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DIY Refractor

kbrown

By kbrown
in DIY Astronomer

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kbrown

kbrown

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Finally got all the parts done and assembled to get this in a working order. Turns out the focal length is more like 500mm. Remains to be confirmed when I get to do some astrometry. 

I had the objective fully AR coated too (all four surfaces) . I also took the easier route and bought a 2" star diagonal rather than making one myself. That actually extended the optical path a lot more than anticipated. But I managed to find a sweet spot in the tube length so I can easily switch between visual and imaging use. 

The OTA weighs just under 1400g without the finder and star diagonal. Should be ok with my star adventurer even with a CCD and accessories. Can't wait to get to test this ?

IMG_20180917_194447.jpg

IMG_20180917_194813.jpg

IMG_20180917_222020.jpg

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kbrown

kbrown

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Found the necessary bits and bobs to attach my dslr to the scope. Not an ideal match but at least I can get started. It's cloudy and rainy here at the moment so proper testing has to wait.

Did some tests shooting some lights on a crane in a distance though and it did reveal a couple of issues. Stray light was reflecting in the objective cell and focuser tube so I flocked them. The other issue was that I wasn't able to get the edges of the image in focus. Not sure if this is due to too big sensor (full frame canon 6d) or field curvature. I probably should get a field flattener anyway. Was thinking of this:

https://www.firstlightoptics.com/reducersflatteners/hotech-sca-field-flattener.html

IMG_20180920_211940.jpg

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kbrown

kbrown

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45 minutes ago, Peter Drew said:

There is a distinct possibility that the lens cell and lens are the wrong way round, its design give a rather confusing appearance. It could account for the optical anomalies that you have described.    ?

I did wonder which end of the cell should be facing forward when I received the objective from you. Confusing appearance indeed. But since the lens pack can be put in it either way I decided this is the way it goes in my tube. :)

The crown and flint can only be put together one way. If one or the other is the wrong way you'd have glass touching glass even with the thin spacer in there. I also made sure the markings on the edge of the lenses align.

For the sake of making sure I just did try putting the lens pack in the cell the other way and was not able to achieve focus at all.

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Stub Mandrel

Stub Mandrel

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8 hours ago, kbrown said:

The crown and flint can only be put together one way. If one or the other is the wrong way you'd have glass touching glass even with the thin spacer in there. I also made sure the markings on the edge of the lenses align.

The bi-convex element goes at the front, the 'hollow' element goes at the back. Usually this means the 'rounder' surface is at the front.

 

<edit> I just found this, wish I had found it earlier!

http://irpoyser.co.uk/how-to-build-a-telescope/

It explains that the lens at the back is a 'meniscus' lens.

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kbrown

kbrown

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2 hours ago, Stub Mandrel said:

If your scope is f5-f6 (and I think it is) this one I have is a fair bit cheaper:

https://www.firstlightoptics.com/reducersflatteners/ovl-field-flattener.html

Thank you! I did look at this one. However if my guestimated focal length of 500mm is correct then this would be f/7.1. Still need to confirm this...

 

2 hours ago, Stub Mandrel said:

The bi-convex element goes at the front, the 'hollow' element goes at the back. Usually this means the 'rounder' surface is at the front.

 

<edit> I just found this, wish I had found it earlier!

http://irpoyser.co.uk/how-to-build-a-telescope/

It explains that the lens at the back is a 'meniscus' lens.

This objective seems to be a flint leading Fraunhofer APO as in the first image on this page: http://www.donzoptix.co.uk/ASTRO/OBJKTVS/Objktvs.html

If I turn the objective the other way, it just won't focus at all. I think the blurriness in the edges is just a combination of too big sensor for the scope and field curvature. If the FL is 500mm then with my Canon 6d the FOV would be pretty big: 4.10° x 2.73°

I'll post some test images later.

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kbrown

kbrown

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D'oh... you win... Turns out not only did I have the lenses in the wrong order in the cell, the crown was also reversed. ?‍♂️

Went through all the combinations and indeed the double convex goes in front and with the lenses the right way around and aligned I'm able to focus the all parts of the view but not at the same time. Hopefully the field flattener I ordered takes care of the rest.

This test image has the center in focus but the edges are slightly blurred.

IMG_8069.thumb.jpg.51d54485f2ccd364f3462bf8506da3e3.jpg

 

Slight turn on the focuser puts the edges in focus but the center is slightly blurred.

IMG_8070.thumb.jpg.d1c0c4a26ddf82a131acfe3484a037a3.jpg

 

Thanks for your help!!!

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Stub Mandrel

Stub Mandrel

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21 minutes ago, davidc135 said:

Congrats on this project. Where did you get the lens surfaces coated? Ah, scientific mirrors I've just noticed. David

They've just done my 10" mirror and secondary, waiting for them to arrive!

I can't see anti-reflection coatings on their website? My dad is trying to get me to take a WW2 gunsight with a ~2 1/2" to 3" objective and a HUGE avici prism, none of which appear to be coated but could be fun to get coated and put in an astro scope...

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kbrown

kbrown

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19 hours ago, Stub Mandrel said:

I can't see anti-reflection coatings on their website?

No they don't advertise it in there so I asked Terry directly. His reply was:

"Yes we do anti-reflection coat objective lenses. We coat them with a standard broadband coating which on average reduces the reflection down to 2%."

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kbrown

kbrown

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1 hour ago, Stub Mandrel said:

I assume the prices are in broadly line with the mirror coating prices?

I guess so. I've had my 10" mirror Re-coated by him before and it was roughly the double price to this job (two lenses / four surfaces). 

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  • 1 month later...
kbrown

kbrown

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Finally got to test this thing properly. Here's Andromeda shot with my unmodded Canon 6D attached to the scope with the field flattener. I cropped the original image to 3k x 3k then resampled down for web. There was only tiny bit of vignetting in the corners of the full image and everything was in focus. The polar alignment or balance was probably a little of as the stars are no quite round. Other than that I'm pretty pleased so far :)

Andromeda_LRGB_resample.jpg

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kbrown

kbrown

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1 hour ago, feilimb said:

Well done that is a brilliant result, it must be so rewarding having brought the whole scope together from the start :) Did you get some astrometry in the end to determine the focal length?

Thank you. Indeed I am pleased. Will take me a little more work to get it usable with my Atik and Filterwheel. Shouldn't be too hard and the result should be even better.

Indeed I did get the focal length from PixInsight's Photometric Color Calibration which said it is ~497mm. Not too far off from my 500mm guess :)

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kbrown

kbrown

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3 hours ago, Merlin said:

Is there an easy way to work out where the glare stops are positioned?

Do you mean baffling? I didn't put any in this scope but I did add flocking material in front and behind the lens in the cell and in the dew shield (not in the pics) and also inside the focuser tube. Since the objective lens is now fully AR coated and the main tube is black, I didn't feel there was any need for additional baffling. 

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