First night with GS-680 - Few questions

[Krumlov (Gavin)]

Hi guys,

Just finished a couple of hours with my brand new GS-680. Very happy overall, highlights were Saturn and the jewel box. Could not believe I found Saturn first go and managed to watch it for a while with the 6mm EP.

Now a few questions. I didn't use the fan attached and it was pretty damp out. Is the fan just for cooling the mirror prior to use or should you run it all the time?

What is the point of the adjustable position of the side tension knobs? Is it for balance if you have more stuff at the top? I have just put mine in the middle? (Couldn't believe it came with NO instructions!)

My 30mm 2" EP will not focus if it is slid all the way in, I have to have it out about 5mm. Is this normal?

Whilst the view of Saturn was great, actually a bit bigger than I expected using the 30mm EP, but it was all white (quite bright actually). I thought I would see some colour but from some other posts I read will this only happen if I use better EPs or filters?

I have a 2" 30mm EP and a 1.25" 25mm. Apart from the magnification difference are there any particular pros and cons between 2" and 1.25" EPs?

I also have an astrosystems collimator. The instructions at the start are quite complex and I skipped to the end to just get the primary and secondary mirrors aligned. For anyone that owns one did you go through the whole process as described in the book step by step? Stars seemed sharp to me so is that I should worry about in the end?

Thanks in advance for the advice.

[Krumlov (Gavin)]

anyone?

[sopticals (Stephen)]

Hi Krumlov

Your existing eyepieces are good for low power wide field views but for planets you need more magnification. My suggestion to you would be to aquire the GSO I.25" barlows the 2x and 3x ED to compliment your 25mm Plossl.The outlay for these two items is very reasonable and they perform well. Or you could go for the 2"ED 2x barlow to use with your 30mm 2" ocular (this barlow is a little more expensive but has a removable 1,25" eyepiece adapter included for use with 1.25" oculars).

Best regards
Stephen

[joecool (Mark)]

Hello,
I like to adjust my side tension knobs for balance so the scope stays in all positions with the heavier 2" eyepieces in place with the tension off. When using smaller eyepieces on objects like Saturn I have a bit of tension to slow the scope down when I nudge it to keep Saturn in view, and this will keep the scope from tipping up too as it will now be out of balance.

I also got the Astrosystems LightPipe/SightTube and Autocollimator. I followed their instructions pretty much, but skipped marking the secondary. I also have a Hotech SCA Crosshair laser collimator. After measuring the secondary for being central in the spider (with offset already built into the secondary holder), I fired the laser at the secondary to find it missed the center by about 1/2inch. I could see this in the lightpipe but it only looked like a few mm out! My problem was fixed by slipping 2mm of card under one side of the focuser.

The scope will come to collimation visually without doing much with the alignment of the secondary. Pinpoint stars will show this, but the view will not be as bright as it really should. And planets viewed at high power will not look nearly as sharp as they should.

Check out these 2 sites and cross reference them with the Astrosystems instructions. They show that you need to have everything aligned to within a few mm of the center spot, so for the brightest views, you need to initially do a bit more, but only do it once as the mirrors will not move much, only tilt and change shape due to temperature and gravity.
http://www.propermotion.com/jwreed/A.../Collimate.htm
http://web.telia.com/~u41105032/kolli/kolli.html

Use the fan to cool the mirror to ideally within 0.5C of ambient temp. This can take hours, so for the sharpest views set up early and get the fan running. It can usually be turned off once cooled enough. If you have dew forming on the primary then just having moving air over the mirror can prevent it, so see if this fan moves air over the top of the mirror and if it causes any vibration.

Using a barlowed 20mm I find gives much better views than my 10mm straight. Better eye relief so I'm not straining to center the view and squinting up to the eyepiece. I will be needing filters too for Saturn. Naked eye observation is new to me too, but Saturn is far too bright for me to see any colour.

Mark.

[Krumlov (Gavin)]

Thanks Mark.

Originally Posted by joecool

I like to adjust my side tension knobs for balance so the scope stays in all positions with the heavier 2" eyepieces in place with the tension off.

Ok so that is for balance. I will see how I go over time and decide whether I need to move these off the zero point.

Check out these 2 sites and cross reference them with the Astrosystems instructions.

Ok they were helpful. My telescope came with a mark on the primary so I not planning on marking it again either. The next step in the instructions is "Mark and offset the secondary" but it also says this has no noticeable effects in a visual telescope so I am going to skip that.
The next step is mark the tube opposite to the focuser, I am a little worried about drilling a hole in the side of my new telescope!

Then since I skipped the mark and offset the secondary step I am not sure I need to do the next step which is "Offset and square the spider"?

The remaining steps seem straight forward although requiring me to dismantle the thing!

If you have dew forming on the primary then just having moving air over the mirror can prevent it, so see if this fan moves air over the top of the mirror and if it causes any vibration.

Ok so it is OK to leave the fan running as long as there is no vibration.

Stephen I am surprised you say I need more magnification. Saturn was moving fast enough through the FOV with my 6mm I thought?

Anyone else have any comments thoughts on my 2" EP not focussing when slid all the way in or the pros and cons of a 2" over a 1.25" EP?

Cheers

[joecool (Mark)]

Yes, marking the secondary is a bit iffy. I'm worried about getting it clean again too. Marking the Primary does not matter as it's center is blocked by the secondary when using an eyepiece. So long as your original center spot is very close to the center, then a de-focused star at high magnification will show nice concentric rings. Only re-spot it if it is obviously out.

Marking opposite the focuser just aligns the focuser to the tube. As I said, my focuser needed 2mm on one side to straighten it up as it was 1/2inch off center when looking through the LightPipe at the secondary. Using the LightPipe you will see if the secondary is not centered in the focuser, so that should be close enough.

At f/6 and for visual use you can ignore offsetting the secondary. So OK to skip that.
But, measure that it is central to the tube and that the vanes are square to the tube so they block the minimal amount of light down the tube. The vanes are usually adjustable with thumbscrews???

Oops. I missed the bit where you said you used the 6mm too. that's probably a good choice for planets, but also as you have found longer eyepieces give nicer views and coupled with a barlow can give you high magnification with wider fields of view. Quite often longer eyepieces have wider fields of apparent view due to their design. I don't mind saying it confuses the heck out of me.

I have a 30mm in 2" and a 26mm in 1.25". You would think that on the face of it that they deliver almost the same view due to similar magnification. But no way! The 30mm in 2" has a very large apparent field of view.

Just found this -
http://jayandwanda.com/digiscope/vignette/afov.html
"AFOV is the field of view through a telescope eyepiece at it appears to the eye. AFOV is determined by multiplying the actual field of view by the scope's magnification. For instance, a scope yielding a 2 degree field of view at 20x magnification would have an AFOV of 40 degrees.

Most eyepiece makers supply AFOV information for their eyepieces. As a general rule, Plossl designs are usually limited to no more than a 50 degree AFOV. Also, at focal lengths much greater than about 30mm, the barrel of 1.25" eyepieces will tend to limit the AFOV to something less than 50 degrees. For instance, a 40mm eyepiece with a 1.25" mount is usually limited to around 44 degrees. Copyright © 2003 by Jay Turberville"

Mark.

[Krumlov (Gavin)]

Originally Posted by joecool

then a de-focused star at high magnification will show nice concentric rings. Only re-spot it if it is obviously out.

I will try this is a few days when I get back from work.

Using the LightPipe you will see if the secondary is not centered in the focuser, so that should be close enough.

I don't have a Lightpipe I have the laser collimator with barlow attachment. I need to look at the instructions again to see how this is done.

At f/6 and for visual use you can ignore offsetting the secondary. So OK to skip that.
But, measure that it is central to the tube and that the vanes are square to the tube so they block the minimal amount of light down the tube. The vanes are usually adjustable with thumbscrews???

Yep I need to go back and have a good look at that part of the process.

Thanks for the FOV link. I will do some more research into that when I feel the need to upgrade EPs. For now I just want to get the best out of what I have got and get a feel for it.

Do you have the problem that I described about my 2" not focussing if it is fully in the focuser?

Cheers.

[joecool (Mark)]

You will need some kind of peep sight to center your eye in the focuser to be sure the secondary is centered relative to the focuser. The 2 sites I linked before have some info on this. A film cannister with a tiny hole in its center will get you close, and a machined tube will get you closer. Astrosystems mention this in your instructions at the point of rotationally centering the secondary.

Sounds like you need a focuser with a longer draw tube and/or with more travel. So long as the eyepiece is centered and not going to fall out then it should not be a big deal. You can get extensions for eyepieces which screw into the filter threads to increase the tube of the eyepiece. My own GSO scope has the opposite problem. I do not have enough inward travel on the focuser for barlows and cameras. I will need a VERY slim focuser with long travel or something like this to correct for the problem- http://www.siebertoptics.com/Camera%...0Adapters.html
Apparently I can also shorten my trusses??? Need to look into that.

Mark.

[Matty (Matt Fowler)]

Hi Gavin
Totally new to this game as well but am having the same issue with 2 inch EP on my new SW 14 dob. My 40mm wild field piece would not focus at all. Got myself a extension sleeve but found it wouldnt fit in the focuser, it was too wide by a good 2mm. After some experimentation with another two found one that fits the bill. So make sure the shop knows exactly what size one you need as it was really frustrating having to go back and forth till i got the right one. Cost me a week and a half of observing time. Was worth it in the end though fantastic super bright wide field veiwing at approx 30x. Hope his helps
Cheers Matt

[Gem (Grant)]

Originally Posted by Matty

Hi Gavin
Totally new to this game as well but am having the same issue with 2 inch EP on my new SW 14 dob.

Astronomy really has changed over the years. The quality of telescopes for beginners is really amazing. It is good to see!

Good luck all!