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Muc

Achromat & Semi-Apo filters

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Hi,

After reading through the details of these three filters, I am a little confused, do they not essentialy perform the same function?

https://www.firstlightoptics.com/achromat-semi-apo-filters.html

Ive a Tal 100R and when veiwing Mars its very washed out and bright with some flarey-ness  going on.

Also some double stars are not splitting due to abberations form the primary. Its happening on what are listed as relatively easy cases (Eta Cassiopeiae).

So trying to choose wisely and very confused.....

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I tried all the semi-apo filters and found them to be lacking.

For CA a cheap light yellow filter was just as good.

I did see a slight  contrast benefit from the Baader Neodymium filter.

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Is the Tal100r new to you or are these abberations a new occurence? The Tal100r does give a kind of very pale yellow cast on images but for an achromat it, well mine anyway, performs very well. The Tal100r objective lens cell is collimatable. I have a Baader semi-apo filter and I've only tried it in a fairly limited way in my ST120. I had no expectation that it would work any kind of wonder on the CA but first impressions are that stars look a little "tighter" more pleasing to the eye but thats a long way from being a scientific analysis! Looking forward to trying it in the Tal and my Vixen achro.

 

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Posted (edited)

I bought the scope 2nd hand about 2 years ago, but have never really used it propely until recenlty. I honestly  cant say how the collimation is, Im not too up on it, I should probably read more on star tests and get a handle on it.
 

Generally viewing DSO (M57, Dumbell) is great, and star fields look lovely but when their is a brght object then the resoloution issue kind of shows. I am do wear glassess normally, I find it more comfrotable to remove them when viewing, so my own vision may be a convern also.

 

How would one perform basic collimation on a refractor?

 

I am aware that its very easy to start throwing money at issues also when you can just relax and enjoy the view that you do get. I might just get a cheap moon or yellow filter and give that a go

Edited by Muc
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Get a piece of paper draw a circle the same size as the objective place this over the front of your scope then get a laser collimator turn it on mark the paper where the red light hits turn the laser 180° and do the same again then do the same again without the diagonal. If you have good collimation when there is no diagonal in but not with the diagonal hopefully you can adjust the diagonal to gain the same results. 

Like this 

IMG-20200330-WA0003.jpg

IMG-20200330-WA0003.jpg

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Cheers, I'll take a look at that

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Here's some daytime images comparing various fringe killer filters:

spacer.png

Most folks find the Baader Contrast Booster the most aggressive.

And this one shows how much of the spectrum different filters suppress:

spacer.png

The Contrast Booster works so well because it effectively cuts all violet and a bit of green-yellow-orange in the middle to reduce color shift.

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I've owned a couple of TAL 100's and at no time felt the need to filter the small amount of CA that they produced.

 

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Posted (edited)
13 minutes ago, John said:

I've owned a couple of TAL 100's and at no time felt the need to filter the small amount of CA that they produced.

 

I agree.

Edited by Saganite

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The easiest way to conduct a basic collimation test is to view Polaris at high power, around 200x and rack it in and out of focus.  If the image is nice and circular and no flaring at best focus you have little to worry about.  The test described by Wookie is a check for the alignment of the focuser, it's also important  but is not a test of the collimation of the objective.  Collimation of a Tal100 objective is a tricky business best left to someone with experience.  I have a Tal100 and a similar Vixen 4" F10, both have a small amount of CA but not enough for corrective filtration for my taste.       🙂 

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Someday I should try comparing the color correction differences between my f/5 ST80 achro, my f/6 72ED, and my f6.6 FPL-53 triplet 90mm on various subjects.  How much contrast on various subjects is gained with each step up in color correction?  I know that spherical correction also plays a big role as well.  I'll have to figure out how to separate the effects of the two of them first.

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I always used a Fringe Killer in my 6" f/8 achromat but never even considered a filter for my Vixen 90mm f/11.

The Vixen has always delighted me with its almost, violet free views. Both 'scopes have fixed cells.

Peter's advice is sound. Test, by all means, but change nothing you don't understand.

Laser collimators and Cheshire Eyepieces are themselves subject to errors until proven otherwise.

 

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After reading all the helpful advise and infomraiton on this thread, my plans came to an unfortunate stop.

The Tal took a drop of a kitchen table onto a tiled kitchen floor :(

The outside body looks good but the focuser is mostly jammed. It moves a little but only a small  amount of its usual travel. And what movement is there is horribly knotchy and grating.

Only the 25mm Plossl will come to some degree of focus and im not sure if that image is as good as it was.

My heart is a little broken at the minute. Particularly given the stock levels of anything to replace it

 

Edited by Muc
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Heck, thats sad news. The unfortunate thing is that the 100r focuser and diagonal is something of a bespoke item and not easily replaced, mind you, Tals are solidly engineered and maybe if you know someone with engineering skills they can get it corrected. 

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That's bad luck.  You might find that you have a broken small bearing that the focus tube rides on, they can be quite brittle and a sharp blow could split one.  They are replaceable if necessary. 

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There are screws where the rear black section meets the main tube. You can see them in the photo below. Should I try open to look at focuser or is that a bad move? 

IMG-20201015-WA0004.jpg

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You'll probably want to remove the focuser from the tube as you suggest.  Next, see if there is a way to remove the plate holding the pinion against the rack.  Once the pinion is removed, the focuser tube should slide out.  Do all this over a cookie sheet or other lipped pan to catch loose parts.  Once the focuser tube is out, you can inspect everything for damage.  Hopefully, nothing is permanently damaged.

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