Care to do a experiment

[weixing]

Hi,
Recently, someone ask me this question again... "Does a smaller apeture APO can out-perform a bigger aperture SCT?? Because he read some review saying that a 80mm APO perform better than a 8" SCT."

Below is my understanding why people will have such a view:
the 80mm APO will give a more pleasing look than a 8" SCT, because the 8" SCT will grather more light and cause the atmostphere turbulence or the imperfect sky to be more obvious than in the 80mm APO.

If my above understanding is correct, the below will be true:
a smaller aperture scope will give a more pleasing view than a larger aperture scope using the same magnification.

So I will do the below experiment during my next observation in Singapore:
I will make a mask with a clear aperture of 4" to convert my 6" F5 Newtonian to a 4" F7.5 Newtonian. I will then compare the view of the Jupiter, Saturn and Moon at 100x and 200x.

I expect the 4" Newtonian will give a more pleasing, but dimer view than the 6" Newtonian under imperfect sky.

By the way, I hope more people will also conduct the same experiment using their scope and report the result here... especially those who has an APO or those who have a smaller APO and a larger SCT/MCT/Newtonian.

I hope I can conduct the experiment by next week and I will report the result here once I completed the above experiment.

Have a nice day.

PS: May be I was just eating too much and had nothing better to do... Ha Ha

[IceAngel]

really love to see this experiment being conducted.. I wish to see the result from this experiment soon, it can clear most of the beginners doubts including me about the big and small aperture scopes and the difference between them... :oops:

*To Weixing: inform me when you going to get this experiment conducted, can I??? if i am free, i will definitely go~!!!! just drop me an sms

[VinSnr]

Hi,
Recently, someone ask me this question again... "Does a smaller apeture APO can out-perform a bigger aperture SCT?? Because he read some review saying that a 80mm APO perform better than a 8" SCT."

Below is my understanding why people will have such a view:
the 80mm APO will give a more pleasing look than a 8" SCT, because the 8" SCT will grather more light and cause the atmostphere turbulence or the imperfect sky to be more obvious than in the 80mm APO.

If my above understanding is correct, the below will be true:
a smaller aperture scope will give a more pleasing view than a larger aperture scope using the same magnification.

So I will do the below experiment during my next observation in Singapore:
I will make a mask with a clear aperture of 4" to convert my 6" F5 Newtonian to a 4" F7.5 Newtonian. I will then compare the view of the Jupiter, Saturn and Moon at 100x and 200x.

I expect the 4" Newtonian will give a more pleasing, but dimer view than the 6" Newtonian under imperfect sky.

By the way, I hope more people will also conduct the same experiment using their scope and report the result here... especially those who has an APO or those who have a smaller APO and a larger SCT/MCT/Newtonian.

I hope I can conduct the experiment by next week and I will report the result here once I completed the above experiment.

Have a nice day.

PS: May be I was just eating too much and had nothing better to do... Ha Ha

Weixing, I don't think your 4" masked down newt is gonna look better than your original 6" Newt. Not only your view is dimmer, but you might even see lesser details.

Why then is people saying an 80mm APO gives more pleasing view than the 8" SCT? I have both scopes and I can tell you that the 8" SCT will eat the 80mm APO in deep sky but may lose out to the 80mm APO on planets.

The reason why a smaller APO could be better is the lens surface figure and lack of central obstruction. APO normally cost way more and has better lens quality than those mass produced SCTs. When comes to planetary details, contrast is very important and that's where the smaller APO has an advantage.

But when comes to deep sky...forget about the small APO. The 8" SCT is gonna show more stars and aliens if any.

[ariefm71]

Weixing, would it be possible to convert my C8 into a C5 using the mask? always want to compare the performance of 8" SCT against 5" SCT on DSOs.

Arief

[VinSnr]

Weixing, would it be possible to convert my C8 into a C5 using the mask? always want to compare the performance of 8" SCT against 5" SCT on DSOs.

Arief

the central obstruction size of the 8" is different from the 5" SCT. How are you gonna convert it to a 5" SCT?

Also for SCT, the corrector lens is matched with the primary mirror. I am not too sure what's gonna happen to your image quality if you remove part of that matching!

[IceAngel]

eh..... so is that experiment not going to be accurate?? as the central obstruction is different, the bigger the aperture, to bigger the central obstruction??

if the central obstruction is bigger and the aperture is masked small(example 6" SCT), the light gathering will be lower than the actual size(example 4" SCT)?

[IceAngel]

if i compare my 93mm arhcomat and the 80mm APO (the aperture looks not much difference to me),

1: will the Field of View of the APO better than the 93mm on DSO?? (if i wanna take some DSO picture)

2: then for planets viewing, will the 93mm archomat be better for camera image effects?? or the APO will have lesser false colour than 93mm... as i can see my two sets of moon photos, can still see some blue outline in the photo..

[ariefm71]

the central obstruction size of the 8" is different from the 5" SCT. How are you gonna convert it to a 5" SCT?

dunno, maybe i was just drinking to much

Anyone with a C5 available for a joint observation this weekend? The weight and size of the 8" SCT starts to take toll on old ppl like me :? (I normally observe from a multi-storey carpark nearby, need to bring the scope, wedge, and tripod through the stairs)

[weixing]

Hi,

The reason why a smaller APO could be better is the lens surface figure and lack of central obstruction.

If you use an off-axis mask (so became a 90mm or so unobstructed SCT) on your 8" SCT, will the view of your unobstructed SCT now match the 80mm APO?? Will the contrast improve, since there is now no obstruction??

Have a nice day.

[VinSnr]

if i compare my 93mm arhcomat and the 80mm APO (the aperture looks not much difference to me),

1: will the Field of View of the APO better than the 93mm on DSO?? (if i wanna take some DSO picture)

2: then for planets viewing, will the 93mm archomat be better for camera image effects?? or the APO will have lesser false colour than 93mm... as i can see my two sets of moon photos, can still see some blue outline in the photo..

before I can answer this, what is the focal length of your 93 mm scope?