8" f/4...coma???

Post by **acc** » Sun Jun 10, 2007 2:34 pm

VinSnr wrote:
bro....how long is the tail of the "comet"?

As a very rough gauge, maybe a few times the radius of the first/second diffraction ring depending on severity.

Post by **VinSnr** » Sun Jun 10, 2007 3:01 pm

weixing wrote:Hi,
Bro..yeah the secondary is 2.5" x 3.5". What is the size of the secondary for the 8" f/5 version?
The size for the 8" F5 is 50mm... around the minimum secondary size for a typical 8" F5. My Celestron 8" F5 also come with a 50mm secondary, but there is around 1mm around the edge not aluminized, so the actual size is smaller than 50mm. An accident to the secondary mirror "encourage" me to change the secondary mirror, so I change to a slightly larger secondary mirror, but not too large that degrade the planetary performance. So I get a 53.34mm (2.1") secondary mirror which give me around 3.4mm of 100% illuminated image at 27% obstruction.

Have a nice day.

bro, don't you have to change the secondary holder as well?

Post by **weixing** » Sun Jun 10, 2007 3:09 pm

Hi,

bro, don't you have to change the secondary holder as well?

No... I can't find a suitable holder and spider, so decided to used back the original holder... I just remove the original mirror (which was hold by a very strong double sided tape

) and install the new mirror on the original secondary holder using RV Silicone. It's works very well.

Have a nice day.

Post by **VinSnr** » Sun Jun 10, 2007 3:56 pm

weixing wrote:Hi,
bro, don't you have to change the secondary holder as well?
No... I can't find a suitable holder and spider, so decided to used back the original holder... I just remove the original mirror (which was hold by a very strong double sided tape ) and install the new mirror on the original secondary holder using RV Silicone. It's works very well.

Have a nice day.

Wouldn't the holder which holds the 50mm mirror smaller than your 53.3mm mirror? How could your new mirror fit into the holder?

Also, what is RV silicone and where to get?

Post by **weixing** » Sun Jun 10, 2007 4:27 pm

Hi,

Wouldn't the holder which holds the 50mm mirror smaller than your 53.3mm mirror?

The original holder is also smaller than the original 50mm mirror. The picture of my secondary mirror is here: http://www.singastro.org/viewtopic.php?t=4492

How could your new mirror fit into the holder?

I use the RTV (is RTV not RV... I miss out the 'T') silicone to stick the secondary mirror to the holder as recommend by most ATM website. I follow the protostar instruction, but I think I applied a bit too much of silicone as I'm a bit worry that the secondary mirror will drop... first time doing this... hee hee

Also, what is RV silicone and where to get?

I get from a local DIY shop... the brand and full product name is "WESSBOND Clear RTV Silicone Adhesive/Sealant"

Have a nice day.

Post by **VinSnr** » Sun Jun 10, 2007 5:50 pm

I see what you mean now.

The newer GSO scopes have a holder that fit the secondary mirror exactly. So the side of the secondary is covered by the holder. Good thing is that it looks quite professional, bad thing is, you can't use other sizes.

Canopus Lim · Post by **Canopus Lim** » Sun Jun 10, 2007 5:54 pm

To actually really see coma, you need a good eyepiece. Mostly cheap wide angle eyepiece are not well corrected for such fast scope and hence what you would see off-axis is a combination of astigmatism and coma. Astigmatism of the eyepiece is the dominant one. When these combine, you get seagulls towards the edge instead of 'comets'. If you use good eyepieces (well corrected ones), you will see more coma and hence you see stars with 'tails' towards the edge and not stars that look like seagulls. Anyway seagulls are far worse than comets! The star itself will be round but there will be a short 'tail' to them at the edge of the FOV. The 'tail' will be pointing to the centre of the FOV. Anyway, the tail is faint, if you look at faint objects like DSOs, it is mostly not noticeable. If you let a bright star drift across the FOV, you can see the 'evolving' comet.... haa.

One reason why I bought a f/5 scope is that I do not want to buy a paracorr. Well there is coma on a f/5 scope but to me it is still ok... at least I find the stars look sharp across majority of the FOV on a wide angle eyepiece. With a paracorr it will be sharper, but I would lose true field, less contrast on galaxies and money (which is the biggest issue..haa)

kayheem · Post by **kayheem** » Sun Jun 10, 2007 10:32 pm

VinSnr, RTV is also known to us Singaporeans as silicone sealant, like the ones they use on aquariums and sealing the sink around the kitchen work top.

CKH

rlow · Post by **rlow** » Mon Jun 11, 2007 12:11 pm

My eyes are quite sensitive to coma at f/5, ie, I am already affected by coma in my 11" Astrosky or the 12.5" Obby & 12.5" Portaball. Hence, for my 11", I find it a necessity to use the TV Paracorr with my eyepieces 20XW, 14XL, 10XW etc for specific DSOs like open and globular clusters etc. Once I have used the Paracorr, I can't live without it.

The 30XW and 40XW are already well corrected without the Paracorr. Incidentally, the Paracorr helps to 'balance' my shorter focal-length XWs with the 30XW & 40XW.

You should not be seeing coma on-axis when you are doing a star test, and measuring coma is not so straightforward as it varies across the field and gets more severe towards the edge of field.

I have look through the R200SS f/4 and the coma is ... well...er... bad, just to put it mildly.

rlow · Post by **rlow** » Mon Jun 11, 2007 12:20 pm

VinSnr, maybe can we have a chance to look through your GSO 8"f/4? I can bring along the Paracorr for you to see if there is any difference.