12 replies to this topic

[richr]

I've been using my Askar 700mm APO for six months now with excellent results. Last week I set it up with the 0.6X reducer for a shot of the Tulip Nebula. Preview exposures were sharp from corner to corner. I needed to rotate the camera to adjust the composition, but after loosening the rotation ring and re-tightening it, the stars on the edges and in the corners were stretched away from center. I tried loosening and re-tightening the rotation knob again to make sure it was seated properly but got the same result. What could be the cause of this? One person I asked about this thinks I need another spacer between the reducer and sensor, another person says I introduced sensor tilt when I loosened the screw on the rotation ring. Here is an image from that night.

Attached Thumbnails

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[afd33]

It looks like you need to add more spacers to increase the backfocus. It’s possible there’s tilt too, but all four corners tells me backfocus is at least part of it.

https://images.app.g...bcZmkeZPjvTbm97

Edited by afd33, 21 July 2024 - 10:15 AM.

[Tapio]

0.6x reducer really has tight tolerance for correct back focus.
And it can be difficult if the setup is not screwed.

[ccaissie]

Not a photo guy, but the "stretching" is symmetrical in all 4 corners, or nearly so.  

 

That means it's not tilt introduced by any unequal screw tightening.

 

Foucus is sharp in the center, but not the edges?  Not a focus issue....refocus the edges and see the center is now out?

 

I'd say the addition of the reducer has introduced off axis astigmatism or curvature of the field.

[Bill G.]

What Tapio said. Backfocus is off.

Set as accurately to the spec listed for the reducer as you can and then fine tune using the image link Tapio provided. You may find that if set very close to the spec, you will be good to go. ...but don't be surprised if it isn't.....

Bill G.

Edited by Bill G., 21 July 2024 - 06:30 PM.

[Oort Cloud]

Also, adding or removing filters from the light path can change BF point. Usually small increments, but the faster you go (lower f/#) the more sensitive the system becomes to misalignments/spacing/focus/tilt/etc, and tiny differences can lead to significant changes.
Tolerance is the cost of speed.

[mrmusclesrocks]

Yeah like everyone said, add a small spacer, maybe 5 or 10mm and that should do the trick. 

[archiebald]

I've been using my Askar 700mm APO for six months now with excellent results. Last week I set it up with the 0.6X reducer for a shot of the Tulip Nebula. Preview exposures were sharp from corner to corner. I needed to rotate the camera to adjust the composition, but after loosening the rotation ring and re-tightening it, the stars on the edges and in the corners were stretched away from center. I tried loosening and re-tightening the rotation knob again to make sure it was seated properly but got the same result. What could be the cause of this? One person I asked about this thinks I need another spacer between the reducer and sensor, another person says I introduced sensor tilt when I loosened the screw on the rotation ring. Here is an image from that night.

You might find it easier in the long run to invest in one of these, they come in two flavors.

Askar Adjustable Backfocus Adapter

 

https://youtu.be/0tpxRjg-4kc

 

I recently bought an Askar 120 with 1.0 and 0.8 flat/reducers.  With the 1.0, I got perfect stars so this adapter is not required, haven't tried the 0.8 yet due to cloud, but if I do find anything a bit off, I'll be going for the adjustable adapter.  Far easier than messing around with spacers and fine tuning might change depending on filters in use.

 

With the 0.6 for the Askar 103 causing extra sensitivity, the adjustable adapter might be worthwhile and way easier than fiddling with spacers and shims.

Edited by archiebald, 22 July 2024 - 03:40 AM.

[Oort Cloud]

Yeah like everyone said, add a small spacer, maybe 5 or 10mm and that should do the trick.

With a .6x reducer, it'd be more like 0.5mm or 1mm. Maybe even just a few delrin spacers.

[matt_astro_tx]

The resolution of the image you posted is too small for me to run a sensor tilt assessment on it, but as others have said based on the stars in the corners you need additional backspacing.  Here are some diagrams that help.

 

[Kerry D. Green]

With a .6x reducer, it'd be more like 0.5mm or 1mm. Maybe even just a few delrin spacers.

I was going to make this same comment.  The spacing can be very touchy.  I like to play with these kind of things (and new software configurations) on full moon nights.

[bbasiaga]

Tilt cannot be accurately assed until you have fixed the back spacing. Everyone here obsessed over tilt...save worrying about that until last, and then only IF you are really unhappy with your flat field results AND have eliminated all other mechanical causes. Once you start adjusting tilt, you will never stop.

Brian

[Oort Cloud]

Tilt cannot be accurately assed until you have fixed the back spacing. Everyone here obsessed over tilt...save worrying about that until last, and then only IF you are really unhappy with your flat field results AND have eliminated all other mechanical causes. Once you start adjusting tilt, you will never stop.

Brian

Yup.

Then I bought a Petzval.

Never again.