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

[VinSnr]

I tested the GSO 8" f/4 last night at Turf City and I didn't notice there was a coma problem at all. I am not sure why people keep saying the coma is so damn bad at f/4.

I used two eyepieces, 32mm TV Plossl and 5.2mm Pentax XL.

I did not notice any severe coma in the FOV. Object at the center was tack-sharp. Stars were probably a little soft at the last 10-20% of the edge, but you have to look for it. I expected the coma would "jump" at you but it did not.

Using the 5.2mm XL, Jupiter was wonderful. Lots of bands and festoon. Certainly better than any 8" SCT I have seen. Those bands are not as "obvious" as the 12.5" Portaball that I saw few weeks ago, but they are there if you look for it and let your eyes adapt to it.

Did a lot of video on those popular M objects, the view on the screen was WoW! (YewKay can testify). Got to see to believe it. Globular shows yellow, blue, red, green stars, very nice. Wanted to use DSLR but forgot to bring an adaptor.

All in all, it's a great scope for US$375. Superb value for money. The finder is good and the focuser is way better than those older R&P focuser although there is still a slight shift at high power.

[acc]

I am one of those who when in the dark, says "I see coma".

It varies with severity according to observing conditions:
-I see significantly more coma at F4.7 than at F5
-not an issue at moderate to high power (>150x)
-most annoying at low-moderate power (~100) using some ultra-wide angle eps on a F4.7 or faster scope. In terms of coma suppression on a F4.7 scope based on my own experience (YMMV)
1) TV T2 20mm (best)
2) Meade 5k 24mm UWA
3) TV T4 22mm (worst, basically unuseable without parracorr)
-not an issue at very low power (the coma is there but not magnified as much, so its not distracting unless you are looking out for it)

[Canopus Lim]

It should be noticeable at focus if you look carefully. If you defocus the stars, you can see that the shape is comatic and becomes more severe towards the edges. The 32mm TV plossl is well corrected and the field is not so large (being 50 degree AFOV) and at lower magnification so the coma is not that obvious. Also the 5mm XL is at higher magnification, so the field is smaller, coma will not be too obvious. Coma will be obvious if the true field is very wide.

I notice coma on my f/5 scope but I would say it is still ok if one is not too fussy. Generally the field is sharp. The edges will have 'tail' like stars due to coma. I will not buy the paracorr since it is so expensive and it will cause balance issues and loss of light throughput with it.

[VinSnr]

It should be noticeable at focus if you look carefully. If you defocus the stars, you can see that the shape is comatic and becomes more severe towards the edges. The 32mm TV plossl is well corrected and the field is not so large (being 50 degree AFOV) and at lower magnification so the coma is not that obvious. Also the 5mm XL is at higher magnification, so the field is smaller, coma will not be too obvious. Coma will be obvious if the true field is very wide.

I notice coma on my f/5 scope but I would say it is still ok if one is not too fussy. Generally the field is sharp. The edges will have 'tail' like stars due to coma. I will not buy the paracorr since it is so expensive and it will cause balance issues and loss of light throughput with it.

When I was using the 5.2mm XL to check for collimation, the shape of the defocus star on both side of focus is pretty round to me. The "donut" (black shadow of the secondary) tend to shift a little from one side of focus to another. I guess the focuser has a little shift in it.

I didn't notice the defocus star changed to a comatic shape.

Probably my eyes are not too sensitive to coma. I was expecting the stars to be oval-shape like but there isn't.

[weixing]

Hi,

Using the 5.2mm XL, Jupiter was wonderful. Lots of bands and festoon. Certainly better than any 8" SCT I have seen.

The GSO 8" F4, unlike the Vixen R200SS 8" F4 scope, is actually a visual F4 scope... it's use the minimum secondary size for a 8" F4 (63mm). Since the central obstruction (31.5%) is still smaller than a typical SCT, it should perform better than a SCT especially it got less optical element (no corrector plate and no diagonal).

All in all, it's a great scope for US$375. Superb value for money.

Anyway, seem like the GSO 8" F4 is a good scope and good value. Where did you get it from?

Have a nice day.

[VinSnr]

-I see significantly more coma at F4.7 than at F5
-not an issue at moderate to high power (>150x)
-most annoying at low-moderate power (~100) using some ultra-wide angle eps on a F4.7 or faster scope. In terms of coma suppression on a F4.7 scope based on my own experience (YMMV)
1) TV T2 20mm (best)
2) Meade 5k 24mm UWA
3) TV T4 22mm (worst, basically unuseable without parracorr)
-not an issue at very low power (the coma is there but not magnified as much, so its not distracting unless you are looking out for it)

from f/5 to f/4.7 also can see??? So sensitive huh?

Can elaborate what you see or not? Are you talking about edges or center?

[acc]

Ya lor, seems like there was a pronounced increase in coma when moving from F5 to F4.7.

Coma is very apparent to me. To try to see it for yourself, try this experiment,

WARNING, THE EXPERIMENT BELOW MAY LEAD TO COMPULSIVE PURCHASES OF EXPENSIVE EQUIPMENT. TRY AT YOUR OWN RISK!

-select an eyepiece that gives you 50x-100x magnification (preferably a cheap, wide-angle ep).
-centre your FOV on a bright star and focus to achieve a sharp image (the star will be a pin-point of light at this stage).
-move the star towards the edge of the FOV.
As you carry out the last step, you should begin to see the star being transformed into a comet... (the head of the comet is the star and it will look as if the comet is pointing towards the centre of the FOV).

But VinSnr if you don't see coma than don't try the experiment lor... some things are better left unseen...

[VinSnr]

Ya lor, seems like there was a pronounced increase in coma when moving from F5 to F4.7.

Coma is very apparent to me. To try to see it for yourself, try this experiment,

WARNING, THE EXPERIMENT BELOW MAY LEAD TO COMPULSIVE PURCHASES OF EXPENSIVE EQUIPMENT. TRY AT YOUR OWN RISK!

-select an eyepiece that gives you 50x-100x magnification (preferably a cheap, wide-angle ep).
-centre your FOV on a bright star and focus to achieve a sharp image (the star will be a pin-point of light at this stage).
-move the star towards the edge of the FOV.
As you carry out the last step, you should begin to see the star being transformed into a comet... (the head of the comet is the star and it will look as if the comet is pointing towards the centre of the FOV).

But VinSnr if you don't see coma than don't try the experiment lor... some things are better left unseen...

bro....how long is the tail of the "comet"?

[VinSnr]

Hi,

Using the 5.2mm XL, Jupiter was wonderful. Lots of bands and festoon. Certainly better than any 8" SCT I have seen.

The GSO 8" F4, unlike the Vixen R200SS 8" F4 scope, is actually a visual F4 scope... it's use the minimum secondary size for a 8" F4 (63mm). Since the central obstruction (31.5%) is still smaller than a typical SCT, it should perform better than a SCT especially it got less optical element (no corrector plate and no diagonal).

All in all, it's a great scope for US$375. Superb value for money.

Anyway, seem like the GSO 8" F4 is a good scope and good value. Where did you get it from?

Have a nice day.

Bro..yeah the secondary is 2.5" x 3.5". What is the size of the secondary for the 8" f/5 version?

Thinking of changing the front spider to curve ones and use a 2.6" mirror instead. Not sure worth it or not. Upgrade itself is nearly as expensive as the scope..hahaha

[weixing]

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.