ghosting!

[steviebee]

Not sure if this is what it is called. I have a SW star travel 102mm. I was out last night just having a look at the Moon and I was getting a ghost effect( not sure if that's what it is called). It is as though another Moon was opposite like a mirror image. The only way I could lose this was to move the position of my eye on the lens. I was using the standard 25mm lens that came with it and a 45* diagonal. Viewing through it in daylight is very good. Would a Coma corrector help? I did take it apart last year as there was some blooming on the lens inside, but all seemed well with it after that. I do suffer from a lot of light pollution, could this be the cause. I used this scope in France in February (after cleaning) and again all seemed well.

[Knighty2112]

I'm assuming your 45 degree diagonal is a standard prism type one supplied with the scope? Check to make sure the prism is not loose, or is able to move at all in its housing as I had an old stock Celestron prism diagonal that used to move about slightly and cause similar issues. 

Edited September 9, 2016 by Knighty2112

[steviebee]

Thank you I will have a look now!

Regards.

Steve.

[michael8554]

Hi Steve

Hard to describe this, but the moon is a very bright object, so can cause reflections on the various optical surfaces, visible on the black background.

This will also be happening during the day, but there's no black background to see them against.

A neutral density moon filter might help.

And Knighty's point about the diagonal is also as likely.

Michael

[steviebee]

Hi Michael, I have checked the diagonal and that seems ok. I also have a Moon filter fitted. I am wondering if I may have not seated the lens right when I cleaned them, but I am sure it was alright after. It just seems to be the last couple of weeks. I do live in a very light polluted area, perhaps this could be the problem! As you can probably gather I'm a total newbie to this. My SW 200p on the other hand is great, but that is only used in France.

 

Regards.

 

Steve.

 

[Alan64]

On 9/9/2016 at 04:19, steviebee said:

Not sure if this is what it is called. I have a SW star travel 102mm. I was out last night just having a look at the Moon and I was getting a ghost effect( not sure if that's what it is called). It is as though another Moon was opposite like a mirror image. The only way I could lose this was to move the position of my eye on the lens. I was using the standard 25mm lens that came with it and a 45* diagonal. Viewing through it in daylight is very good. Would a Coma corrector help? I did take it apart last year as there was some blooming on the lens inside, but all seemed well with it after that. I do suffer from a lot of light pollution, could this be the cause. I used this scope in France in February (after cleaning) and again all seemed well.

The 45° Amici diagonal is really only suitable for daytime/terrestrial use; for observing birds and what-not.

Consider a 90° star-diagonal for astronomical use at night...

1.25"... http://www.rothervalleyoptics.co.uk/antares-90-star-diagonal-125.html

2"... http://www.rothervalleyoptics.co.uk/altair-astro-2-mirror-star-diagonal.html

...for examples.

[Mak the Night]

Interestingly, 90° Amici prisms can work well for astronomy and I often use one for lunar viewing. 45° prism diagonals are more for terrestrial use.

Edited September 11, 2016 by Mak the Night

[Knighty2112]

1 hour ago, Mak the Night said:

Interestingly, 90° Amici prisms can work well for astronomy and I often use one for lunar viewing. 45° prism diagonals are more for terrestrial use.

Somebody needs to tell Skywatcher about this then, as they supply quite a few of their telescopes with a 45 degree diagonal, ones which are clearly designed for astronomical rather than terrestrial use.

Of course one of the first upgrades I got for my Skywatcher Startravel 120mm f5 'frac was get a good 90 degree diagonal.  

[Mak the Night]

34 minutes ago, Knighty2112 said:

Somebody needs to tell Skywatcher about this then, as they supply quite a few of their telescopes with a 45 degree diagonal, ones which are clearly designed for astronomical rather than terrestrial use.

Of course one of the first upgrades I got for my Skywatcher Startravel 120mm f5 'frac was get a good 90 degree diagonal.  

I think Synta tend to supply 45° diagonals with small scopes with alt-az mounts that can often be used terrestrially. Most EQ mounted scopes have regular diagonals supplied.

[steviebee]

OK, so a 90* is on my shopping list. Should I go for the 2" as would be nice to put my camera on it, or would the 1.25 be better. Also (newbie question here) what is the difference between a diagonal and an erecting prism? Would the diagonal be better?

[happy-kat]

Erecting prism gives right way up views which you want for day time. Night use it doesn't really matter what way up views are so 90° degree star diagonal doesn't correct the image orientation.

Edited September 12, 2016 by happy-kat

[steviebee]

Right got it, thanks happy-kat, much appreciated.

[Mak the Night]

I regularly use a 90° Amici prism (I have several including an Orion).

I prefer non-mirror reversed views, especially for planetary/lunar. Although the upside down world of the Newtonian is perfectly acceptable to me, I find mirror reversal is a bit weird.

Before you dismiss Amici prisms completely due to the prevailing entrenched mythology, read the link below.

 

http://www.telescope.com/Articles/Equipment/The-Advantages-of-Observing-with-Correct-Image-Diagonal/c/9/sc/192/p/106656.uts

[Louis D]

On 9/9/2016 at 04:19, steviebee said:

The only way I could lose this was to move the position of my eye on the lens.

Have you seen this same ghosting using other eyepieces?  How about the same eyepiece in your Skywatcher 200P?  Based on the above sentence, it is possible you're seeing a reflection of the moon's image reflected off of your cornea back into the eyepiece.  Try to narrow down which element is introducing the reflection.

[steviebee]

 

Startravel 102.

Sorry I have not got back to you all, but have just finished a 12 night stint at work. I was out last night though. I have cleaned the lens (in scope) again and refitted them making sure I did not over tighten them. I used a new to me LET 2" 28mm lens with an extension tube ( could not get focus without this). This was perfect in daylight viewing. I was happy with the night time results except when viewing altair (very bright) it had like a comets tail. This tail was there to varying degrees no matter where I positioned it in the lens. I have a SW 2" dielectric 90* diagonal coming later in the week. All other stars where good. Perhaps a filter (any suggestions) would help to reduce the light. I am also considering a Crayford duel speed focuser as there seems to be a lot of play with the one on it ( standard from factory). Just want to say thanks to all you for the help you are giving me.

 

Regards.

 

Steve.

[Peter Drew]

Steve. Play in the stock focuser tube is adjustable. There are two very small grub screws that need a tiny Allen key to adjust.

[steviebee]

Thank you Peter, I will have a look now. I have been looking at various posts on here. Do you think it could be Chromatic Aberration? If so is it possible to reduce it. On another post here it was described perfectly as a star having a comets tail.

[steviebee]

Just checked the grub screws and each took about 3 turns to make contact. No play now, happy bunny. Hopefully the clouds will clear tonight for a test. If not I will have to wait until the weekend.  Thank you again for that, and all the help I've had on this.

[Louis D]

14 hours ago, steviebee said:

Thank you Peter, I will have a look now. I have been looking at various posts on here. Do you think it could be Chromatic Aberration? If so is it possible to reduce it. On another post here it was described perfectly as a star having a comets tail.

Were you using the 45 degree Amici prism diagonal?  They can cause a diffraction spike when a star crosses the center line.

Is the comet tail the same color as the star or is it more of a rainbow?  If the former, it's coma.  If the latter, it's chromatic aberration.

Does it grow longer the farther the star is from the center?  Does it always point back to the center as the star is moved around the FOV?  How does it look on either side of best focus?  Which way does it point when the star is dead center?  Have you swapped eyepieces to make sure it's the eyepiece and not miscollimated or pinched optics?

[steviebee]

Hi Louis D, I will try to answer all your questions to the best of my ability.

1st I was not using a 45 degree diagonal with the 2" 28mm lens, just an extension tube that came with my SW 200p to enable focus. I will be getting one (a 90 degree) tomorrow.

2nd The comet tail is the same colour as the star (suggesting coma?)

3rd Yes it does seem to grow longer the further away from the center, and yes it always points back to the center of the star as it is moved around the FOV.

4th The tail looks a bit blurry either side of focus, and always points down away from the center of the star.

5th I have used my 25mm (1.25) lens that came with my SW 200p on a 45 degree erecting prism which I got for terrestrial viewing and it is the same as all above.

 

I bought the scope second hand last year for £50, it came without lens's, no 2" to 1.25" reducer, no diagonal and no finder scope. It does not owe me a lot, but I do like it. I do not want to throw too much money at it as I would sooner just go buy a new one. ( I will raid my 12" Dob fund if necessary). I am sure it was fine before I took the lens's out to clean them as there was a couple of milky colored spots on the inside lens. I ensured they and the spacers all went back in the right order. I am not sure if it can be colminated as there are no screws holding the lens's in just a plastic threaded disc. Hope this helps. I have tightened the focuser up now as it was very wobbly, but will not be able to use it again until Saturday as back to work tonight.

 

Steve.

[Louis D]

Is this similar to your scope?  If so, it's an f5 parabolic mirror and that's probably where your coma is coming from, although it seems a bit excessive from your description.  If you were in the US, I'd recommend you watch for a second hand GSO coma corrector to clean up most of this for not a lot of money (usually under $90).  However, I've yet to find anyone selling it in Europe under any brand.  Maybe someone more familiar with European retailers could chime in here.

[Louis D]

Is this similar to your scope?  If so, it's an f5 parabolic mirror and that's probably where your coma is coming from, although it seems a bit excessive from your description.  If you were in the US, I'd recommend you watch for a second hand GSO coma corrector to clean up most of this for not a lot of money (usually under $90).  However, I've yet to find anyone selling it in Europe under any brand.  Maybe someone more familiar with European retailers could chime in here.

[Peter Drew]

28 minutes ago, Louis D said:

Is this similar to your scope?  If so, it's an f5 parabolic mirror and that's probably where your coma is coming from, although it seems a bit excessive from your description.  If you were in the US, I'd recommend you watch for a second hand GSO coma corrector to clean up most of this for not a lot of money (usually under $90).  However, I've yet to find anyone selling it in Europe under any brand.  Maybe someone more familiar with European retailers could chime in here.

Louis D.  The OP is describing a problem with a refractor.  

[philj]

On ‎09‎/‎09‎/‎2016 at 22:43, steviebee said:

Hi Michael, I have checked the diagonal and that seems ok. I also have a Moon filter fitted. I am wondering if I may have not seated the lens right when I cleaned them, but I am sure it was alright after. It just seems to be the last couple of weeks. I do live in a very light polluted area, perhaps this could be the problem! As you can probably gather I'm a total newbie to this. My SW 200p on the other hand is great, but that is only used in France.

 

Regards.

 

Steve.

 

Hi Steve

Ghost image is typical of tilted objective elements/incorrect spacing. If as you say above you cleaned the lens which are you referring to.

When I 1st got my 4" F15 I knew it could perform but on bright objects there was a ghost of the object in view. I had a look at the objective collimation, all was well, then I took a closer look and someone in the past had stripped the objective and replaced one of the foil spacers with a bit of cigarette packet foil which was far thinner than the other 2. I replaced all 3 with correct thickness and voila no ghost image.

Check your spacers are in place at 120 degrees and also that the objective elements are seated correctly before buying anymore kit

Edited September 27, 2016 by philj

[Louis D]

3 hours ago, Peter Drew said:

Louis D.  The OP is describing a problem with a refractor.  

Ah yes, I see that now in the original posting.

22 minutes ago, philj said:

Hi Steve

Ghost image is typical of tilted objective elements/incorrect spacing. If as you say above you cleaned the lens which are you referring to.

When I 1st got my 4" F15 I knew it could perform but on bright objects there was a ghost of the object in view. I had a look at the objective collimation, all was well, then I took a closer look and someone in the past had stripped the objective and replaced one of the foil spacers with a bit of cigarette packet foil which was far thinner than the other 2. I replaced all 3 with correct thickness and voila no ghost image.

Check your spacers are in place at 120 degrees and also that the objective elements are seated correctly before buying anymore kit

You're probably correct about the objective.  My only other thought is that a short refractor will have field curvature that looks like coma until the star is refocused.