need advice on small SCT issues

[BenM]

I am trying out a C5. I've only spent about an hour for two nights getting it up and going and I have some questions about it the answers to which don't occur to me.

To summarize briefly, I see:

Chromatic abberation

Diffraction rings

bizarre spots

Let me describe each in more detail.

This is a brand new C5 spotting scope (OTA). I mounted it on a Manfrotto 401 Junior geared head and Manfrotto tripod. The goal is a field scope, mount, and eyepieces I can put in a shoulder bag with a camera tripod strapped outside. This setup has some compromises but if it works out, it will be a 5" field scope (not bad).

I replaced the 45 correct image 1.25 diagonal with a Televue short SCT adapter (2" visual back) and the requisite Televue 2" Everbrite diagonal. Optically, I don't need 2" but I'm using this because it better supports the large Ethos eyepieces. This "short" unit is .7" shorter than a standard 2" diagonal to clear fork mounts. In my case it just lets keep the scope compact and doesn't exacerbate the already long focal length.

The OTA alone has a 1250mm focal length, and I'm guessing with the short diagonal I'm getting roughly 1350mm. I have an Ethos 13mm and 6mm for about 103x and 225x (I am using the primary mirror focuser).

I start my evening with Jupiter because it's visible before any stars are. The view at 100x is bright. I can see banding and four moons. I see what looks to be chromatic abberation. The top right (in the field) of Jupiter is fringed in blue and the lower left is fringed in red. Going to 225x, I see the same. I wasn't expecting this with an SCT. From what I've read, I'm led to doubt that the eyepieces are causing this. Is it the atmosphere?

With the 13mm, transit time in the field of view is reasonably long. With the 6, you can really see we're whizzing around. I was pleased that it worked out at all since this focal length is borderline on a rig like this. As it turns out over the course of two days observations, the 6mm only worked about 50% of the time, and for the rest it was too much. I might choose a Radian in this size for those times it does work, but probably not an Ethos.

The Ethos eyepieces have been quite uncomfortable. The 6 being the worst. At first I thought it was the lack of eye relief because I was getting eyelashes when I tried to close in to get the whole apparent field and generally having trouble viewing. I realized that it is actually the small exit pupils that are causing the blackouts and discomfort. This is made worse by my pupils not being dialated (I didn't have time, and there were lights on nearby). So small exit pupils, small entry pupils, and eye position was quite touchy. After experimenting, I can see that eye relief is actually not bad at all although you do lose some of the field as you back away.

Now for the strange spots I'm seeing. The whole field is scattered with these little dark spots with a ring around them. They are not attached to the sky image but to some other surface. I thought they might be eye floaters but when I move my eyes laterally, the spots stay attached to a surface that is apparently in the scope or eyepiece. Also, they have a very uniform circular shape, not like bits of gel like you would expect from floaters. They move around separately from the sky image but not with my eye either. They're worse at higher magnification. It's almost like I'm looking in a microscope and seeing something that shouldn't be on the slide.

I pointed the scope at a star. At one point I used Sirius and other times I don't know which one it was. I checked collimation with the focuser. The diffraction rings seemed concentric out of focus both ways but as I approach focus, I get a little coma off to one side. I took a shot at fixing it. I was able to get the obstruction off center in the rings and then put it back in the center both ways, perhaps a little better than it was before. As best as I can focus, I still get coma and some diffraction "waves" coming off one side of the star. I wasn't able to fiddle with it any more but I'm at a loss for what else I could do. Having centered the obstruction in the diffraction rings in both directions, I'm at the limit of my screwdriver skills.

The first night I was able to get good views of the Orion nebula in the 13 and 6mm. The trapezium was easily resolved and the nebulosity was quite bright. The second night I had even less time and clouds were beginning to obscure the view. I could see the trapezium well enough to know I was looking at it but I couldn't resolve the four stars due to the clouds. The scope and eyepieces were also starting to dew (I had left it out to cool while I walked the dog).

The 401 geared head is "adaquately" stable and tracks smoothly even at 225x. However, my hands are not so stable and cranking on the knobs as necessitated by the short field transit times does induce vibration that interferes with the view. The gears themselves are quite smooth once I'm turning them but the initial hand hit on the knob shakes the field pretty good. It smooths out for most of the transit and then I have to crank again. It is a compromise for weight and bulk. I use the tripod at a low and wide setting while sitting. It would not be adaquate at standing height. I need it to be low enough for my children anyway which is why I'm not using a Newtonian. Where I am not so happy is how the pivot axis are all well below the scope's center of gravity. Even if perfectly balanced fore and aft, the scope will flop around hopelessly if the gears are disengaged. If I decide to go with this kind of scope, I would probably put it on a Vixen Porta II or something similar. It's double the weight of my Manfrotto setup but would be worth it if it gives me an altitude axis through the scope's center of gravity. I want a free scope that can be jammed around the sky but obviously need fine control for half-degree fields of view.

In any event, if someone has some ideas about the strange spots I'm seeing and the coma, I would appreciate advice.

[CGolder]

Hello,

Obvious questions for me are... how long has the scope been cooling down? (spots) and are you sure the scope is collimated correctly (de fraction rings) my C6 defraction rings can change a lot depending on focus level, it seems central and then i focus and the rings become more obvious and require adjustment

I know it sounds obvious but i am unsure on your level of experience with SCT's so thought i would ask the question.

Coma is worse when collimation is incorrect if its off to one side considerably then i would say its sounds likely. you cant eliminate it completely but it helps and when correct is hardly noticable.

[FraserClarke]

Good long description

Colours: How high was Jupiter above the horizon? The red/blue colour separation is an effect the atmosphere can produce at low altitudes.

Spots: Sounds like it might be dust spots on a surface close to the focus. Have a look at either the first lens of the eyepieces, or the diagonal fold mirror

Diffraction rings: Sounds like the collimation is very slightly off. However, before you do anything to try and fix it, make sure that the telescope is properly cooled to ambient. Leave it at least 30-60 min to make sure you're not seeing a temporary thermal effect. When collimating the SCT, make sure you put the star in the middle of the field. The adjustments on the secondary can be very small indeed -- I can see the effects of 1/64th of a turn on the secondary screws with a 16-inch meade... I have no experience with a C5 though.

[BenM]

Thanks for the advice. I am sure the diffraction is due to being out of collimation. I just didn't think I could get it any better than I did and it's still not satisfactory. I'll have to try some more.

Also, I'm rather sure the spots are eye floaters now. Pointing the scope at a blue sky background with the 6mm eyepiece, I can see lots of them. If I tap the scope, the spots don't move. I have good vision and healthy eyes so I guess the exit pupil around .56mm is just too small for my eyes. I knew there was a limit for most people around .4mm but I guess mine is a little higher.

Jupiter was not particularly low, but certainly there was some atmosphere. Looking at other objects that typically show CA in uncorrected glass, I don't see any.

[Merlin66]

There's nothing in the design of a SCT which would cause Chromatic aberrations - certainly the correcting lens is not issue - too thin and usually very low curvatures.....