I'm just getting used to my new refractor, a nifty little triplet APO. It takes nice photos, but I've noticed some elongation in a corner of my shots. I've attached one, and it looks like the stars in the middle are pretty god, but in the corners they're getting quite stretched, outwards from the middle.
I had a yarn with one of the experts from ASV who suggested maybe some slumping in the train somewhere. I think this is quite likely, as when I've got the DSLR at infinity focus it's racked out quite a way, and along with the flattener (which weighs 500g alone) there's quite a load on the focus tube.
My question (and this is where the noob-itdity is truly revealed) is whether I'm not locking the train up right when I'm exposing. The second photo shows the focus rack and the end of the tube. There are a number of screws, which I've marked. Obviously screw C is the focus lock, and this was locked up nice and firm. But the other two I didn't have tightened. Screw B locks the rotation of the focus train so I can orient the camera as I want. But I don't even know what screw B does! My guess is that it wedges the focus tube to prevent slumping, but that's only a guess. Does anyone know?
I haven't had a chance to take more test subs with it all locked up (it's cloudy in Melbourne today). Does anyone know if this would help (I guess it can't hurt!)
Oh, by the way, the scope is a Sharpstar 107-PH that I got from Cris at Astronomy Alive. I'm liking it so far.
Hi Bill, the stars are stretched in all of the corners so either the flattener isn't well matched to the scope or your camera chip isn't at the correct distance from the flattener.
Hi Bill, the stars are stretched in all of the corners so either the flattener isn't well matched to the scope or your camera chip isn't at the correct distance from the flattener.
Thanks Tony, you might be right - the reducer/flattener is a Sky Rover 0.8X, and it has a sign on it saying "For 115APO". The scope is a 107APO. Cris was of the opinion that it was close enough and it was the right flattener for the job - that the difference wasn't going to be significant. Was he wrong? How close is close enough?
I think you might have some slump, regardless of whether the flattener is the right one, because the stars at the bottom right corner seem to be more elongated than those in the other three corners.
raymo
The slump is minor compared to the poor correction though, assuming correct chip to reducer distance. Telescope Service in Germany recommend a version of the Riccardi reducer or their 2.5" flattener for that scope.
TS also sell the same reducer Bill has and they offer it for a large range of different focal lengths - doesn't mean it will work well at different focal lengths though.
To my eye you need to move the camera further away from the flattener. Attached are images that guys from TS once used to illustrate problems with incorrect spacing. The image on the left shows star shapes when the camera is too close (your case it seems), and the other shows distorted stars when the camera is too far from the corrector. Hope it helps.
To my eye you need to move the camera further away from the flattener. Attached are images that guys from TS once used to illustrate problems with incorrect spacing. The image on the left shows star shapes when the camera is too close (your case it seems), and the other shows distorted stars when the camera is too far from the corrector. Hope it helps.
Well, while it's a pain to have to go out and buy more bits and pieces to get a decent image, I can adjust for the camera being too close to the reducer by adding an M48 spacer between the reducer and the camera. If the camera were too close to the reducer that'd be a pain.
The way it is at the moment (see photo) is the reducer ends in an M48, on to which I have put an M48 to M42 stepdown, to then attach to the Pentax t-ring (lucky Canon/Nikon users, they get to find t-rings with an M48 thread).
The question now, of course, is how much - 1mm, 25mm or something in between? Is there a way of estimating, or do I just go and buy a series of spacers?
I do not know much about reducer/flatteners, but I thought that flatteners are designed for a specific FL. For a flattener to work correctly, aperture does not matter, but the FL does.
Sky Rover 115 has a focal length of 805 mm, while your telescope has a FL=696 mm. This might lead to imperfect field correction even with correct spacing.
I had a focuser made by the same company as your Sharpstar- the compression rings on the focuser are awful and need to be replaced. They do NOT hold anything properly, and this will account for the "slump". The 2" insert is also a sloppy fit.
The focuser itself is OK, but you'd be best served replacing it with another option if you want to get into imaging seriously. I managed to tune mine, but it still was so-so at best.
Your spacing also seems off, and using a reducer not specifically designed for the scope will mean you will need to fiddle to get it right.
I had a focuser made by the same company as your Sharpstar- the compression rings on the focuser are awful and need to be replaced. They do NOT hold anything properly, and this will account for the "slump". The 2" insert is also a sloppy fit.
The focuser itself is OK, but you'd be best served replacing it with another option if you want to get into imaging seriously. I managed to tune mine, but it still was so-so at best.
Your spacing also seems off, and using a reducer not specifically designed for the scope will mean you will need to fiddle to get it right.
Um, ok, you're telling me that I've been sold a pup?
Just need to fiddle and tinker. The compression rings are cheap enough to replace, and a little time with the focuser will smooth it up a lot. I won't comment on the "generic" (?) reducer, as I have nil experience with reducers not designed for my scopes, let alone that particular reducer.
We all need to fiddle with focusers to get them right - heck, I have adjusted every Takahashi focuser I have had except one. Some take a lot, some take little. Surprisingly, the WORST focuser I ever had was one rendition of a Feathertouch - just could not get it right no matter what.
Tinkering is half the fun - keeps us from complacency
Looking at that I would estimate about 5mm, just looking at the elongation in all of the corners.
As for screw B, I think that is a tension screw which allows you to put tension on the tube from more than one direction. When you tighten the bottom screw it tends to lift the tube a slight amount, screw B allows you to push down as well
Um, ok, you're telling me that I've been sold a pup?
No, those focusers are quite good except for the 2" compression ring fitting. If you're using a screw on fitting for the reducer you bypass the weak point of the focuser. Experienced imagers become increasingly pernickety about all aspects of imaging, something us novices don't need to worry about - yet
I do not know much about reducer/flatteners, but I thought that flatteners are designed for a specific FL. For a flattener to work correctly, aperture does not matter, but the FL does.
Sky Rover 115 has a focal length of 805 mm, while your telescope has a FL=696 mm. This might lead to imperfect field correction even with correct spacing.
Clearly, I do not know much about reducer/flatteners
If the seller insisted that this reducer is suitable, then it probably is. Also the link provided by Tony suggests that this reducer can be used with various FLs. It looks like 2mm extra should get you there.
Having said that, I remember a conversation with a telescope manufacturer who explained to me that quality flatteners are designed for a specific FL. It might not be applicable to reducer/flatteners as much, or at all :-)
No, those focusers are quite good except for the 2" compression ring fitting. If you're using a screw on fitting for the reducer you bypass the weak point of the focuser. Experienced imagers become increasingly pernickety about all aspects of imaging, something us novices don't need to worry about - yet
OK, that's good, as I don't use the compression rings. The reducer fits onto the focuser using an M68 thread. The photo shows the train - there's a 3" focuser on the end of the tube, connected to the flattener using an M68.
I had a look at the link you sent (thanks for that, by the way, it's useful), and did a bit of a bodgy measuring. Working left-to-right in the photo, chip to flange on a Pentax-K is about 45.5mm (Wikipedia), the t-ring is about 9mm (not including threads). The step-down (up?) M42 - M48 is 5mm (plus thread), and it looks like between 1-2mm of extra thread gap where the step down doesn't quite cover the male M48 on the reducer. All up, looks like I've got about 61mm flattener to chip. That's kinda in the right ball park for the 0.79X on the link.
If I assume (from a previous post) that the chip is too close to the flattener I'll probably only need a few mm extra space. The original thought (slump) might be handled by tightening everything up a bit.
I'm pretty happy with the scope overall though. It got this:
I was curious about suitability of a reducer/flattener at various focal lengths. After a bit of reading have found that although these are designed to correct curvature for a specific focal length, some variation will still yield satisfying results, but the field may not be optimally corrected further away from the optical axis, so it may not be suitable for larger sensors.
Since this particular corrector was designed for 115 Skyrover with 805mm FL, my crude calculations indicate that it should work okay with telescopes between 650mm to 950mm FL, in particular if the sensor is not too large, but the spacing needs to be corrected by a few mm depending of the FL.
Please take all of the above with a pinch of salt :-)
Bill, I've got the same scope and I'm also finding it good as well. I added a Sharpsky Pro motorised focuser to it to control the focusing from the computer. The focuser that come with the unit is good and holds position well.
I've got a reducer that I'd like to get set up with the scope, but I haven't got the correct spacing as yet - I need to shave some mm off which is a bit hard with the FW and OAG in the mix as well. Theoretically I can do it, but it's getting the "shorter" connectors between some pieces that's the key.
I am having issues with dew and I've now got a dew strap. The Sharpsky Pro unit comes with a heater controller as well, so once I learn to RTFM and work out how to operate the heater controls, I should hopefully have the dew issues sorted as well.
Bill, I've got the same scope and I'm also finding it good as well. I added a Sharpsky Pro motorised focuser to it to control the focusing from the computer. The focuser that come with the unit is good and holds position well.
I've got a reducer that I'd like to get set up with the scope, but I haven't got the correct spacing as yet - I need to shave some mm off which is a bit hard with the FW and OAG in the mix as well. Theoretically I can do it, but it's getting the "shorter" connectors between some pieces that's the key.
I am having issues with dew and I've now got a dew strap. The Sharpsky Pro unit comes with a heater controller as well, so once I learn to RTFM and work out how to operate the heater controls, I should hopefully have the dew issues sorted as well.
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?
I don't use an OAG, using an autoguider instead, so my train isn't quite as long, I guess. I think I'm going to be adding millimetres. I think I'll end up doing it by replacing an M48-M42 stepdown with a slightly longer one. There are a large number available, none all that expensive (mostly around $15), and all different depths.
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'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.
Strangely enough, and this might be not using the scope to its best abilities, I don't use it for visual work, and I don't have any (decent) eyepieces, I just use it for photography.
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
and work out how to operate the heater controls, I should hopefully have the dew issues sorted as well.
