TS Optics (TSRCflat 3) flattener for GSO RC's

[Paul Haese]

Having spent quite some time and expense buying adapters in trialling this flattener to determine its ability to flatten the GSO RC12 field and via advice from TS Optics I have come to the conclusion that the flattener does not seem to work on large format sensors. It might work well with smaller sensors but larger format sensors don't seem to be corrected properly.

Upon looking at the flattener it has a 3" aperture and that would indicate it to be ideal for use on large sensors. It has a good stated back focus but I have discovered that is most likely a flattener for a refractor. The flattener looks exactly like the one TS Optics sells for refractors and it looks exactly the same as the one sold by Meade for their refractors. There is a possibility though that there is a different prescription for the refractor flatteners but some how I doubt it. There is nothing out of the ordinary about refractor flatteners being used for GSO RC's. I have used a Tak flattener on my GSO RC8 before and it worked well on an 8300 sensor. That flattener would not work with my current setup.

I found initially that the stated back focus of 106mm produced worse results than without the flattener in play. I was advised to move the sensor further back from 106mm to 109mm. This had the effect of making the field curvature look much worse. Reported curvature went from 37% to 45%-47% on various sub exposures. I was then advised to move to 103mm and found that this improved things a bit but did not eliminate the field curvature beyond 30%.

I have made sure my mirror distance is very close to optimum as stated by GSO under their documentation. Star shapes in the corners of the images without the flattener look like field curvature.

I have included two image that are crops of the top left of the frame of the same field of view for comparison at a back focus distance of 103mm. The first is with the flattener in place and the second is without the flattener. As you can see there is in fact a slightly worse correction with the flattener in place.

So it would be my assessment that this flattener might not work as a flattener for large sensors on GSO RC's. Feel free to ask or discuss.

[DJT (David)]

Hi Paul

Thanks for sharing your findings on that. Are you aware if GSO are going to come up with an effective flattener in the future?

I found with the RC8 that when I used a TS flattener, again refractor touted for RC's, I got a really bad hot spot in the centre that covered a good third of the sub. It did a good job of flattening the field and I am assuming I could handle it through flat fielding but it was really obtrusive so I removed it from the imaging train pretty much staright away. Did you encounter a similar issue with the larger one in your testing?

[Paul Haese]

Originally Posted by DJT

Hi Paul

Thanks for sharing your findings on that. Are you aware if GSO are going to come up with an effective flattener in the future?

I found with the RC8 that when I used a TS flattener, again refractor touted for RC's, I got a really bad hot spot in the centre that covered a good third of the sub. It did a good job of flattening the field and I am assuming I could handle it through flat fielding but it was really obtrusive so I removed it from the imaging train pretty much staright away. Did you encounter a similar issue with the larger one in your testing?

As I understand it, GSO are planning on doing a flattener soon, one dedicated for their scopes. Currently GSO are moving premises and that has delayed operations for now, though plenty of stock is available I am told. I am hoping this can be made in the next year as other dedicated flatteners are very expensive.

I had not noticed a hot spot but I was not specifically looking either.

[gregbradley]

Unless the hot spot/bright centre of your lights is really bad you can flat field them. I've done this with a few scopes with correctors namely Tak BRC250 and CDK 17.

It does put pressure on your flats, darks and biases being high quality. A bit off and you'll get poor results.

What worked for me was dusk flats or flats taken with roof closed and a white cloth cover on the tube. I expose to about 22 to 26,000 ADU with a 16803 camera. Camera in focus and same orientations as used for imaging.

Take quite a few. Then mean combine in CCDstack no bias subtract.

I make a good quality bias, same temperature as lights and quite a few.
Min/Max combine. I subtract the bias when I apply the flats and darks and use the bias for both flats and darks (so the darks can be scaled).

A high quality dark, again quite a few and min/max combine.

Doing the above worked very nicely.

For some strange reason subtracting the bias whilst making the master flat gave poor results and odd corrections. I don't know why. Greg.

[DJT (David)]

Thanks Bradley, will give that a go when the summer kicks in. Have spent too much time tinkering this year with not a lot to show for it.

I did a set of daylight flats the other weekend and they have worked really well especially with the NB filters which took for ever to get to a reasonable ADU with a light panel.

[John K]

Hi Paul,

Would any of the correctors from Deep Sky Instruments work on the GSO RC's?

http://rcopticalsystems.com/accessories/ffc_70mm.html

[Paul Haese]

Originally Posted by John K

Hi Paul,

Would any of the correctors from Deep Sky Instruments work on the GSO RC's?

http://rcopticalsystems.com/accessories/ffc_70mm.html

Thanks John, I have made contact with them just now. I will wait to hear back. Hopefully it will be less expensive than the ASA one.

[gregbradley]

They used to be around USD$1200.

Greg.

[Paul Haese]

Originally Posted by gregbradley

They used to be around USD$1200.

Greg.

That is cheaper than 3000 AUD at least.