First light with the Starfield 102 ED

[Tiny Clanger]

5 hours ago, quasar117 said:

...  as before the Star appeared to be 'hairy' around the circumference and not pinpoint like the stars in the background starfield. Also if my eye was not perfectly centered the star would distort and create a spike.

In-focus revealed concentric rings. However, out-focus I could not discern any rings it just appeared as a solid white circle. Is this normal?

For comparison I dusted off my old department store "toy' refractor. I was surprised that when focused on a star, the star appeared more pinpoint than viewing through the Starfield 102. Very strange.

Whilst I'm happy with the lunar views, I still felt something is not quite right when viewing stars. Could it be just the seeing again?

 

 

I wonder if what you are seeing is astigmatism in your eye showing up in the Starfield, but not in your 'toy' refractor ?

As I understand it (i.e. not very much) any astigmatism your eye may have will show more with a setup's greater exit pupil. If  the old 'toy' 'scope has a higher f number vs the Starfield, perhaps the smaller exit pupil it gives allowed you to see pinpoint stars ... it could also be the reason you never saw the problem in a mak.

 

[quasar117]

1 hour ago, Tiny Clanger said:

I wonder if what you are seeing is astigmatism in your eye showing up in the Starfield, but not in your 'toy' refractor ?

As I understand it (i.e. not very much) any astigmatism your eye may have will show more with a setup's greater exit pupil. If  the old 'toy' 'scope has a higher f number vs the Starfield, perhaps the smaller exit pupil it gives allowed you to see pinpoint stars ... it could also be the reason you never saw the problem in a mak.

 

That's definitely a plausible answer. The old refractor is a 60mm Simmons generic department store telescope. Can't remember  the focal length/ratio  but guessing its around F/9.

As it happens I'm going for a eye test this afternoon so will ask the question about astigmatism.

[Tiny Clanger]

30 minutes ago, quasar117 said:

That's definitely a plausible answer. The old refractor is a 60mm Simmons generic department store telescope. Can't remember  the focal length/ratio  but guessing its around F/9.

As it happens I'm going for a eye test this afternoon so will ask the question about astigmatism.

Timely !

Not sure if it's a good thing if the answer is your eye or not ... it would absolve the new 'scope tho' !

Edited April 13 by Tiny Clanger
Fat finger syndrome

[quasar117]

Apparently my eyesight has improved since my last check in 2021! Also minimal  signs of astigmatism - so I can probably rule out my vision affecting it.

Hoping it's a clear night tonight.

[Tiny Clanger]

OK, so cross that possibility off the list.

You tried various eyepieces, and it showed in all of them. Could you try removing the diagonal and seeing if a straight through view still shows the problem ?

The only other kit related (rather than seeing related) possibility I can think of is something inside the 'scope protruding into the optical path and causing diffraction (similar to the spikes caused by the  secondary supports in a Newtonian)

 

[quasar117]

20 minutes ago, Tiny Clanger said:

OK, so cross that possibility off the list.

You tried various eyepieces, and it showed in all of them. Could you try removing the diagonal and seeing if a straight through view still shows the problem ?

The only other kit related (rather than seeing related) possibility I can think of is something inside the 'scope protruding into the optical path and causing diffraction (similar to the spikes caused by the  secondary supports in a Newtonian)

 

Yes I'm going try directly mounting the EPs to eliminate the diagonal from the equation.

The sky isn't looking great here at the moment, looks like a good chance of rain soon. 

Should I see an airy disk both inside and outside of focus? As yesterday I could only see it when racking the focuser in.

[John]

10 minutes ago, quasar117 said:

...Should I see an airy disk both inside and outside of focus? As yesterday I could only see it when racking the focuser in.

If the seeing is not too good, the pattern of diffraction rings around the airy disk will not be well defined on one side of sharp focus as it is on the other. 

This test needs to be done at quite high magnification - around 200x for a 4 inch scope is good.

This is what a decent 4 inch refractor star test should look like under very good seeing conditions:

Under less than good seeing conditions, the image on one side of focus may well look "mushy" rather than showing well defined diffraction rings. You might still get a nice in focus image though.

Polaris is a good test star. 

Edited April 13 by John

[dweller25]

9 minutes ago, quasar117 said:

Yes I'm going try directly mounting the EPs to eliminate the diagonal from the equation.

The sky isn't looking great here at the moment, looks like a good chance of rain soon. 

Should I see an airy disk both inside and outside of focus? As yesterday I could only see it when racking the focuser in.

Pick a very high star for best seeing

[dweller25]

Are you seeing any of these ?

[quasar117]

16 minutes ago, John said:

If the seeing is not too good, the pattern of diffraction rings around the airy disk will not be well defined on one side of sharp focus as it is on the other. 

This test needs to be done at quite high magnification - around 200x for a 4 inch scope is good.

This is what a decent 4 inch refractor star test should look like under very good seeing conditions:

Under less than good seeing conditions, the image on one side of focus may well look "mushy" rather than showing well defined diffraction rings. You might still get a nice in focus image though.

Polaris is a good test star. 

So last night on Polaris I could get good diffraction rings in intra focus but no notable ring separation in extra focus.

The intra focus image was stable. 

[quasar117]

16 minutes ago, dweller25 said:

Are you seeing any of these ?

The image I was seeing on Polaris in intra focus was as per the top left example.  The bigger I made the image the more rings appeared.

[dweller25]

See here

https://www.cloudynights.com/topic/611161-doubts-about-taka-fc100d-star-test/

But - all that matters is the image at perfect focus 👍

Edited April 13 by dweller25

[quasar117]

The closest example to what I'm seeing when in focus on a star is as per this image although the black spikes are more severe in this image than what I'm seeing. 

[dweller25]

@quasar117

I have just been out with the Takahashi 4” F/8 refractor.

Jupiter at 11’ high is too low for any meaningful observation now for me.

I spent some time looking at Aldeberan which was 24’ high at x150 power.

It appeared bright, unsharp and unsteady, the dimmer stars in the same field of view were more stable and sharper.

At perfect focus Aldeberan showed an unsteady airy disk, either side of focus did show an unstable airy pattern that was identical.

This is just a data point for you to compare with your observations 👍

To summarise, this is what I would expect from poor seeing and low altitude observing.

Had to observe from the conservatory as I have a bad back, so had a very limited view and could not look any higher !

Edited April 13 by dweller25

[dweller25]

13 hours ago, quasar117 said:

The closest example to what I'm seeing when in focus on a star is as per this image although the black spikes are more severe in this image than what I'm seeing. 

Yes, apart from the spikes that is how Aldeberan looked tonight compared to the dimmer stars in the same field of view - but with an airy disk.

Edited April 14 by dweller25

[Sunshine]

Be patient and don’t rush to judgment too soon, both Tak scopes I have owned, one being a TSA 102 took several sessions before condition ls were right and they showed what they could do.

[TiffsAndAstro]

1 hour ago, Sunshine said:

Be patient and don’t rush to judgment too soon, both Tak scopes I have owned, one being a TSA 102 took several sessions before condition ls were right and they showed what they could do.

I am definitely in no rush.

I need a lot more time just doing this my first proper run with everything going. Auto guiding is a bit janky but there is some wind gusts. Really clear though

[Louis D]

IIRC, rings on one side of focus and smoother on the other side indicates spherical aberration of the objective.  Refer to telescopeѲptics.net Refracting Lens Objective page for more information.  I've grabbed a snippet of the spherical aberration explanation below:

[dweller25]

@quasar117

Here is a useful summary of optical tests.

BUT do bear in mind that what really counts is how the views at perfect focus look.

[quasar117]

Sorry for the slow reply. I've come down with the flu the past couple of days. Which is typically, as I bet there will be good seeing tonight 🙃

[quasar117]

20 hours ago, Louis D said:

IIRC, rings on one side of focus and smoother on the other side indicates spherical aberration of the objective.  Refer to telescopeѲptics.net Refracting Lens Objective page for more information.  I've grabbed a snippet of the spherical aberration explanation below:

So if I'm seeing rings on intra focus but misty rings in extra focus the scope could be under corrected for spherical aberration?

With good seeing I should see the focused star appear as an airy disk that is pin sharp?

Could an artificial star test (torch with pinhole in the centre of covered lens) achieve the same results or is that a waste of time?

[Mr Spock]

Just do a star test. The 102 I had was perfect inside and out, and showed a classic airy disc.

[quasar117]

Tonight was clear and steady to my eye. The star I selected was Dubhe; due it being high in the sky and relatively bright.

Intra focus revealed a nice diffraction pattern of concentric rings. In extra focus I could just about see some contrast between rings but the whole thing appears very milky and misty.

In focus I could see the airy disk which was quite steady- so can assume seeing was good. However, it was accompanied by flaring around the edge shooting off at symmetrical angles. I believe this is either astigmatism or spherical aberration? This feature could also be seen in intra and extra focus. Those airy disks did not appear oval but round. I believe astigmatism would have shown these to be oval. So am slightly confused at this.

Briefly observed Jupiter before it set. Some banding was just about discernible. Jupiter also showed this symmetrical flaring.

I wanted to test without the diagonal but I discovered I need a 2" extension to achieve focus with an EPs in that configuration.

So my next step is to buy one and test without the diagonal.

I was really hoping I would have been happy with what I would see tonight but alas it didn't come true.

I'll keep trying though.

[Elp]

Have you tried another refractor during the same night for comparison?

[Mr Spock]

Which diagonal are you using?