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F/15 Istar

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#1 Rutilus

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Posted 16 October 2013 - 04:13 PM

At long last my observatory is now clear of items that I have been storing for someone,
and for the past few weeks I have been able to test my 6inch f/15.
The lens is a 150mm f/15 Classic Achromatic from Istar Opical. I went for the lens only as the scope
needed to be made to some special requirements. Basically the front tube and lens retracts
into the rear tube section which reduces the length to a little over 4 feet when the scope is not in use.

Due to an accident in the workshop, (i.e. yours truly knock a plank of wood over and smashed
the baffle system) I have been using the scope with no baffles in place.
I also constructed this scope for Lunar, planetary and double star observing. For DSO
I use my C9.25.

Star Test: Testing shows the lens to be a very well corrected visual objective.
Nothing unusual or out of place detected in the star test.
Bright stars such as Vega and Altair show a small but noticeable amount of C.A.
around the star. The C.A. is certainly a lot less than that seen with my 6" f/8 Achromat.
All-in-all, the lens behaves much as I expected an f/15 to behave. i.e. there is C.A.
but it is a small amount and not a problem for me.

Lunar: The lens has given me some of the best lunar views I have ever had.
Detail is exceedingly crisp and sharp. At powers above 100x, you can see a very tiny sliver
of blue fringing running along the limb.
What has struck me the most with the lens is the contrast it produces, it is excellent.
The moon appears on a jet black background, different coloration hues are easily visible
on the Lunar surface. Last night I was observing Plato, with six of the craterlets/spotmarkings
easily visible, the crater floor was filled with albedo markings.

Double-Stars: Well, where do you start. The lens is a first class double star splitter.
At powers from 200 to 500x it is well on top of its game, producing an excellent spurious disk
and faint first diffraction ring. It is very good at detecting close binary stars with different magnitude differences.
Each time I have observed 99 Her, the fainter companion instantly shows itself , even in poor seeing
conditions I have had no problems seeing it.

Jupiter: I have been getting quite a few early morning observations before going to work.
First off, there is C.A visible around the disk, not a large amount but it is there.
With the planet I have tried using a yellow filter, blue 80A, Semi-Apo and contrast booster filter,
but I prefer the un-filtered view. Visible details on the planet have been excellent, with much detail
visible within the belts, the GRS showed darker detail within the spot. Festoons are nicely seen,
as well as small white ovals in the southern region of the planet.
The North and South polar regions have shown clear mottling patterns. When the seeing
conditions are good the whole planet erupts into a wealth of detail, I don't think I would be able
draw to it all as there is just so much to see.

I have used aperture masks of 127mm and 100mm on the scope and had very good results.
With the 4 inch mask, the views of Jupiter easily match the best views I had of the planet
with my Tak TSA-102.
Really looking forward to observing Mars with this scope.

The scope cost me £580 GBP to build and I have been really pleased by the views it has delivered.
Now I really must get those baffles fitted to the tube.
Here it is in the observatory.

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#2 wormstar

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Posted 16 October 2013 - 04:21 PM

Looks interesting, can you post a daylight pic?

#3 De Lorme

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Posted 16 October 2013 - 04:47 PM

Take a Look at Russell23 review "CA Reduction filters"
The Longpass on the Moon and Deep sky is great. The Moon
and Sky Glow coupled to the Longpass makes them even better.
Congratulations! On your 6"F/15 build. De Lorme

#4 John Huntley

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Posted 16 October 2013 - 08:15 PM

Very interesting scope and report :)

I can't wait until I get my Istar Perseus 6" F/12 properly mounted !.

#5 Planemo

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Posted 05 November 2013 - 07:47 PM

Stick a camera in that thing and lets see some stuff!!!

#6 BillP

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Posted 06 November 2013 - 11:01 AM

The amount of color blur predicted for your scope without masking should be a little more than what an 80mm f/11 refractor produces, which is really very little. So would expect great results from yours like you posted. When you mask it to 102mm it operates at about f/22, and the color blur for that places it very near APO performance so good to hear your confirmation that it is operating masked akin to your TSA. A great setup you have there!

#7 Muffin Research

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Posted 06 November 2013 - 11:17 AM

So would you rather mask it down or use a filter for tweaking out the remaining color?
Wouldn't the Extra 50mm account for better resolution or is that lost on the blurry colors?

#8 Rutilus

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Posted 06 November 2013 - 05:19 PM

Not got a daylight shot on the mount, only got this at the moment.

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#9 Rutilus

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Posted 06 November 2013 - 05:53 PM

So would you rather mask it down or use a filter for tweaking out the remaining color?
Wouldn't the Extra 50mm account for better resolution or is that lost on the blurry colors?


For me the residual colour is a non issue, I still prefer the non-fitered view. The difference between 150mm and 100mm on resolution
is like night and day. My Tak (in 7 years) never ever gave me the detailed views that this scope shows at full aperture.
The cratelets in plato are so sharp and clearly defined, with my Tak they tended (apart from the largest one)to just appear as bright spot markings.
At full aperture I clearly observed 6 without even struggling to see them, at 4 inch aperture I got the big one and during moments of
better seeing managed another 2, but they just appeared as spot markings at full aperture they are striking in their appearance.

Over the last couple of weeks I've been observing Jupiter and the detailed views have been superb, we had the scope up against a 5 inch APO
and were extremely pleased with the Istar. It confirmed to me that for me to get better visual eyepeice views from a refractor would involve
spending a seriously large wedge of cash.
Still not fitted any baffles to the tube yet.

#10 KarlL

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Posted 06 November 2013 - 07:07 PM

Rutilus -

Very nice job. I've been batting around Richard Berry's 6" f/15 project for some time. I need to decide and buy the wood. That'll force me to commit. :grin:

Peace,

Karl

#11 Muffin Research

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Posted 06 November 2013 - 11:36 PM

For me the residual colour is a non issue,


good, kinda what I was hoping for.
Except for 3"-ers I've never looked through achromatics.
Only reflectors and apo's.
And my color perception is skewed anyway, I've got some Daltonism and well my hue's are off.

#12 Messyone

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Posted 07 November 2013 - 04:46 AM

Really nice. Long FL achro's are it for me too. I have high hopes for my f12 build after a look through it at trees, and blue sky background today. Very little CA. Baffles should make a difference, would like to see a before and after compare with and without baffles, is that possible?
Matt

#13 Rutilus

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Posted 09 November 2013 - 01:09 PM

Hi Matt
Yes when I get the new ones fitted I will report, but I have to say that even without the baffles this lens has excellent contrast.
Here a shot of the baffles before the accident.

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#14 Rutilus

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Posted 10 November 2013 - 03:58 AM

Got my first look at Mars with the Istar this morning, well chuffed. Seeing conditions were the not the best and Mars is small (around 5.1 Arc-seconds I think), however a North polar cap with a dark band running along its southern edge clearly seen. At the south of the planet there was a dark band (mare Sirenum?)running from the preceding to the following edge of Mars. Also running down the Preceding edge was what looked like a thin dark vertical streak.

Jupiter also gave up good detail inspite of the seeing, the GRS showed darker areas within it, superb rifts/activity following the GRS.
To give some idea of how much C.A. the scopes puts out around Jupiter, I used the moons as a guide. If you can imagine Jupiter surrounded by a dark blue fringe on a superb jet black background then the amount of the C.A. extending out from the limb was extremely close in size to the size of Ganymede at 225x power.

I Also tried a couple of filters again. With the Semi-Apo Icould still detect a tiny amount of Blue fringe, what I did not like was the fact that it made the Jet black background appear with a greyish tint.

The Contrast booster did a better job by removing the blue fringe, detail on the planet was still good and the background was blacker than with the S.A.

However I still prefer the un-filtered view, how Jupiter appears on a Jet black background reminds me very much of the view with the TSA-102.

#15 Rutilus

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Posted 10 November 2013 - 05:42 PM

Managed a shot of the moon this evening, that was quite low down in the Western sky with average seeing conditions.
I'm not an imager, as you will probaly tell from my photo.
This was a single frame shot taken at prime focus with a Canon 350D, exposure 1/100th Second at ISO 400.
All I Have done is re-size the shot for posting.

Had to play around with the cameara to get as much of the moon on the camera chip, which is why the orientation
of the moon is different to how it appeared in the sky.

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#16 Rutilus

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Posted 10 November 2013 - 05:46 PM

This is a section from the full size image, showing the C.A. along the lunar limb. The image on the left is from the Northern region, with the
southern limb shown on the right.
The blue C.A. band is how I see it visually on the lunar limb.

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