's you don't ask!

A black dot on Jupiter, eh? May I ask what magnification you were using? There's a good reason to believe that your black-dot may have been a shadow from one of Jupiter's moons transiting the planet. Photograph below.

Dave

 

[MalusDB]

7 minutes ago, Dave In Vermont said:

First off - Greetings and welcome to SGL - it's nice that you've found us! Secondly - no need to apologize! We love helping folks find answers to their astro-related questions. It's how we all learn. As someone else coined: The only stupid questions are the one's you don't ask!

A black dot on Jupiter, eh? May I ask what magnification you were using? There's a good reason to believe that your black-dot may have been a shadow from one of Jupiter's moons transiting the planet. Photograph below.

Dave

 

Thanks for the warm welcome! I'm 100% certain it wasn't that, however. I could clearly see the stem from the secondary mirror that holds it in place, and it covered about 2/3rds of the planet while centred. Unfortunately I can't provide photos, I'm literally that new lol. Is there a known cause for dobs to have this issue, the secondary impeding the view of the scope at the higher limits of its mag?

 

Eamonn

 

EDIT: I decided to just add in what I saw to the picture you provided. Needless to say I wasn't seeing anything that crisp and clean but, then again, I wasn't really seeing much of anything for it to be that crisp and clean!

Edited April 4, 2016 by MalusDB
epic photoshop incoming

[Dave In Vermont]

No - this is not normal by any stretch. Having spikes on a star from the spider (as it's called due to it's 3 or 4 arms) is normal, but it shouldn't be blocking-out the object you're looking at.

Frankly - I'm stumped. I'd look into the collimation of the optics. 'Collimate' is the word meaning to align in optics. The primary-mirror and the secondary-mirror should aligned center-to-center. I'll link you to one of the best guides to this by Astro_Baby, who is a member here:

http://www.astro-baby.com/collimation/astro%20babys%20collimation%20guide.htm

Have a good read,

Dave

[MalusDB]

I'll make sure to get a good read through that Dave, thanks for the help! Hopefully I can come back and say it's working fine after this

 

Thanks,

Eamonn

[Mak the Night]

23 minutes ago, MalusDB said:

I have a 10mm and 25mm wideview, the stock ones that came with it. I actually seem to notice it at higher magnification, but I can see it using both. Hoping it wasn't damaged in transit, although if it was clear enough to get a look at the moon maybe I could figure out more clearly what is going on. I took a look down the focuser with the eyepiece removed to make sure it's collimated right, and it seems fine, but I noted there's only the one screw attached to the secondary anyway, and no spider, so changing anything there would be beyond me for sure. Is it considered a normal occurence to see the secondary like this? Once I brought Jupiter up to the point it near enough filled the eyepiece the secondary was covering at least 2/3rds of the planet.

 

EDIT: Thanks for taking the time to reply to me btw, appreciate it

Those little Dob's have a 300mm focal length. That means the 10mm eyepiece will give you a 30x magnification. The 25mm will give you a 12x magnification. At 30x Jupiter won't be much more than a tiny disc. My guess is that you hadn't focused properly and therefore were seeing the secondary. I wouldn't expect too much from that little Dob. You should get some pretty good lunar views though. I've seen the Orion Nebula and the Moon with a 76mm SW Dob at about 53x. I was using a 9mm William Optics eyepiece with a Barlow element threaded into it essentially making it a 5.6mm eyepiece. I've seen Jupiter with the SW 76. Jupiter was a tiny disc, but the four Galilean moons could be seen as tiny points of light.

Edited April 4, 2016 by Mak the Night

[MalusDB]

Okay, the more I think about that, the more sense it makes too. I hope I get some clear skies in the next hour or so, would love to see if it'll give me clear views of the moon. I'll honestly be happy with that, I didn't expect magic from this thing, just for it to at least not block my view haha. Thanks Mak! And by the way Dave, I've been looking at astro baby's site and its a goldmine! Bookmarked already. Thank you both for the great help tonight, can't wait to start contributing to this community!

Many thanks,

Eamonn.

[Dave In Vermont]

Thank you for the breakdown on this scope, MTK. Now I see what's going on here - he's de-focused into a 'black-hole' image of the planet!

Please pass me the Homer-Simpson Award!

Dave

[MalusDB]

So I basically focused beyond the capacity of the telescope? Man I'm hardcore

 

[Mak the Night]

15 minutes ago, MalusDB said:

Okay, the more I think about that, the more sense it makes too. I hope I get some clear skies in the next hour or so, would love to see if it'll give me clear views of the moon. I'll honestly be happy with that, I didn't expect magic from this thing, just for it to at least not block my view haha. Thanks Mak! And by the way Dave, I've been looking at astro baby's site and its a goldmine! Bookmarked already. Thank you both for the great help tonight, can't wait to start contributing to this community!

Many thanks,

Eamonn.

You're welcome. Those little Dob's can be great fun just as long as you realise they can have limitations. You should get some decent views of the Moon though. It won't rise over the UK until around 05:00 BST today though lol. I got some good lunar viewing a couple of days ago on a tabletop Dob mounted Mak.

Check these freeware programs out for lunar/planetary positions:

http://www.stellarium.org/en_GB/

I started a thread about my lunar observation:

https://play.google.com/store/apps/details?id=com.celestron.skyportal

Edited April 4, 2016 by Mak the Night

[MalusDB]

Guys I finally got a good night, and was able to get an ACTUAL look at jupiter. it was tiny but i could make out 4 of the moons, 2 which couldn't be resolved. No more black dot problem, I learned my limits with this scope. Hoping for a clear sky at moonrise! Thanks again for the help!

[JohnP292]

Yup....that was my problem too!

[GracetH]

Hi everyone! Another newbie here. Have really enjoyed this discussion. It’s taken me 2 years to gain the courage to try my scope again and making slow but significant progress. Having said that, I also see a large black bob of the secondary mirror when I try and look at Jupiter. I’ve got the Skywatcher Heritage 130p. I think I’ve collimated it as I focused on a tree. I saw the full moon last night and it was beautiful! Much less luck with a Jupiter. I have to say I had a good giggle at the suggestion that the black blob I was seeing was the shadow of Jupiter’s moon. How did you resolve your issue!? I don’t have a barlow, not sure if that matters. Maybe won’t be able to see much with the 10mm and 25mm eye pieces I have? I’m based in London so light might not be great…But had a clear view of Jupiter quite high in the sky with no luck. Thanks for your help! 

[wimvb]

If you see a donut like object, bright ring with black centre, that’s most likely a focusing issue. Daytime focusing on a distant object will get you close, but is not enough for stars and planets. The moon is so large that even with focus off, you can still see some details. The easiest way to get good focus is to buy or make a Bahtinov mask.

https://www.firstlightoptics.com/bahtinov-focus-masks/starsharp-bahtinov-focus-masks.html

Edited September 10, 2022 by wimvb

[bomberbaz]

On 10/09/2022 at 08:32, GracetH said:

Hi everyone! Another newbie here. Have really enjoyed this discussion. It’s taken me 2 years to gain the courage to try my scope again and making slow but significant progress. Having said that, I also see a large black bob of the secondary mirror when I try and look at Jupiter. I’ve got the Skywatcher Heritage 130p. I think I’ve collimated it as I focused on a tree. I saw the full moon last night and it was beautiful! Much less luck with a Jupiter. I have to say I had a good giggle at the suggestion that the black blob I was seeing was the shadow of Jupiter’s moon. How did you resolve your issue!? I don’t have a barlow, not sure if that matters. Maybe won’t be able to see much with the 10mm and 25mm eye pieces I have? I’m based in London so light might not be great…But had a clear view of Jupiter quite high in the sky with no luck. Thanks for your help! 

Sounds like a focusing issue. If you are seeing a large ish fuzzy blob with a black shadow then that is the problem. 

Try adjusting the focuser until all you see is a small crisp dot, this is jupiter. The jovian moons should also come into focus and the secondary mirror shadow will be gone. 

Try it on the moon then swing round to jupiter without altering focus and you will be there.

Note. Using the 25mm on the moon may show the secondary, it did in the 100p i used to have.