I got my first decent view of Mars last night thanks to a break in the weather, it was nice and crisp in spite of street lights and neigbours garden lights etc. The big problem was that it was so bright I could see very little detail of the surface due to the brightness, would a neutral density filter fix this and if so at what percentage 50-25 etc?
Another question also is while doing the star test thingy ie put a star out of focus and check the Airy disc, while doing this last night I had very symmetrical rings around the star but I did notice that they were bent out of shape around the primary mirror clips, the circles sort of bent around the clips at these points, Is this normal?
If you have distortion around the clips, then the clips are too tight on the primary. You can loosen the clips a little without compromising the mirror's safety. You might like to place something soft between the clip and mirror to cushion the grip.
The one problem with most newtonians (I'm assuming your's is a newt.) is thier fast focal ratio, anything from f/6 and lower in number is fast. This makes most newtonians to be considered by some not to be 'good' planetary scopes. Refractors and cassegrains have typically slower f/ratios. This serves to reduce glare on bright objects, in the same way a diaphram does in a camera.
The solution is just like that in a camera- stop down your newtonian.
It is a 'trick' more commonly done with big dobs to have a secondary opening to the primary so that the fast newt. becomes a slow newt. You can do this by getting your hands on a waxed vegie box from a fruit and vege shop, open it out and cut a panel that will cover the opening of your scope. Then it is a matter of cutting out a circle in this panel to give you an effective f/ratio of anything slower than f/10.
You can have this cut-out sit between the vanes of your spider so you don't have the secondary casting a shadow, or you can take the secondary into consideration in your apeture calculations and centre the cutout over your secondary.
The pic shows these 'off-set' holes in the mirror cover to a 20" dob.
Yes I do have a Dob a 8" skywatcher, it has a stop in the aperture cover the opening is about 50mm but using this also shrank the image using a 10mm ep. Would a ND filter solve the glare problem?
I'm assuming that loosening the clips would be best done at night while using the scope so as to achieve the desired result. Is there a risk of miscollimating the scope while doing this? I am waiting on collimation tools so would rather wait till I have them if this is so.
You will need to collimate the scope once you loosen the clips, so it doesn't matter if you do it now or then. It wouldn't be so gross the miscollimation anyway to inhibit you from using the scope.
A 50mm apeture on the scope cover is there as a solar apeture to use the projection method of viewing the sun, not for reducing glare. You will need a larger opening than this.
I would suggest a 4" opening to start with for an f/12 ratio. However, with the secondary in the way (I would do the apeture cut-out over the centre of the scope with your 8" scope), this would reduce the f/ratio to something like f/14 (assuming the central obstruction is something like 50mm). This effective ratio is still very good. You can always make it larger. It is harder to add material back.
You can always make up a few masks of different diameters, smaller and larger, the one box will provide anything at least four masks. You can then decide which works best for you.
The reason for using the waxed boxes is this makes them more water resistant for when the night dews up.
The Neutral Density (ND) filters may work. The box won't cost anything as an immediate solution. Better off spending the money on a good nebula filter than ND, IMO. Far more useful.
It will tell you all you need to know about them, nebula, colour, etc., their features and applications.
From there you can look at a few brands, like Lumicon, and Baader and GSO. GSO are very cheap, but I have no experience with them. If you do most of your observing from light polluted skies, a genral purpose nebula filter. Other than that, the Oxygen III, Hydrogen Alpha and Beta and Ultra High Contrast filters are more specialised as they are suited to different types of nebulae as some are reflection and others emission, so they glow at different wavelengths, hence the specialization in filteration.
I'm looking at getting my hands on a UHC and OIII, but once I finish my current project. I already have a Lumicon Deep Sky filter.
However, you need to know what you are doing as they really drop the intensity of the image. You may be dissapointed at first, but the details are there. Be patient.
One or two colour filters might also help with the planets. An 80A blue will help bring up the Great Red Spot on Jupiter, as it is a rather faint-salmony colour which can make it difficult to see. Check out the chart, and then only select ONE or TWO, no more. You won't use them too many times.
There is currently a discussion on this topic in the Eyepiece and Filter forum:
Diaphram, Iris, same thing. it is the 'curtin' which you see open and close in the camera lens, sometimes it uses five panels, sometimes over a dozen, depending on the lens.
I have had an OIII filter for years. Never use it. I can never see anything through it. I once looked at a star with it and all I saw was a faint red dot - totally useless.
What are you supposed to use them on?
Itis a Lumicon Nebula line transmission %
Oxygen 496 nm 93
Oxygen 501 nm 93
Hydrogen beta 486 nm 0.4
What does that all mean???
The numbers refer to the % transmission of particular wavelengths of light. Wavelengths of light are in the nanometer range (very short). We see these wavelengths as the different colour of the spectrum (rainbow).
The oxygen/hydrogen bit refers to the fact that when the gases are stimulated by electromagnetic radiation, like heat or light or ultraviolet, etc, radiation, gases, here Oxygen & Hydrogen, in turn 'glow' at specific wavelengths. Different gas, different glow colour.
These filters allow these specific wavelengths of light to be transmitted, blocking out pretty much everything else. This way the sky-glow is reduced to zero, and everything else the same except for those specific wavelengths. That is why any stars that are visible through the filter appear greenish, and nebulae are able to then stand out.
So, the OIII filter transmitts:
* 93% of 496 nm wavelength, specific to Oxygen
* 93% of 501 nm wavelength, specific to Oxygen
* 40% of 486 nm wavelength, specific to Hydrogen.
You would use this filter to help you better see nebulae like M42 (Orion nebula), eta Carina neb., etc.
These filters are NO GOOD for galaxies as galaxies glow across the entire light specturm
I use my OIII filter primarily to view and confirm Planetary Nebs, use a regular Neb filter for other nebs. Main use is to confirm PNs, it dims everything else but the PN stays bright, that confirms what I am looking at then mostly examine at high mag without any filter.
Very useful filter!
thanks to a break in the weather, it was nice and crisp in spite of street lights and neigbours garden lights etc. The big problem was that it was so bright I could see very little detail of the surface due to the brightness, would a neutral density filter fix this and if so at what percentage 50-25 etc?
I can never see anything through it. I once looked at a star with it and all I saw was a faint red dot - totally useless.
I'll try it out on M42,as that one is bang overhead at present!
