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Actual Field of View

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#1 Rob55

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Posted 13 January 2013 - 03:54 PM

I have been doing this stuff just two weeks and have less than 3 hours looking through my scope. Everything uses the 25 mm EP for 40x. I want to learn how to judge distance and star hop to find dimmer objects. Dickinson in "Night Watch" trains on the big dipper to give this lesson, but that is below my horizon. I'm going to attempt to apply his work to Gemini.

My manual and math say the actual field of view I have with my 25mm eyepiece is 1.25 degrees. When I look towards Gemini and put Castor right on the top of my lens until it deforms and drifts out of view I swear I can see Pollux deformed on the bottom of the lense just coming into view. I have to shift my pupil and cannot see both stars at the same time. That's 4.5 degrees according to the charts. Am I hallucinating. Granted neither are in view as I peer comfortably into the lens. Jupiter and his moons just fit comfortably in the FoV that I see when peering straight into the lens and nudge the DEC knob every few seconds.

Stars look consistantly in focus until they get to within three or four diameters distance from the edge of the lens. Then they start to deform into a butter bean shape until forming a thin parenthesis and dissapearing out of view. To see this effect I have to be peering at the edge and cannot really see the middle of the lens.

The shorter EPs in my signature do not arrive until next week. I plan to spend more time around Castor and Pollux as they are currently the most comfortable to view. What can I expect to see out of the 6.5mm and how much faster will I have to crank the DEC knob?

thanks

Rob

#2 Dennis_S253

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Posted 13 January 2013 - 04:34 PM

Looks like what your seeing is Castor and p Gem.
Thats according to Stellarium.

#3 lamplight

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Posted 13 January 2013 - 05:51 PM

Hi Rob

The 6.5 will definitely require more movement of your scope to keep centered as you guessed. How much faster? faster. ;) And yes your field of view will be much smaller as well (im too lazy to lookup the math but its not hard) , this also contributing to more scope movement.

If it becomes an issue they make some motors relatively affordable for the cg-4 that comes with that scope.

In your finder scope you should be able to see both in the same view, and noting their brightness relative to surrounding stars, that will help clarify the surrounding stars.

Hopefully your finder scope is aligned right?

#4 Pharquart

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Posted 13 January 2013 - 06:16 PM

You probably have this formula, but true field of view (TFoV) is apparent field of view (AFoV) divided by magnification. The XLT 102 has a 1000mm focal length, and most Plossl eyepieces have a 50 degree AFoV. So at 40x with your 25mm Plossl, you should indeed see a 1.25 degree TFoV. The 6.3mm Plossl will provide about 160x, for a 0.31 degree TFoV. (That 6.3mm Plossl will also have really short eye relief...I hope you don't require glasses to view through it!)

The stars "move" about 1 degree in 4 minutes. In your 25mm eyepiece, it takes about 5 minutes for a star to go from one edge to the other, straight across a diagonal. In the 6.3mm, it will only take 75 seconds. If you pony up the cash to purchase an 82 degree 6.3mm eyepiece, you'll get 0.5 degrees and 2 minutes of drift time.

I have been observing for a few years now, and I still get confused about what I see in the eyepiece or finder compared to what's on a chart. For some reason, I seem to have trouble differentiating magnitudes. I know the star is supposed to be bright, but the star certainly looks bright in that eyepiece. Just not bright enough, I guess.

Brian

#5 Jon Isaacs

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Posted 13 January 2013 - 06:44 PM

When I look towards Gemini and put Castor right on the top of my lens until it deforms and drifts out of view I swear I can see Pollux deformed on the bottom of the lense just coming into view. I have to shift my pupil and cannot see both stars at the same time.



Rob:

You have gotten some good help so far. Here's a couple of more things to try:

- When the stars seem like both are in the field of view, look through the finder with both eyes open. You should be able to see both Castor and Pollux naked eye. What do you see. (This assumes you have a right straight through finder.)

- When you move from one of these two stars to the other, again look through the finder, is one of them Pollux?

The "Actual Field of View" is generally referred to as the "True Field of View" and can be calculated a couple of ways but TFoV = AFoV / Mag is sufficiently accurate for this and indicates a 1.25 degree TFoV.

Jon

#6 Pharquart

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Posted 13 January 2013 - 07:27 PM

What can I expect to see out of the 6.5mm and how much faster will I have to crank the DEC knob?


One more thought: how fast will you have to crank the knob? At the exact same speed. If you want to keep the object perfectly centered, you'd slowly turn the knob (and it's actually the right ascension--RA--knob and not the declination--DEC--knob) to match the speed of the turning earth. Regardless of the eyepiece you use, the earth turns at the same speed, so you'd have to turn the RA knob at the same rate. The object moves across the field faster at higher powers, but a turn of the knob moves the field by just as much more. Most observers turn the knob to move the object past center, then let it drift through center to the other side, then adjust again. By this measurement, you have to turn the knob more often, not faster.

Hairsplitting, maybe. But it's better to fix an error early in the learning process to help you down the right mental path!

Brian

#7 Dennis_S253

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Posted 13 January 2013 - 07:52 PM

Hey Rob, Have you figuired out the orientation of the scope and the finder? My 6x30 straight through finder rotates everything 180 degree's. One thing to try is to goto "Alnitak" the bottom star in Orions belt. As Jon said, Look through the finder with both eye's open. As your getting it in focus "it's like magic" the way the star glides across the FOV of the finder and bingo. But you will notice in the finder that the 2 bright stars above Alnitak look below it. If so, then your finder is showing things turned 180 degree's. I'm not sure what your scope does.

#8 lamplight

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Posted 13 January 2013 - 10:50 PM

What can I expect to see out of the 6.5mm and how much faster will I have to crank the DEC knob?


One more thought: how fast will you have to crank the knob? At the exact same speed. If you want to keep the object perfectly centered, you'd slowly turn the knob (and it's actually the right ascension--RA--knob and not the declination--DEC--knob) to match the speed of the turning earth. Regardless of the eyepiece you use, the earth turns at the same speed, so you'd have to turn the RA knob at the same rate. The object moves across the field faster at higher powers, but a turn of the knob moves the field by just as much more. Most observers turn the knob to move the object past center, then let it drift through center to the other side, then adjust again. By this measurement, you have to turn the knob more often, not faster.

Hairsplitting, maybe. But it's better to fix an error early in the learning process to help you down the right mental path!

Brian


Good call didn't think of that !

#9 Rob55

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Posted 15 January 2013 - 07:20 PM

Thanks

I knew something wasn't right. I'll get better at this.

#10 Rob55

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Posted 15 January 2013 - 07:39 PM

I'm sure I will pony up the cash sooner or later. 2" diagonal and eps are on the list, but I recon that is a different forum.

#11 Rob55

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Posted 15 January 2013 - 07:42 PM

RA knob I'll learn. Makes perfect sense speed of movment doesn't change. Field of view changes. I move to DC in 3 weeks. I hope to link up with some group. Appreciate everyone's help.






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