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Possibly a daft question about images & eyepieces
https://popastro.com/forum/viewtopic.php?f=14&t=20803
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Author:  SueGS [ Tue Jul 24, 2018 2:07 pm ]
Post subject:  Possibly a daft question about images & eyepieces

Hi @ the SPA forum

I have been playing around with a Celestron First Scope (FL = 300 mm; Apert = 76 mm), my iPhone 6 & a Svbony Metal Universal Camera Mount Telescope Digital Camera Adaptor (which is very fiddly to set up!).

I took out my 2x Barlow lens, inserted a Celestron 20 mm FL len with adapter/iPhone attached only to discover a black blob (please see dreadful images attached) in the centre of the image. I could also see the blob if I looked directly into the EP & looked 'around' the FoV, but it wasn't centrally placed. I cleaned everything I could get my hands on, but the problem remained: no amount of tiddling with the focus got rid of it. After faffing around & trying a Meade 40 mm FL EP & getting the same result, I decided to try again plus Barlow lens - the blob 'disappeared'. I've never seen this blob before, but then never used the scope without the Barlow.

Am I right - probably not - that this is some kind of weird, optical, focussing issue/abberation (sorry, not a very technical explanation) with 'long' FL lenses plus such a short 'scope? I've done an internet search & the nearest thing I can find is that the blob is a 'image' of the secondary mirror.

Did O-level physics 40-ish years ago, so have forgotten much of the optics stuff - I'm so embarrassed :roll:

I'm keen to learn anything astro-related so can anyone shed some light on this mysterious blob?

Many thanks in advance.

BTW: I do a have a larger Newtonian reflector in storage awaiting the dark winter skies :)
The little scope is for playing with &looking at the moon :P

Attachments:
IMG_2828.JPG
IMG_2828.JPG [ 37.63 KiB | Viewed 1747 times ]
IMG_2811 (1).jpg
IMG_2811 (1).jpg [ 48.44 KiB | Viewed 1747 times ]

Author:  brian livesey [ Tue Jul 24, 2018 2:30 pm ]
Post subject:  Re: Possibly a daft question about images & eyepieces

Here's a daft reply. There was a scifi film in the 1950s called "The Blob", but I can't remember how they got rid of the wobbly alien. Maybe the desperate townsfolk added custard and ate it. :lol:
Sorry to seem to be frivolous Sue, but it's a bit of a mystery. Maybe it's due to the exit pupil being too big and producing the equivalent of the "kidney bean" effect.

Author:  SueGS [ Tue Jul 24, 2018 2:43 pm ]
Post subject:  Re: Possibly a daft question about images & eyepieces

Hi Brian

I think you might be right about exit pupil effect :)

Otherwise I might spray it with sea water as per the Quatermass :roll:

Thanks for taking time to reply.

Author:  David Frydman [ Tue Jul 24, 2018 3:43 pm ]
Post subject:  Re: Possibly a daft question about images & eyepieces

The Blob is a famous movie because of the early appearance of actor Steve McQueen.
The special effects are very basic.

The blob in the scope could be a UFO or maybe an eclipse.

More likely the diagonal.

I think that these small scopes have big diagonals relative to the objective.

An 80mm f/5 refractor may work better.

Regards,
David

Author:  Brian [ Tue Jul 24, 2018 6:24 pm ]
Post subject:  Re: Possibly a daft question about images & eyepieces

Is this a reflector you are using? So no diagonal? From what you say I think this is the case. Looks to me like its a focusing problem, the background is not in focus and the blob should not be seen at true focus.

Found this:
https://www.youtube.com/watch?v=CiQoAxby8Rk

regards,

Author:  SueGS [ Tue Jul 24, 2018 6:37 pm ]
Post subject:  Re: Possibly a daft question about images & eyepieces

Hi Brian

yes, it's a reflector... shall check out thUtube thing later.

Thank a lot :)

Author:  mike a feist [ Wed Jul 25, 2018 5:56 am ]
Post subject:  Re: Possibly a daft question about images & eyepieces

Low magnification on very small instruments with a secondary mirror obstruction , either in Newtonian or Maksutov-type configuration, always produce this very annoying shadow. Using a higher magnification improves the problem but the image will them be poor. Even on a large reflector with a secondary obstruction it is impossible to use a very low power eyepiece without this annoying blob appearing. Regards maf.

Author:  SueGS [ Wed Jul 25, 2018 6:43 am ]
Post subject:  Re: Possibly a daft question about images & eyepieces

Hi Mike
Thanks for your input - much appreciated.
I noticed the shadow ‘disappear’ at higher magnification.
Just glad that there’s nothing gone wrong with my ‘scope.

Author:  SueGS [ Wed Jul 25, 2018 6:50 am ]
Post subject:  Re: Possibly a daft question about images & eyepieces

Hi Mike
Thanks for your input - much appreciated.
I noticed the shadow ‘disappear’ at higher magnification.
Just glad that there’s nothing gone wrong with my ‘scope.

Author:  skyhawk [ Wed Jul 25, 2018 10:17 am ]
Post subject:  Re: Possibly a daft question about images & eyepieces

There is NO SUCH thing as a DAFT question, just remember that.

Author:  SueGS [ Wed Jul 25, 2018 11:37 am ]
Post subject:  Re: Possibly a daft question about images & eyepieces

Thanks Skyhawk :) I was just a bit embarrassed in case I was doing something stupid :oops:

Author:  skyhawk [ Wed Jul 25, 2018 4:52 pm ]
Post subject:  Re: Possibly a daft question about images & eyepieces

Three weeks ago (I am a PRO photographer and NUJ REGISTERED journalist) and shoot for a glossy magazine, I went out with the intent to shoot Kites (birds).

D4s, D810, 150-600 Sport lens, arrived 40 miles away, NO CF card in cameras.

AND my son had taken the spares out of the car when cleaning

And YOU may feel STUPID :) :) :)

Author:  SueGS [ Wed Jul 25, 2018 5:18 pm ]
Post subject:  Re: Possibly a daft question about images & eyepieces

:lol: :lol: :lol:

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