Slump in a refractor

[skogpingvin (Bill)]

Originally Posted by Atmos

The best thing you could get yourself for focusing is a bahtinov mask, they're only about $30 or something and they'll make life so much easier for you

Thanks for that Atmos. I actually had one that I made myself for an old 6" scope, and I never actually got around to using it, for some reason. I'll probably make another one for the 4" eventually...

Bill

[lazjen (Chris)]

Originally Posted by skogpingvin

Very interested to hear from someone who has the same scope. Sounds like you've got a different reducer. What do the stars in the corners of your field look like?

With the camera (ASI1600MM-C) and the filters I've got (1.25"), I get slight vignetting. I think if I could get a smaller connector I'd be able to get rid of it. That masks some of the current errors. I had a thought this morning that I might remove the filter wheel and check the field is flat with no reducer in line - and fix things as required.

If that's ok, I'll add in my reducer. I'm trying to get this reducer to work: http://www.teleskop-express.de/shop/...Teleskope.html In a bit of insanity I thought I'd try for 0.6. If I get it to work, the scope will be at f3.9. So far, I haven't been able to get the spacing right with all the gear - but I might do some tests without some bits (like the FW again) to get the magic 55mm required and see what the field is like.

Originally Posted by skogpingvin

I focus using my fingers. I know it's old school, but my technique is that I take an exposure of a handy cluster or something, then look at the preview zoomed to the max, then adjust and take another. While tracking, if you take a number of exposures, you can flick through them all at max zoom, and get a pretty good understanding of what the best one is. You then have to get back to it, which is the trick...

I got sick of manual focusing pretty quick. Any scope I buy from now on either has a motorised focuser or will get one.

Originally Posted by skogpingvin

I'm planning a visit to a dark sky site next weekend for Centaurus A (weather permitting, of course) and at this stage they're forecasting freezing temperatures both Friday and Saturday. I don't have any dew zapping equipment, so I'm going to take my daughter's hair dryer! A few seconds blowing on low power and warm heat down the dew hood between subs should do the trick, and I'm hoping it won't upset the optics too much. I'll get a couple of dew straps, but I have to pay for school fees and a final payment on a bathroom before that's going to be possible.

Good idea all round.

[skogpingvin (Bill)]

Originally Posted by lazjen

I'll add in my reducer. I'm trying to get this reducer to work: http://www.teleskop-express.de/shop/...Teleskope.html

Woah, 0.6 is a big correction! Am I reading that right, and the corrector gets slotted into the focus sleeve and clamped using the compression screws? For mine you take the compression screws off the focuser to reveal an M68 thread, and the corrector goes on that M68. That way there's one fewer potential slump point, like casstony said down the thread a bit.

The great thing about that corrector is that the distance from the corrector to the DSLR chip is fixed at 55mm, which is (at least for my Pentax) exactly the distance from the flange to the chip plus the width of my t-ring, and the t-ring goes straight onto the flattener using the M42 thread so there's no worry about the back focus.

Bill

[Slawomir (Suavi)]

Wow, refractor working at f/3.9...orthogonality n spacing need to be extremely precise at that speed. If it works it will be awesome. There will be light loss away from optical axis for sure though.

Fingers crossed and good luck

[lazjen (Chris)]

Originally Posted by skogpingvin

Woah, 0.6 is a big correction! Am I reading that right, and the corrector gets slotted into the focus sleeve and clamped using the compression screws? For mine you take the compression screws off the focuser to reveal an M68 thread, and the corrector goes on that M68. That way there's one fewer potential slump point, like casstony said down the thread a bit.

The great thing about that corrector is that the distance from the corrector to the DSLR chip is fixed at 55mm, which is (at least for my Pentax) exactly the distance from the flange to the chip plus the width of my t-ring, and the t-ring goes straight onto the flattener using the M42 thread so there's no worry about the back focus.

You can either use the compression ring, or you can use the thread. I can't remember exactly what it is, but I suspect M48. I've got an adapter that steps down to that.

[lazjen (Chris)]

Originally Posted by Slawomir

Wow, refractor working at f/3.9...orthogonality n spacing need to be extremely precise at that speed. If it works it will be awesome. There will be light loss away from optical axis for sure though.

Fingers crossed and good luck

I did mention it was a bit of insanity.

I'm already vignetting in the corners without a reducer, so there's some loss of field already. I'll just have to see how much when I get it there with the reducer.

Of course the weather is ok for testing right now, but I am not. Typical.

[skogpingvin (Bill)]

Originally Posted by lazjen

You can either use the compression ring, or you can use the thread.

Use the thread, Luke!

[lazjen (Chris)]

Originally Posted by skogpingvin

Use the thread, Luke!

I should have mentioned that - yes, naturally I am using the thread.