7 replies to this topic

[RadiantDust]

I recently purchased Celestron's C8-N reflector, which comes bundled with an AVX mount (I realize the AVX mount is not quite strong enough to guide an 8" reflector with good enough accuracy for long exposure images, but I plan to upgrade to an EQ6-R at some point). My long-term goal with this scope is to learn to do differential photometry and eventually try to detect exoplanet transits. However, I'm still somewhat unsure about what all I'll need to add to it to make that possible. For my main imaging camera I've decided most likely on the ZWO ASI533MM, mainly due to the lack of amp glow, decent well depth, and square sensor. I'm considering using an OAG with the ZWO ASI432MM as a guide camera. Will I also need to invest in a coma corrector? Additionally, the OTA comes with a rack and pinion style focuser, which I've noticed during several nights seems to have quite a lot of stiction (and also needs to be almost fully racked out to achieve focus, though that may be due to poor collimation). Should I try to find a crayford-style focuser instead and if so, should I install an autofocuser or can I still get good data with just a manual one?

[StupendousMan]

Will you need a coma corrector? 

 

If your goal is to perform exoplanet transit photometry, you will need to make precise measurements

of at least two or three stars: the target, and one (or two, or three ...) other stars of similar

brightness; say, within one magnitude of the target either way.  So, the question is -- how

large a field of view does your system require to provide a high probability that

any random target will have one (or two or three) other stars of similar brightness

visible simultaneously?

 

Take a look at a list of likely targets, and note the typical magnitude; call it "M".   Take a few

test shots with your system, and measure the size of the field which shows

no appreciable coma, and then the size of the field including the comatic regions.

 

Finally, using a good planetarium program or star catalog, figure out the chances

that 2 (or 3) stars of magnitude "M" will fall inside the coma-free field.

If the chances are good, then you won't need a coma corrector.

If not ... then you might.

[RadiantDust]

So I'll have to get the cameras first before knowing if I'll need a coma corrector then? That does mean I don't have to spend as much money in one go at least. Honestly I'm still a bit overwhelmed. Just getting the AVX mount to behave has been frustrating (despite having what I think is an okay rough polar alignment, the GoTo 3-star alignment process has yet to work, and the mount never seems to know where it is. Here's hoping a factory reset and fresh drivers solve the issue on the next clear night).

 

I'll definitely go through the Swarthmore database and pick out a few of the deeper transits and take a look at the starfields, maybe even see if I can take a look at them visually.

[pvdv]

1) I'd definitely go for the coma corrector.

2) i'd upgrade the mount at once - you will need good tracking for 4-5 hours and often a fairly quick and accurate meridian flip for transits.
IMHO, as far as hardware is concerned, this is critical. You can do transits with a 100mm telephoto lens on a good mount, you can't do them on a larger scope with a so-so mount.
There is nothing worse than a capricious mount.

3) unless the mirrors have been moved to extreme positions, collimating will not change the focus position meaningfully. The coma corrector will help in that respect in the sense that you will have a fixed backfocus (around 55mm) behind it and will be able to use a 'standard' imaging train made of supplied adapters and/or common OAG/filter holders.

 

4) an auto-focus would be a nice add-on, you could refocus manually, but it will be very tedious. The focus will change with temperature and the meridian flip.

[SeymoreStars]

Are you a member of the AAVSO? If not join. Here's last years list of courses offered, 2025's courses will be posted in January.

 

 

 

 

 

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[SeymoreStars]

This website can show you the resolution and field of view of various scope + cameras combinations - https://astronomy.to.../field_of_view/

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[SeymoreStars]

1) I'd definitely go for the coma corrector.

2) i'd upgrade the mount at once - you will need good tracking for 4-5 hours and often a fairly quick and accurate meridian flip for transits.
IMHO, as far as hardware is concerned, this is critical. You can do transits with a 100mm telephoto lens on a good mount, you can't do them on a larger scope with a so-so mount.
There is nothing worse than a capricious mount.

3) unless the mirrors have been moved to extreme positions, collimating will not change the focus position meaningfully. The coma corrector will help in that respect in the sense that you will have a fixed backfocus (around 55mm) behind it and will be able to use a 'standard' imaging train made of supplied adapters and/or common OAG/filter holders.

 

4) an auto-focus would be a nice add-on, you could refocus manually, but it will be very tedious. The focus will change with temperature and the meridian flip.

Put your monmey where your "mount" is.

[rutherfordt]

I'm going to offer slightly different advice than the previous posters-- try it with what you have-- you already have a mount and a telescope so only need a camera-- the 533 will work well, although so will a lot of others.  You do not need to buy a bigger, more expensive mount for your initial attempts-- what you have will work just fine.  A bigger, heavier mount will make some things easier, and may be a future purchase, but it won't determine whether you detect a transit or not.

 

One thing that you must pay attention too, however, is your polar alignment-- a rough polar alignment won't cut it.  You're going to be tracking a star for several hours so your polar alignment will have to be pretty accurate-- an autoguider will help, but you still need to be pretty close.

 

Meridian flips are not a big deal-- there will just be a gap of a few minutes in the middle of the transit which will affect nothing (make sure that the flip isn't going to happen near the beginning or end of the transit though-- it will be an issue in that case).

 

Autofocusers are nice but aren't really needed for this-- all you are doing is counting photons within an aperture.  Within reason, the size of the star image won't change anything about that.  Try for a good focus, but a perfect focus isn't necessary.

 

As already mentioned, think about joining the AAVSO-- if you are just starting out, the courses will help you a lot and will certainly shorten the learning curve a bit.

 

Don't be afraid of failure (failure is just another way of spelling "learning").  My first transit attempt was in 2007 and I failed at it (I failed at the second attempt too) but I kept at it and have since done hundreds-- it gets easier.

 

Good luck-- once you see that drop in the light curve of the star you followed for hours, you'll be hooked!

 

Tom