Very clear reflection of spider in eyepiece

[Kees Bijker]

Hello all,

New here, and normally not such a fan of internet fora, but this forum showed such promising content I decided to enroll and ask my question.

Several years ago I bought a 10" Meade Newtonian as a project. I used it without motors or even controls, just moving it by hand. I decided to first improve the scope by getting rid of the idiotic plastic focuser. So I bought a proper machined solid aluminium Crayford, from an engineering point of view absolutely gorgeous item.

I have not used the scope much since installing the new focuser, because I happened to move to a light polluted area. Now I am back in the relative dark, I want to use it, so I cleaned it all, reassembled and collimated (with a laser collimator), and started.

 

First I noticed was that some of my eyepieces seem to be close to the end of the focuser movement, maybe due to the focuser needing some adjustment relative to the center of the OTA. Not a problem right now as all my eyepieces reach focal point.

 

The troubling problem I did notice, is a blurred, but clearly visible image of the spider with secondary mirror. Mostly disturbing when viewing the moon, especially during daytime, as it increases contrast of course. At night I don't think it is that bad but I am wondering if I did something wrong with the positioning of the new focuser, the alignment or something else?

I am of the opinion that my next improvement will be the spider, as this thing is very feeble. Maybe I will go for a guitar string design, but not sure yet any seasoned input on this is also appreciated.

 

As comparison I have a Meade 6" as well, and this thing has the solid cast spider instead of the thin plates of my 10" (4mm thick vanes opposed to the 1mm) . The image in both is similar, which I did not expect. I thought my 10" would have to be much better and show less secondary spider image.

So, there it is, do you peeps think this is normal or would I need to look at some stuff to conclude if something is wrong?

Much appreciated,

Kees

 

[ronin]

The problem is that the eyepiece is located too far out from the scope. When this occurs in effect only objects closer to the mirror then the stars+planets will be visible, in the case of a newtonian it is always the spider.

Either the assembly is a bit wrong, or as happens in a skywatcher there is a 2" eyepiece adaptor and a 1.25" eyepiece adaptor and one is inside the other and one needs removing.

Check the focuser, look for the presence of these eye piece adaptors, you may have purchased one and not realised there are 2 and just lect them together. The "problem" is that they are constructed to fit insode one another for shipping so when people buy them they are a nice neat package and it is too easy to not realise that just one has to be in the focueser at any one time. The additional distance moves the eyepiece back and so the spider and secondary becomes semi visible. Even if you can focus on infinity you will be at one extreme of the ficuser travel.

If you have rebuilt the scope then it could be that the mirror is now located too low in the scope (on the mirror cell), you will have to move the mirror "up" a bit which will push the focal plane outwards a corresponding amount which effectively places the eyepiece closer to the focal plane.

If you have rebuilt the scope then it is just too easy to adjust bits until you see the secondary and so start to set up the scope for or with that in mind, which is wrong. Equally it is not easy to reassemble and rebuild and scope outside at night using the moon. Suppose you could work out the difference in the focal plane and put a spacer in of that length then reassemble using the secondary but will almost certainly go wrong somewhere.

When the eyepiece is at the right location then you should see a change in the apparent size of the vanes, if the vanes are specifically visible at all. So that aspect has to be addressed aftetr the eyepiece(s) are at the correct location with respect to the primary focal plane.

If you have to move the mirror upwards a quick measure of the movement would be to focus on the moon then move the focuser to the mid point of it's travel, that change is approximately the amount the mirror would have to be shifted upwards.

[Kees Bijker]

Dear Ronin,

Thanks for your answer, I will research this issue as I never heard of this issue before. So to recap, what you are saying is that when the full focal length of the primary mirror is divided by a too short a primary to secondary and too long a distance from secondary to eyepiece, you get this problem?

The thing is, the distance from primary to secondary did not change from original.....so how did it change from putting a new Crayford focuser on change the whole thing?

 

Anyway I will look at this and post any outcome of tests here of course.

Regards,

[Stu]

I think Ronin is answering assuming you cannot reach focus with the new crayford, but from reading your post it seems that all your eyepieces do reach focus, just at a position very close to the inward limit, is that correct?

It sounds to me just like the normal diffraction spikes you see off bright objects such as the moon, planets and stars. It is quite normal. Many find it perfectly acceptable, others find it distracting. Options for reducing the effect include circular and curved vane spiders. These do not remove the diffraction but smear it around evenly so it is less noticeable. My understanding is that wider secondary supports produce wider, but dimmer spikes. The thinner ones on the 10" will produce thinner but brighter diffraction spikes, I think!

The only confusion is that you seem not to have noticed it before changing the focuser which doesn't make sense to me.

On you focus issue, you can give yourself a little more infocus by collimating the mirror towards the top end of its travel.

[Louis D]

On 8/7/2016 at 00:41, Kees Bijker said:

The troubling problem I did notice, is a blurred, but clearly visible image of the spider with secondary mirror.

How about posting a picture through the eyepiece centered on the moon so we can be sure of what it is you are seeing?  Any decent cell phone camera should work for this purpose.  Also, a picture of the eyepiece in the focuser when focused on the moon.