Problem with Skywatcher 250p flextube

[speed1268]

Hi All, I am new to this but after buying our daughter a Skyhawk 1145 synscan last Christmas,and after visiting the Kielder Observatory felt we needed something bigger!. I recently got a Skywatcher 250p flextube after reading various advice here.

 After set up though i can see only a lot of fuzziness. i contacted the guy i bought it from and have after much faffing with the old laser collimater(ie collimating the collimater!!)

 Have been out tonight and can see stars now but still not in true focus.

Things i have done.

1. Left it outside to cool down.

2. Have tried to collimate it, but am not sure the Seben is accurate to be honest.

3.There is some dust on the primary mirror which i have NOT touched!

If anyone has any advice i would be grateful,including anyone that might service these scopes around the South Bradford area

Thanks speed

[Schorhr]

Hello Speed,

what eyepiece are you using, does it get into focus at all (stars getting smaller and bigger again) or don't you even reach near focus?

Even if you eyeball collimation, it should get into focus, even if contrast isn't ideal... As long as the secondary isn't pointing in the wrong direction ;-)

[speed1268]

Hi Thanks for the quick reply. I have tried what looks like a higher quality than we had 20, but returned to the trusted 25 we used on the skyhawk. Originally, As an initial test looking at the moon the 250p couldn't get a view i went back to the Skyhawk and once again had an excellent view.

 Could dust cause this? Or is the mirror about light gathering?

Ta speed

[Schorhr]

Hello,

dust can reduce the contrast but even if it's quite visible, dust particles only cover a small area of the optics (total surface wise).

Large amount of dust and even scratches would not prevent the telescope from reaching focus. Perhaps a galon of pudding would ;-)

Try during the day to focus a chimney or tower or power line (something as far away as possible), and see if you can reach focus.

If you can't, move the eyepiece further out of the focuser by hand. Some telescope&eypieces may need extension tubes. Rarely you can't screw it in far enough.

On a star the telescope should show a "doughnut" that get's smaller as you get closer to focus, and then larger again (intra/extra focal). Sometimes getting into focus can be a bit tricky especially on higher magnifications.

Try other eyepieces.

Usualy a dirty and even not well collimated mirror won't cause you to not reach focus like that, only if it's really, really off (as in, secondary mirror turned sideways after a rough transport).

If you look through the focuser and you can see your eye's reflection on the primary that means it can't be THAT bad though.

Does your primary mirror have a middle marking? Can you see it through the focuser tube?

[Reeny]

Dust shouldn't be a problem.

I have just checked to see if the condensation has cleared from the 200P dob mirror, and found a cobweb inside the tube.

It is likely be the collimation.

With mine it was the secondary mirror which was ever so slightly out.

It made a big difference to mine.

Make sure you are using all of the required adaptors on the 2" focuser.

It won't focus in if you are working beyond the travel limits of the mechanism. 

The adaptors add 50mm to the length of the focuser tube.

[Dude_with_the_tube]

if it is the original skywatcher focuser you are using, without the 2" extension and the 1.25" adapter none of your Ep`s will come into focus as there is not sufficient back travel. Put them both in and try to focus on some distant object during the day as that will make the things easier to tinker around.Even with a collimation being off you should still be able to focus.

Dust is deffo not the issue.

[JamesF]

if it is the original skywatcher focuser you are using, without the 2" extension and the 1.25" adapter none of your Ep`s will come into focus as there is not sufficient back travel. Put them both in and try to focus on some distant object during the day as that will make the things easier to tinker around.Even with a collimation being off you should still be able to focus.

Dust is deffo not the issue.

Agreed.  I think lack of outward focuser travel is likely to be the problem.  I took this pic for another posting recently, but look how far my focuser is away from the tube (it is effectively a 250P flextube, though assembled from parts)

James

[JamesF]

From memory, each of the two blocks after the tube adaptor are 1" thick, btw.  So the eyepiece ends up being about 4" from the tube.  I'd drag it out and measure, but I've had a glass or three and I don't trust myself to do so safely now

James

[stev74]

As already said it's a fairly common mistake that's made using both the 2" adapter and the 1.25" adapter at the same time instead of separately in some Skywatcher focusers. Another thing that is very common on the Flextube is dewing of the secondary mirror because it's not a solid tube unlike your smaller scope so it would definitely dew up quicker, that could also explain the problem. Did you physically look at the mirrors when you were out observing?

I was down at the Kielder star camp this weekend and my secondary was dewing up in minutes because of all the moisture in the air at the time although it is easily cleared with a small 12v hairdryer in seconds.  

[brantuk]

Does it have a shroud and/or a dewshield? Are you sure the secondary mirror wasn't getting dewed up? Tonight is really quite moist out there.

[AlexB67]

Hello Speed Welcome   If it is none of the above issues the only other thing that comes to mind. Are you sure you are extruding the tubes fully ?, because there is a second set of holes in the truss tubes at least on some models, and it is easy to lock into them, making the overall scope shorter than what it should be. The scope should be fully extended on the truss tubes as far as they go and lock into the  holes at the far end of the tubes, not the ones just above  ( I made this mistake once shortly after  got mine )

If you have made that  mistake focus should be really bad, you should not even come close to focus and the stars will appear like big rings ( airy disks), at least with the eyepieces I have that is how it looks.

Hope you work it out

[Pete Presland]

Hi welcome to SGL from Pete in Bedfordshire  any luck with the scope? could you post a picture of the out of the focus stars next available clear night?

[lars1967]

Hello from Lars

[speed1268]

Hi All, Firstly a big Thank You for all the advice. It's given me lots to think about and look at.

The guy i bought it from did say about not putting the eyepieces fully in which i have tried to do. I will take some pics of the tube,focuser etc and return soon.

Cheers speed

[glowjet]

Hi Speed and welcome to SGL, hope you get your problem sorted soon

[ultramol]

Good Luck m8 and welcome...Geoff

[speed1268]

Hi, I think we could be getting somewhere.

Things i have done:

1. Checked that tube is fully extended.

2.When i look through the focuser in do see the reflection of my eye centrally.

3.i have extended the focuser as much as i know how, and also the 1.25- 2" eyepiece converter,and i have withdrawn the eyepiece.

4.Initially to check focus i was using a lamp post bur couldn't see it,i then used some trees further away but they wouldn't focus.I then moved the scope and used a target around 200feet away at a guess and this nearly focused.

I am guessing that i need to be even further away from an object using this scope?

Cheers speed

ps Hopefully uploading pic's

[JamesM]

Hi and welcome to the forum

Clear skies

James

[Pig]

Hello & welcome   it looks like you are receiving plenty of advice.

Shaun

[speed1268]

Hi,The focuser seems to like this http://www.opticalvision.co.uk/astronomical_accessories/Focusers/dual-speed_2_crayford_focuser. Has anyone used this type with the 250p?

Thanks again

speed

[Charon]

Welcome aboard.

[Bizibilder]

Hello Speed and welcome to SGL!!

[stev74]

Hi, I think we could be getting somewhere.

Things i have done:

1. Checked that tube is fully extended.

2.When i look through the focuser in do see the reflection of my eye centrally.

3.i have extended the focuser as much as i know how, and also the 1.25- 2" eyepiece converter,and i have withdrawn the eyepiece.

4.Initially to check focus i was using a lamp post bur couldn't see it,i then used some trees further away but they wouldn't focus.I then moved the scope and used a target around 200feet away at a guess and this nearly focused.

I am guessing that i need to be even further away from an object using this scope?

Cheers speed

ps Hopefully uploading pic's

Ok well I think you have narrowed the problem down a bit - as you can see your eyeball reflected through the focuser I'd say the collimation may not be perfect but it won't be that bad. Also as you already have a Astrozap shroud fitted it's less likely to be dew on the secondary although it will still form after a time on certain nights. As the focuser isn't a standard one and the chap you bought it from said he usually has to pull the EP's out of the focuser a bit to achieve focus I'd say that is most likely the problem.

 

Pull the EP out as far as you can with it still being able to be held securely in place with the focuser fully extended and work the focus back in slowly until you achieve focus, use the furthest object on the horizon you can see (well away from the Sun). If this still doesn't work release the the EP from the focusers, while it's fully extended again, your still holding it and looking through it then slowly move it away from the focuser until you get something in focus as the focus point maybe further out by a centimeter or two, if that's the case you may need to buy an extension tube. If you don't have a decent distance to the horizon you can do the same at night on the Moon as it's a big target, try all your EP's starting with the lowest power, as most EP's will focus at a different point you may get lucky with one of your other ones. 

 

Hopefully trying the above will help, I'd also check with the previous owner as to what EP's he owns, as they can be focused it your scope it maybe worth upgrading from the supplied Skywatcher EP's you have.  

[Pete Presland]

not really sure what the problem is, but have you tried using a barlow?

[knobby]

Try a Barlow with the element removed, if that works then a short extension will sort it