C8 vs Refractor for planets?

[Roy Challen]

19 hours ago, Lockie said:

Definitely worth throwing a Classical Cassegrain into the mix for consideration, it could be the best compromise of aperture, contrast, sharpness, and cooling time. 

After reading the thread about StellaLyra scopes, I think this definitely sounds possible.

[Peter Drew]

    🙂

[mikeDnight]

1 hour ago, Alkaid said:

Aw...c'mon Mike....my C8 heard that and is now crying in the corner 😜

Kidding aside, the worst enemy for me is UK seeing.  Generally speaking, it is terrible for 99% of the year.  Refractors are just better at dealing with it.  I'm sure that a good SCT, collimated and away from the jet-stream would deliver.

Sorry your C8 is crying Steve. If it makes your C8 feel any better, it was actually a C8 that gave me one of the best views of Jupiter I've ever had. It was an old 80's Orange C8 who's owner was desperately trying to sell on. I reluctantly looked through the scope and was blown away by the view, and so was its owner. He changed his mind about selling it after that.  So some can do a great job and Celestron always seemed to be better than Meade for some reason. I'd love to look through an old Takahashi SCT, but they are as rare as hen's teeth and no longer in production. 

[inFINNity Deck]

On 12/10/2020 at 17:41, inFINNity Deck said:

Next to my C11 I have a SkyWatcher 300PDS Newton (304mm f/4.9) on a Dobson mount, but even though the SCT is not my best friend, I have to say that I find the views trough the SCT more pleasant than with the larger aperture Newton.

Nicolàs

A bit off-topic perhaps (so apologies for that), but during the past few weeks I have been building a bath-interferometer and a few days ago, after having first having tested a Bresser 130 f/5, I decided to place the before mentioned SkyWatcher 300PDS in front of the bath. I took 640 images to calculate the mirror's shape using DFTFringe. These 640 images were taken methodical: the mirror was rotated along its optical axis 8 times, so a set of images was taken for every 45 degrees of mirror-rotation. For each mirror-rotation I also rotated the interferogram 8 times, so the fringes were oriented in 8 steps of 45 degrees. The each fringe-orientation was imaged 10 times to get a good average and to be able to filter out some 50 bad igrams based on RMS (the median RMS of this set-up is about 0.1 lambda with an estimated standard-deviation of 0.02 lambda, so I rejected all igrams with an RMS larger than 0.15 lambda). The following image shows the 8 mirror-rotations in the top row and the same mirror-rotations below it, but then de-rotated:

And this is the final plot (average of above 8 de-rotated ones) of its surface:

The 300PDS has quite a bit of astigmatism and although I did not test the mirror of my C11 SCT, I think I this may well explain why the C11 performs better than my 300PDS Newton.

Nicolàs

 

[vlaiv]

2 hours ago, inFINNity Deck said:

The 300PDS has quite a bit of astigmatism and although I did not test the mirror of my C11 SCT, I think I this may well explain why the C11 performs better than my 300PDS Newton.

Nicolàs

Have you tried Roddier analysis to confirm these results?

It is rather easy to do - you need either artificial star, or you can do it on a real star in good seeing and planetary camera.

Test consists of shooting defocused pattern both sides of focus and then analyzing it in software written for that purpose WinRoddier.

Only difference to above will be that it tests system performance not the mirror itself (but secondary should not make much difference if it is good).

[inFINNity Deck]

1 hour ago, vlaiv said:

Have you tried Roddier analysis to confirm these results?

It is rather easy to do - you need either artificial star, or you can do it on a real star in good seeing and planetary camera.

Test consists of shooting defocused pattern both sides of focus and then analyzing it in software written for that purpose WinRoddier.

Only difference to above will be that it tests system performance not the mirror itself (but secondary should not make much difference if it is good).

Hi Vladimir,

no, this method was not known to me (yet). I am going to dive into that. Funny thing is that the main purpose of this 300PDS is to use it as an artificial star, although I did create a Dobson mount for it in order to use it for outreach in the field. When used as an artificial star a 9 micron fibre optic cable sits centred in the focuser, which is focused at infinity and in that way it does a great job. 🙂

Nicolàs

 

PS: just found your post regarding the Roddier test, very interesting stuff indeed:

 

Edited December 6, 2020 by inFINNity Deck

[Paz]

I've got a C8 and a 102mm f7 refractor. For me the C8 is harder work but the extra aperture means it simply beats the refractor when the set up and the seeing is good. The refractor presents cleaner images but the C8 presents more detail (and goes deeper on dim targets).

However for me the workload matters and so the refractor gets used a lot more than the C8.

[John]

With the planets low down (eg: Saturn and Jupiter currently) I've found that my refractors "cut through" the unsteady seeing better than my 12 inch dob does.