Solar observing filter, entire aperture vs small cover cap

[wok]

I bought a solar filter film to use on a dobsonian 8'', so I was looking on ways to use it. And I see some people just cover the entire scope aperture, and others only the small 2'' cover cap.

I would prefer just covering the small front cap since I could buy an extra cap, cut the front and build the filter there, plus I'd have extra film to use on other stuff. But I don't know if the view would be much worse than when covering the entire scope aperture with the filter.

Is there a notable better view or other advantage when covering the entire aperture vs just the small cap?

[JOC]

FWIW I cover the whole aperture.  You might like to see this thread which shows what lots of people have done with solar film - don't forget to take your finderscope off if it is optical and not covered

 

[michael.h.f.wilkinson]

I prefer to use a full aperture filter on my C8. Stopping it down doesn't gain me anything, I find. I do prefer to use a Herschel wedge on a refractor, but the C8 and full-aperture solar filter can also give excellent detail shots, especially at the moment, when spots tend to be tiny (if present at all)

[Ben the Ignorant]

The small cap would reduce resolution down to 1/4, and brightness down to 1/16 the full aperture. Use the full aperture, and make the filter a few millimeters wider because the light entering the scope forms a cone. 210mm or 220mm should be okay. Try the smaller diameter on daytime landscapes or any nightime sky targets, and the loss will be obvious. 

[Louis D]

3 hours ago, Ben the Ignorant said:

Use the full aperture, and make the filter a few millimeters wider because the light entering the scope forms a cone. 210mm or 220mm should be okay.

I don't want to go off on a tangent, but I'm pretty sure all the rays entering a Newtonian focused at infinity on the sky are parallel.  Any off axis rays are already going to be greatly truncated by the tube length.  The main reason tubes are made bigger than the mirror is to allow for airflow around the mirror at the back.

I cut my solar film mount aperture to the same diameter as the mirror; however, the film needs to be about 1/2" bigger on each edge to allow for secure mounting.

Another argument for parallel rays is that smaller sized Cassegrain scopes cut the hole in the primary to about the same size as the secondary.  That hole is completely in the shadow of the secondary due to parallel rays, so no light gathering is lost.

On the other hand, many compound Cassegrains with full aperture corrector lenses will slightly diverge the incoming light cone, and the primary needs to be larger than the front aperture.  However, a long dewshield of the same diameter as the corrector won't truncate the incoming light bundle thanks to parallel rays.

[Ben the Ignorant]

I know how that could be misunderstood, rays in a newtonian are not easy to picture in the mind's eye, but if the rays were parallel the scope couldn't see something larger than its aperture.

[wok]

5 hours ago, Ben the Ignorant said:

The small cap would reduce resolution down to 1/4, and brightness down to 1/16 the full aperture. Use the full aperture, and make the filter a few millimeters wider because the light entering the scope forms a cone. 210mm or 220mm should be okay. Try the smaller diameter on daytime landscapes or any nightime sky targets, and the loss will be obvious. 

I tried the watching jupiter last time with only the small cap removed, the view was pretty good, it was less bright, but showed a lot more color and the bands seemed more visible and with more detail. Much better than using a blue filter imo. That's why I was wondering if the small cap would be enough for the sun, since it's bright af.

[Louis D]

2 hours ago, wok said:

I tried the watching jupiter last time with only the small cap removed, the view was pretty good, it was less bright, but showed a lot more color and the bands seemed more visible and with more detail. Much better than using a blue filter imo. That's why I was wondering if the small cap would be enough for the sun, since it's bright af.

If you keep the opening between spider vanes, you've created an off-axis aperture mask the converts your obstruction Newtownian into an unobstructed apochromatic telescope.  The result is that less light will be thrown into the outer rings of the airy disk because there is no central obstruction, and your won't have diffraction spikes from the vanes, so the overall net effect is higher contrast because of less stray light at the cost of lower absolute spatial resolution.  If it works for you, go for it.

As far as solar goes, I'd have to make an off axis mask over my whole aperture solar filter to see if there is any noticeable difference.  I'd never thought to try it until now.

[Stu]

4 hours ago, Ben the Ignorant said:

I know how that could be misunderstood, rays in a newtonian are not easy to picture in the mind's eye, but if the rays were parallel the scope couldn't see something larger than its aperture.

Can you explain this a bit more?  Light rays from distant objects such as the sun and beyond can be assumed to be parallel so I’m not sure what you are saying?

http://blogs.discovermagazine.com/badastronomy/2011/11/02/crepuscular-rays-are-parallel/#.WrP3qyKnyEc

[Ricochet]

Light rays from a single point are parallel. Light rays observed from two different points are not parallel, but converging. In the case of two light rays from opposite edges of the sun (or moon) the difference in the angle must be 0.5°, as that is the apparent angular diameter of the sun (and moon). 

[Stu]

15 minutes ago, Ricochet said:

Light rays from a single point are parallel. Light rays observed from two different points are not parallel, but converging. In the case of two light rays from opposite edges of the sun (or moon) the difference in the angle must be 0.5°, as that is the apparent angular diameter of the sun (and moon). 

They may be separated by 0.5 degrees but they will not be converging

EDIT No, fair enough, from different sides of the sun they will be converging!

[Ben the Ignorant]

Ricochet, you explained it for me, thanks!

[wok]

Another related question, regarding the finderscope. Can I just leave it on the scope if I keep it with the plastic cover caps on?

[cletrac1922]

18 hours ago, wok said:

I bought a solar filter film to use on a dobsonian 8'', so I was looking on ways to use it. And I see some people just cover the entire scope aperture, and others only the small 2'' cover cap.

I would prefer just covering the small front cap since I could buy an extra cap, cut the front and build the filter there, plus I'd have extra film to use on other stuff. But I don't know if the view would be much worse than when covering the entire scope aperture with the filter.

Is there a notable better view or other advantage when covering the entire aperture vs just the small cap?

I just cover the small 2" cap, and fit 8'' lid to scope

Have 10" dob, and works well

Attached pic taken with phone camera to eyepiece

Solar eclipse down under in 2012. Where I was had a 80% eclipse

John

 

 

 

 

[Louis D]

3 hours ago, Stu said:

They may be separated by 0.5 degrees but they will not be converging

EDIT No, fair enough, from different sides of the sun they will be converging!

All of the light bundles are parallel, but some may be approaching from oblique angles which then causes them to be focused off to the side of the point of focus for the light bundle approaching directly on axis.  Due to them being off axis, coma is the resultant aberration.  So yes, some rays of obliquely approaching bundles will be truncated, though I doubt for solar it would have much visible effect if the solar filer is not cut slightly larger than the mirror diameter.

[SilverAstro]

11 hours ago, wok said:

I tried the watching jupiter last time with only the small cap removed, the view was pretty good, it was less bright, but showed a lot more color and the bands seemed more visible and with more detail. Much better than using a blue filter imo. That's why I was wondering if the small cap would be enough for the sun, since it's bright af.

Yes!

There are many reasons* for a large aperture, two of them are light gathering (not a problem with the Sun!) and resolution (detail, maximum useful magnification etc)

If you have found the detail on Jupiter to be as good or better with the small cap then this will also apply to your view of the Sun.  QED !  Go with it!   Later you may want to try a large filter - for times of good atmospheric clarity, perhaps. (assuming some other things are ok !)

*There are many reasons why the view with the smaller aperture can sometimes be good, atmospheric disturbances being only one, but with a larger aperture moments of good seeing can be caught.

Re. the finder, yes you can leave it on provided the front cap is in place and secure.

 

[SilverAstro]

** collimation and other errors are less important at higher focal ratios - as given by the reduced aperture-   so that is something else to investigate laters ! but for now if you are happy with Jupiter then apply same to the Sun

[cletrac1922]

4 hours ago, wok said:

Another related question, regarding the finderscope. Can I just leave it on the scope if I keep it with the plastic cover caps on?

Will take some pics on the weekend

I have made up a cap for my spotting scope, using cardboard tube and cap off aerosol can

Baader film wedged between the 2

Use it with my ED80 as well

 

 

 

[JOC]

For extra safety I just don't put my finder-scope on as I don't have a filter for it.  I guess if the plastic caps were on it might be OK, but I'd want to be very certain that they were light tight and weren't going to fall off. 

That leads to the question of finding the sun in the sky - it isn't easy, so you might want to buy or make or make a shadow finder for it.  However, I have also discovered a useful trick that leads to me to successful sun finding even without using a finder.  Once I have the OTA with my solar filter in place I take all the EP's out of the focus unit and just look through it with the naked eye.  It seems to make finding the sun easier as you can see the entire primary in the secondary without hovering too close to the EP and I find I can find the sun more quickly by moving the scope and looking in like that than using an EP - when the view fills with big disc you know you are there.  I use various shadows caused by lumps on my OTA tube to get the thing in the right direction and then move it up and down until I hit the sun - it always amazes me how quickly I can find it like that.

[Ben the Ignorant]

11 hours ago, wok said:

Another related question, regarding the finderscope. Can I just leave it on the scope if I keep it with the plastic cover caps on?

You can but it's safer to remove it because forgetting to put the cap on just once will fry the crosshairs. Besides, the weight of the solar filter plus the weight of the finder can unbalance you scope, removing the finder will correct that.

Regarding the smaller aperture, your scope has a 200mm main mirror and a secondary about 40mm across (measure that). 200-40=160, and since you can use only half of that, your reduced aperture can be as large as 80mm, way better than the 50mm you're using now, and still impervious to turbulence. This is called an off-axis mask; be careful that it's not in front of a spider vane, and that it's precisely centered between the secondary's edge and the main mirror's edge. It will be like owning an 80mm apo for free!

The mask should be in the lower part of the scope to avoid warm air that sometimes lingers in the upper part of the tube.

[Louis D]

3 hours ago, JOC said:

I use various shadows caused by lumps on my OTA tube to get the thing in the right direction and then move it up and down until I hit the sun - it always amazes me how quickly I can find it like that.

I just keep moving the tube up/down and left/right until the size of the tube's shadow is minimized.  At that point, it is pointing within a degree or two of the sun.  Close enough for me to center it in a 40mm widest field eyepiece, at least.

[SilverAstro]

2 hours ago, Louis D said:

until the size of the tube's shadow is minimized.

Me too and it is easily done with the tube of the finder on a card behind it.

[Paz]

I've experimented over time with full aperture or stopped down with my Newtonian and refractors and have found for bright objects that I often prefer stopped down views at lower magnifications and larger exit pupils but as magnification gets higher and exit pupil gets smaller  there comes a point where full aperture becomes preferable.

[LukeSkywatcher]

Cant really comment as i use a Hershel Wedge for white light observing and a Quark for Ha observing. Both on a 70mm refrac.

Both offer full aperture, so im guessing thats the way to go.