5" SCT + 10mm BCO + Barlow = Blob in middle of FOV

[theironduck]

I got my first telescope yesterday (most exciting!), a Celestron Nexstar 5se (1250mm, f/10), along with a selection of eyepieces: 10mm, 25mm and 2.25x barlow. The weather gods decided not to play nicely - predictable cloudy skies - so I was familiarising myself with the scope and mount by looking at objects in the local neighbourhood: I am now a connaiseur of brick patterns, guttering and street lighting!

The views with the 10mm and 25mm eyepieces on their own were great as was the 25mm plus barlow but something strange happened when I tried to use the barlow with the 10mm eyepiece: I could see a small blob/disk in the centre of the FOV, covering about 1/4 the width, which seemed to have a different focus from the rest, almost like a water droplet. When I focussed on an object the disk/blob would merge with the rest of the FOV. Any ideas as to what's causing this? Is this normal? Am I doing something wrong? Do I need to worry about it?

Thanks!

[John]

The 10mm / 2.25x barlow combination will be giving 281x with your scope - way more than would be useful. 200-250 would be the highest practical power and the higher of those assues perfect conditions, perfectly cooled scope, perfect collimation etc.

On the "blob", using a barlow can show up both floaters in your eye and dust / debris on the field lens of either the eyepiece or the barlow itself so using a blower to puff off any dust from the lenses might help.

It's really too much power for the scope though.

[theironduck]

Thanks for your reply John (a fellow N Somersetter I see). Yes, you are quite right that the 10mm with the 2.25x is probably a bit overenthusiastic for my scope. That said, the barlow is the Baader Classic which is configurable as a 1.3x too and I saw this effect even in that more modest, 162.5x, configuration.

I'm pretty sure the effect wasn't due to a floater or debris in/on a lens (it only showed up when the 10mm and barlow were used together, not when used separately), but I will take a good look tonight.

[John]

I've actually got a Baader 2.25x barlow and the 10mm Baader Classic ortho at the moment but I can't say I've seen this issue when using this combination with my scopes. The two work really well together I think with the barlow in either 1.3x or 2.25x mode

Where abouts in North Somerset are you by the way ? - I'm at Portishead.

[RT65CB-SWL]

...The weather gods decided not to play nicely - predictable cloudy skies - so I was familiarising myself with the scope and mount by looking at objects in the local neighbourhood: I am now a connaiseur of brick patterns, guttering and street lighting!

... so I am not the only one!

Back to subject in question. I get the same/similar result when I couple my Barlow (Klee 2.8x and eyepiece) with my ETX105. (It also happens when I use the ETX105 with a 6mm Radian too). I think it maybe due to the Barlow 'magnifying' the secondary and narrowing the field of view even further. I am no expert on optics/telescope design, so that is my theory.

[Stargazer_00]

Could it be the central obstruction if you were far enough out of focus?

[theironduck]

Where abouts in North Somerset are you by the way ? - I'm at Portishead.

Portishead too - in the Village Quarter!

[theironduck]

Could it be the central obstruction if you were far enough out of focus?

I wondered about that - especially given the geometry. It's interesting that Philip R is seeing it too with a MAK but not John. I'll play around with it again tonight if I can get a better handle on it and maybe try to get some photos.

Philip R

[John]

Portishead too - in the Village Quarter!

I'm on the hill near Highdown School.

We will have to have a chat sometime !

I don't see indications of the secondary of my newtonian when using the barlowed 10mm but it's proportionately quite a lot smaller than that of an SCT or mak.

[theironduck]

I'm on the hill near Highdown School.

We will have to have a chat sometime !

That would be fantastic! Are there any SGL or other astronomy groups around our way? I see the Bristol Astronomical Society do the odd meet at Tyntesfield. I vaguely know / knew some of its members from when I studied at the physics department at Bristol Uni.

I don't see indications of the secondary of my newtonian when using the barlowed 10mm but it's proportionately quite a lot smaller than that of an SCT or mak.

Sounds like it might be a magnification, rather than an equipment, issue then, especially if Philip R is seeing it with his 6mm eyepiece on his ETX105.

[John]

That would be fantastic! Are there any SGL or other astronomy groups around our way? I see the Bristol Astronomical Society do the odd meet at Tyntesfield. I vaguely know / knew some of its members from when I studied at the physics department at Bristol Uni....

Bristol AS and the National Trust still do the public viewing evenings at Tyntesfield - you can get on an e.mail list so thay alert you when one is planned.

99% of the time I'm observing from the garden though. It would be good to meet and have a chat. Drop me a PM if you want to fix something up sometime.

[RT65CB-SWL]

Yes... I forgot to mention that the secondary on a Mak or SCT, ie the central obstuction, can be bigger than the secondary of a Newt.

As said in my post "...I am no expert on optics/telescope design, so that is my theory."

[RT65CB-SWL]

Another thing I forgot to mention...

Many years ago I had a 35mm film SLR and I decided to buy a third party 80-200mm zoom lens. The "focus spot" in the view finder would do something similar on one half. I remember querying this with the retailer from where I purchased the lens from. The assistant told me that it was due to the pupil of the eye being off-centre to the light cone. It sounds very similar and what could be happening is that the higher magnification of Barlow lens/eyepiece, (or combination of both), maybe 'cutting' or 'cropping' the point before it reaches 99.9-100% of focus.

[theironduck]

I had another quick look at the phenomenon and have now spotted the same thing happening, but to a lesser extent, even with the 10mm eyepiece on its own: there is a small ring in the centre of the FOV which appears a bit brighter and seems to focus differently from the rest. It is most noticeable when the image is out of focus so hopefully it won't be visible when I actually get to look at the sky with it (curse you clouds!).

[theironduck]

Just going by the shape - a disk/ring - and the fact that it only appears at higher magnifications (although the 10mm is only 125x so not terribly high) I'm guessing it's related to the obstruction caused by the secondary mirror as Philip R suggests.

[Moonshane]

this sounds like the eye relief (how far away your eye is from the eyepiece to see the whole field) is increasing when using the barlow and you are getting some sort of kidney beaning effect as your eye is not positioned correctly. try the set up again and move your eye back and forth when focused. you'll find a point where the whole field is visible. you should be able to replicate what you saw by moving your eye back and forth.

the exit pupil is also very small at the high mags you were trying (about 0.4mm) so eye positioning is quite critical.

you might be able to use that mag on double stars but not much else.

[Ohmless]

methinking what moonshane said. perhaps more play time is in order before trying to figure it out in the dark.

this sounds like the eye relief (how far away your eye is from the eyepiece to see the whole field) is increasing when using the barlow and you are getting some sort of kidney beaning effect as your eye is not positioned correctly. try the set up again and move your eye back and forth when focused. you'll find a point where the whole field is visible. you should be able to replicate what you saw by moving your eye back and forth.

the exit pupil is also very small at the high mags you were trying (about 0.4mm) so eye positioning is quite critical.

you might be able to use that mag on double stars but not much else.

[Macavity]

During white light solar observation, with a MAK, I get all sorts of "blobs" - Especially when I increase the number of glass elements (barlows, filters) into the chain. If it's daylight and it's diffuse, faint and on axis, maybe an internal reflection. Perhaps (see above!) some effect of exit pupil, a variation on eyepiece blackout, kidney-beaning etc. Scopes are night-time beasts, really - You may well not notice these problems, during dark observations. The eye pupil is a lot wider at night, which often helps.

[Stargazer_00]

During the day on white light solar I find the biggest issue is light from my eyeball reflecting onto the eyepiece.

Nothing a towel can't fix though