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Long Perng 90 APO incoming!


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My big worry was the scope would not perform on planets, that f5.5 was just too extreme. So I am greatly relieved to be getting good views after all.

The extra aperture is really welcome. Instead of feeling held back by the scope's capabilities,  I have the feeling there is more to see if I just wait a little longer for a clearing in the seeing.

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On 26/10/2022 at 20:33, Ags said:

Always prism. I might pack the mirror diagonal and do a shootout on my dark sky trip. 

Given the f5.5 light cone, the prism , if it has 25mm light path, should over-correct by 1/7th wave compared to the mirror.  Interesting to see if it can be noticed.

David

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Yes, the idea is that this cancels undercorrection in the scope. It seems to work for my ZS66, but I haven't actually verified it improves the LP90 views vs a mirror diagonal.

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11 hours ago, Merak said:

Is it a keeper?

Seems it could be a nice size for a EEVA scope.

Just adding I have packed a 0.5 reducer and ASI485MC on my dark sky trip so if I get clear skies (looking doubtful) I may try a little EEVA.

Edited by Ags
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The GSO 0.5 reducer does not work with the LP90 - as expected, as the reducer is advertised for slower focal ratios. It does come to focus at least but almost the entire field is a fishbowl.

Currently shooting some 6-second subs of M31 at 500 mm focal length. It will just be a detail of the core and M32 but still a nice experiment. If I am not too tired I will also shoot M81.

Evaluating the scope for visual DSO performance will have to wait for another day however - my tearducts are blocked and I can't see straight!

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My imaging session was a bust - I managed to shoot unstackable 8 bit SER sequences of M81 and M31. I rescued them with Siril but the resulting tiffs lost all color info??? After stacking in APP it turned out my focus was off. I am trying again at 4 am this morning, should be a clear spell then.

Edited by Ags
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I got up early this morning to have another go at M81/M82, and shot about an hour of 8 second frames. I'm not so happy with the telescope for RGB photographic use, looking at the stars, but this is admittedly with the stars and galaxies sharing the same stretch. If I separated the stars into a separate stretch I think it would look much nicer. Also the image is wierdly colorless, so not entirely sure I didn't mess up at some point in the processing. I'm sleep deprived and liable to err!

The result is very noisy - but I only had an hour of clear sky before dawn twilight, and no further nights at my dark sky location, so no possibility to add more data.

Regarding the stars, focus is extremely critical with this f5.5 telescope, so I am ordering a Bahtinov mask to get that perfect.

Aside from star bloating, the telescope behaved nicely on my AZ-GTI so I expect it will give nice results (to my eye) with a duo band filter, or even in RGB with more careful processing. 

image.png.23de1606609eaedad2031385e8f13978.png

Edited by Ags
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1 hour ago, Ags said:

Also the image is wierdly colorless,

Did you use UV/IR cut filter?

ASI485MC has only AR coated window, so it needs UV/IR cut filter.

This can also be reason for bloated stars.

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2 minutes ago, Ags said:

Yes - I used an Astronomik L3 - so it cuts the extreme blue and red too, not just the IR.

Ah, yes, I was going to mention that - but you obviously know the benefits.

This just shows that there is no magic involved - fast ED doublet is not going to be well corrected for AP, even with aggressive filtering.

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True that. I think more of a visual or narrowband scope. But I will do some more experiments when I get home.

One thing to try is image with the prism diagonal, as that corrects some of the false color.

I can also try my Baader UHC. Or even my Wratten #8. It would be nice to find a hack to get passable RGB as I can't pack two scopes for travel.

Edited by Ags
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Had a detailed view of the Moon and Mars last night. A very good lunar and planetary scope. Some blue haze on the sunlit limb of the Moon - but that could also be from the prism or the cheap barlow. 

Took this picture:

image.thumb.png.73aad9c181a737b9fd81f1c668dd3f74.png

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I also took this snap of Mars with a 2x barlow - 90mm aperture and 1000mm focal length is not the right tool selection, but it is quite close to what I was seeing in the eyepiece:

image.png.88a6ae40d5a811edc0df87972b96f05f.png

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5 hours ago, Ags said:

I also took this snap of Mars with a 2x barlow - 90mm aperture and 1000mm focal length is not the right tool selection, but it is quite close to what I was seeing in the eyepiece:

image.png.88a6ae40d5a811edc0df87972b96f05f.png

Hints of surface detail, maybe Syrtis Major? I think its great for the aperture.

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