Tim Harshman said:
I have a astroblast 103mm reflector from Orion, I bought a Stratus 5mm EP and have the following problem/question.
When viewing Jupiter,Mars and Stars I am getting what I assume is "coma", it is almost impossible to get the distant objects into perfect focus and there is the "coma: looking tail most of the time.
I viewed the moon last night and it was tack sharp clear across the FOV.
I collimated and I think it's pretty close or as close as I can get it.
Doing the "star test" i get a perfect circle.
Any ideas?
Hi Tim:
Here's a few comments and thoughts...
- First, I am assuming this is the Starblast which is a 114mm F/4 Newtonian??? I am not familiar with the Astroblast... This scope has a 456 mm focal length which gives you 91.2X magnification. If the scope is cooled, this is not too much magnification, I have a 76mm F/3.7 that gives good views at 117x.
- The moon is a very forgiving object, it is large, it is bright and the contrast is high. What happens then is the brightness covers up the aberrations, the coma or astigmatism you might see from a star is covered by the brightness around it.
- Collimation at F/4 is critical. The coma free region of an F/4 telescope is 1.1mm. With a 456 mm focal length this translates into a 0.14 degree coma free field of view. With the 5mm Stratus, this means even if everything is perfectly collimated, only the central 20% of the image will be sharp.
The fact that you are seeing coma at the center of the field of view indicates to me that the scope is not perfectly collimated. One thing to do is to look at a star high in the sky and see what happens when you move the star away from the center of the field of view. If it is a collimation/coma question, then in some directions the coma will get worse, in some it will improve. Adjust the primary mirror to move the better images towards the center of the field of view.
- Collimation of an F/4 is not easy. Experienced observers like Alexis Cousin or Rod Mollise get good results with the Starblast because they are know how to collimate their scopes and they have the tools. But for a first time telescope owner, getting the mirrors aligned to tolerances of considerably less than a millimeter is just not easy. Even with the best tools, it is not so easy and I imagine you are collimating your scope with a collimation cap which makes it more difficult.
- So my guess, it's collimation.
- As others have said, F/4 will bring out the worst in an eyepiece, it takes good eyepieces to perform at F/4 though this is mostly off axis, ie away from the center.
I have never viewed through a Stratus but it is my understanding that they are reasonable at F/4, they have a front barlow like section that helps with the fast focal ratios.
- The 1.25 inch Paracorr is pretty rare, it will clean up the view, if the scope is collimated but as you say, yeah, costs more than the scope.
- The Starblast is a funscope and not designed to provide the good planetary and high power views. But, it does turn out that with good eyepieces and good collimation, it is capable of doing a good job, I remember Rod Mollise saying he was operating his at 200x, Alexis can probably give us an idea of what he gets from his.
- Jupiter, low on the horizon, Mars, small and difficult right now. Mars will be best right before Christmas.
Jon Isaacs who has a few F/4 scopes but likes F/5 a whole lot more.
