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Tom Baer
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Reged: 01/05/13

Loc: Seattle, WA
Should stars look like sharp points... new
      #5618187 - 01/11/13 10:59 PM

... in a 4.5 inch f/4 Newtonian reflector (Starblast 4.5)? The best I can focus, stars look a little bit like tiny squashed bugs. Is this a normal limitation of the telescope, or maybe it's in my eyes, or might something else be going on?

I'm interested in learning a bit more about optics, so even if this is normal, I would love to know why.


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lamplight
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Re: Should stars look like sharp points... new [Re: Tom Baer]
      #5618197 - 01/11/13 11:08 PM

Hi Tom and welcome.

Squashed bugs is a new one but I think I know what you mean. You will notice some stars, some. Nights, some girls.. No that's another story..

Sometimes the stars are particularly twinkly. That's when you have a lot of atmosphere interference. That's why people recommend viewing at the zenith (straight up), if you can . stars should be sharper there. Some nights (a lot lately here) everything is fuzzy.. Atmosphere. if you have a lot of moisture in the air as I would bet based on your location that just makes it worse.

Not sure how long you've had it but see if it improves with different nights and if the effect is lessened on overhead stars.

Since it is a reflector I'd definitely check into its collimation. I'm not familiar with particulars of smaller models but I do know the faster the scope the more criticality is. (Yours is fast).

Edited by lamplight (01/11/13 11:11 PM)


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JLovell
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Re: Should stars look like sharp points... new [Re: lamplight]
      #5618205 - 01/11/13 11:17 PM

f4 is pretty fast, and less well corrected (tending to be cheaper too, but not necessarily) eyepieces tend to show their flaws. Collimation is also critical. The "squashed bugs" are probably due to the scope needing better collimation, and the eyepieces showing their flaws.

FYI, around f6 is the general dividing line of fast vs slow, with lower meaning faster.


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Jon Isaacs
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Re: Should stars look like sharp points... new [Re: Tom Baer]
      #5618284 - 01/12/13 12:22 AM

Quote:

... in a 4.5 inch f/4 Newtonian reflector (Starblast 4.5)? The best I can focus, stars look a little bit like tiny squashed bugs. Is this a normal limitation of the telescope, or maybe it's in my eyes, or might something else be going on?

I'm interested in learning a bit more about optics, so even if this is normal, I would love to know why.




Tom:

You didn't mention the eyepieces you are using, I believe the Starblast comes standard with either Explorer II eyepieces or Expanse eyepieces.

There are a variety of possible reasons the stars are not sharp, it could be the collimation/alignment of the scope, it could be the atmospheric stability/seeing, it could be the temperature of the telescope, the eyepieces factor in as do your own eyes. And too, away from the center of the view, an F/4 Newtonian shows an aberration called coma.

Some variety of these various factors add up to "squashed bugs."

The goal is for the stars to be sharp everywhere you look, some telescopes are better at this than others, some telescopes require more forgiving eyepieces to get the good clean views.

Have you collimated it yet?

Jon


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GlennLeDrew
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Re: Should stars look like sharp points... new [Re: Jon Isaacs]
      #5618290 - 01/12/13 12:25 AM

And don't neglect the importance of the exit pupil. At larger exit pupils, your eye's own inherent imperfections, if present, will contribute. You may find the tightest images when the exit pupil is 1.5-2mm.

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dpwoos
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Re: Should stars look like sharp points... new [Re: Tom Baer]
      #5618337 - 01/12/13 12:56 AM

Quote:

... in a 4.5 inch f/4 Newtonian reflector (Starblast 4.5)? The best I can focus, stars look a little bit like tiny squashed bugs. Is this a normal limitation of the telescope, or maybe it's in my eyes, or might something else be going on?

I'm interested in learning a bit more about optics, so even if this is normal, I would love to know why.




You don't say at what magnification(s)?


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Tom Baer
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Reged: 01/05/13

Loc: Seattle, WA
Re: Should stars look like sharp points... new [Re: dpwoos]
      #5618382 - 01/12/13 01:59 AM

Thanks so much everyone!

I just came inside after some experimenting, and I tried all of these combinations on brighter stars high in the sky:
17 mm Plossel (came with the scope), 26x
17 mm plus 2x Shorty Barlow, 53x
6 mm Plossel (came with scope) + Barlow, 150x
6 mm Orion Expanse, 75x
6 mm Orion Expanse + Barlow, 150x

I checked the collimation per Orion's instructions by looking at a star and de-focusing as much as I could, and the blurry blob was perfectly round and the black circle was in the middle.

I have noticed that the "squashed bug" look is almost nonexistant on dim stars. The effect is most obvious looking at bright stars not in the center of the FOV the Expanse eyepiece.

I am wondering if I am seeing a combination of my eye's response to the glare of bright stars, combined with coma. When the stars are near the edge of the FOV in the 6mm eyepiece, the glare definitely has a wedge shape, with the pointy end pointing toward the center of the FOV.

Here is a surprise I got: when I added the Barlow to the 6mm Expanse, the coma (if that's what it is), the wedge effect was reduced consiterably. Could this be because the Barlow is lowering the f ratio by doubling the focal length?



Edited by Tom Baer (01/12/13 02:03 AM)


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Tony Flanders
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Re: Should stars look like sharp points... new [Re: Tom Baer]
      #5618480 - 01/12/13 05:31 AM

Quote:

When I added the Barlow to the 6mm Expanse, the coma (if that's what it is), the wedge effect was reduced consiterably. Could this be because the Barlow is lowering the f ratio by doubling the focal length?




Yes, that's quite likely. If that's the explanation, you should see the same effect when you use the Barlow with your 17-mm Plossl as well.

If you don't see the improvement when using the 17-mm Plossl with the Barlow, the most likely culprit is your own eyes. A 6-mm Plossl plus a Barlow is finally getting down to an exit pupil where most people's eyes operate really well. If you have serious astigmatism, it might well give you distorted stars at a 1.5-mm exit pupil.


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rflinn68
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Re: Should stars look like sharp points... new [Re: Tony Flanders]
      #5618582 - 01/12/13 08:18 AM

Sounds like coma to me. At only 450mm focal length you'll probably want to use your barlow most of the time anyway. Enjoy it that way for a while and have some fun. You may want to purchase a coma corrector someday. Those are neat little scopes.

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Jon Isaacs
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Re: Should stars look like sharp points... new [Re: Tom Baer]
      #5618613 - 01/12/13 08:48 AM

Quote:

I have noticed that the "squashed bug" look is almost nonexistant on dim stars. The effect is most obvious looking at bright stars not in the center of the FOV the Expanse eyepiece.

I am wondering if I am seeing a combination of my eye's response to the glare of bright stars, combined with coma. When the stars are near the edge of the FOV in the 6mm eyepiece, the glare definitely has a wedge shape, with the pointy end pointing toward the center of the FOV.

Here is a surprise I got: when I added the Barlow to the 6mm Expanse, the coma (if that's what it is), the wedge effect was reduced consiterably. Could this be because the Barlow is lowering the f ratio by doubling the focal length?




Tom:

Good testing, good information.

Sharp stars in the very center of the view means the collimation is probably acceptable. The factors you mention may all be part of what you are seeing. There is also another factor, the eyepiece.

Off-axis, that is away from the center, stars that are not sharp can be either coma, definitely present in an F/4 Newtonian, or off-axis astigmatism in the eyepiece. Coma is inherent in a parabolic Newtonian, to eliminate coma, a coma corrector is required.

But it seems like what you are seeing is the off-axis astigmatism in the eyepiece. Most eyepieces are not sharp off-axis at F/4, such a fast focal ratio makes it hard for an eyepiece to provide sharp images off-axis. And generally the off-axis astigmatism in the eyepiece generally is greater than the coma from the mirror.

The fact that the off-axis sharpness improved when you added the Barlow indicates that it is eyepiece that is causing the aberrations. As you correctly reasoned, the Barlow increased the effective focal ratio from F/4 to F/8, this makes it easier on the eyepiece and so the astigmatism is significantly reduced. The Barlow does not affect the actual focal ratio of the telescope so it does not affect the coma.

One can spend large sums of money to buy eyepieces that are essentially perfect across the field of view and with a coma corrector, another large sum of money, the views can be essentially perfect. But such perfection is not needed or affordable.

Those fancy eyepieces, they start out just like you did, they start with a Barlow lens or really a Smyth lens that is very similar to a Barlow. So, you are already on the way to learning how to get the most out of your new scope...

People sometimes refer to a Barlow as a "Poorman's coma corrector." This is not because it reduces the coma but because it reduces the eyepiece astigmatism that is often mistaken for coma.

Jon


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Tony Flanders
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Re: Should stars look like sharp points... new [Re: Jon Isaacs]
      #5618639 - 01/12/13 09:12 AM

Quote:

One can spend large sums of money to buy eyepieces that are essentially perfect across the field of view and with a coma corrector, another large sum of money, the views can be essentially perfect. But such perfection is not needed or affordable.




Indeed! This will "only" cost about 3 times as much as the entire telescope.

The StarBlast is at its best as a low-power browsing instrument. It's equivalent to a pair of giant binoculars that just happens to have the ability to crank up the magnification to yield surprisingly respectable images at 100X or higher.

If you want great image quality, you can achieve much the same thing with an apochromatic refractor in the 80-100 mm range. At a cost, of course.


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Jon Isaacs
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Re: Should stars look like sharp points... new [Re: Tony Flanders]
      #5618676 - 01/12/13 09:35 AM

Quote:

Quote:

One can spend large sums of money to buy eyepieces that are essentially perfect across the field of view and with a coma corrector, another large sum of money, the views can be essentially perfect. But such perfection is not needed or affordable.




Indeed! This will "only" cost about 3 times as much as the entire telescope.

The StarBlast is at its best as a low-power browsing instrument. It's equivalent to a pair of giant binoculars that just happens to have the ability to crank up the magnification to yield surprisingly respectable images at 100X or higher.

If you want great image quality, you can achieve much the same thing with an apochromatic refractor in the 80-100 mm range. At a cost, of course.






Indeed, a $4000 TeleVue NP-101 with a $700 31mm Nagler provides a 4.5 degree TFoV with stars that are sharp from the very center of the view of view right out to the edge. They are great scopes. That's a lot of money and they sell about 100 scopes per year.

The Starblast might not be so perfect but it does a solid job and with tricks like the ones Tom is already figuring out, the Starblast can do some amazing things and show a lot of neat stuff.

In my view, learning to observe is the most important thing one can do, the equipment is secondary.

Jon


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CJK
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Reged: 12/05/12

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Re: Should stars look like sharp points... [Re: Jon Isaacs]
      #5618679 - 01/12/13 09:37 AM

Man, I always learn so much reading this site!

Really want to thank all the knowledgeable folks who regularly post here (and not just in this thread)!

-- Chris


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Achernar
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Re: Should stars look like sharp points... new [Re: Tom Baer]
      #5618745 - 01/12/13 10:26 AM

If the optics are good and the telescope is collimated properly, indeed stars should be sharp points of light when the seeing is good. However, you should also be aware that a Newtonian with a focal ratio of F/4 also has a LOT of coma, an abberation inherit to many telescopes that distorts stars into comets around the outer portions of the field of view. That is why coma correctors live in my 10 and 15-inch Dobsonian's focusers. If you are not getting pinpoint stars anwhere in the field of view, it's because of defective optics, really bad seeing, or the telescope's optics are misaligned. Another possible cause could be your eyes, I have enough astigmatism to make it look like every telescope has defective optics, but when I turn my head the elliptical pattern of a defocused star moves with it. If you see pinpoint stars near the center of the field of view that progressively worsen towards the edge, that is coma. Be aware that many simpler eyepieces don't cope well with an F/4 telescope, and even higher end eyepieces that are well corrected can have trouble too, hence the appearance of coma correctors. But even so, the stock eyepieces that probably came with it are far from ideal for an F/4 telescope. Try an ES 82 or 68 degree eyepiece with a 1.25-inch barrel in it, the huge apparent and actual field of view will impress you.

Taras


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Tom Baer
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Reged: 01/05/13

Loc: Seattle, WA
Re: Should stars look like sharp points... new [Re: Achernar]
      #5619147 - 01/12/13 02:09 PM

This forum is one of my favorite things about having a telescope. Thanks so much for the great information.

I wonder, when I look at a bright star in the telescope or with my naked eyes, and I see what looks like a round splash of glare (like a tiny, bright dandelion), is that in part due to diffraction around the edges of my pupil?

At my age (47) I am sure there are plenty of things happening in my eyes that affect how I see. For instance, I wonder if some of the twinkling I see in the eyepiece is the atmosphere + floaters.


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Achernar
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Re: Should stars look like sharp points... new [Re: Tom Baer]
      #5619170 - 01/12/13 02:26 PM

You're welcome Tom. It's possible, some of the "hairyness" people see on stars originates in their eyes. When I am very tired stars can take on a wierd appearance too. Floaters could also affect what you see as well, I am the same age as you are and sometimes they announce their presence in my vision too.

Taras


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